allfritz123

Thank you for the nice pictures! Beautiful. As you said, we have the same radiator with the big tank on the top as I see the old brace bracket in your pictures like I have on my tank. Your hood also attaches via what I call an eyebolt system at the bottom where it hinges. You have 2 flat washers on it. Mine is the same arrangement! I am trying to locate a suitable CAC. You swapped tank external ports so that you gained additional width to go around the big top tank from what I understand. I am looking into finding a similar CAC. You said your donor truck was a 1988. I can find a CAC from a 1988 on the website below. It looks to have 2 rectangular inlets which looks like a match to yours. It looks like a 2.25 inch core width. I assume you had louvres originally on your original 1978 radiator like our does. At any rate, it can be discarded! https://truckpartsinventory.com/part-details/84693932/new-1988-mack-rdmodel-charge-air-cooler-for-sale Another options I saw but don't know if overall width is wide enough to go beside the old tank? It may have straight out piping which requires no welding? Mounts are different as they use 4 horizontals and maybe it too narrow! https://radiatorpros.com/hd-charge-air-cooler/mack-charge-air-cooler/1983-1991-r-u-models.html My 1985 donor truck engine had a wider core width CAC than 2.25 inch and it feeds the air over the top. It also has the inlet to the manifold at the front of the motor. No way I can make it work. It looks like you were able to find one where it had the inlet further back on ebay. I will search for one like that as it likely makes the piping easier to assemble. I do have the piping from the 1985 and aluminum pieces that made it fit on its original application but it has its horns much narrower and likely designed for a radiator with no large cast top tank. I think my donor engine came from a Cabover (long dipstick and oil filler spout). It also had a very large oil pan on it. Its a 51 quart!

I know one thing that if I could make it work, I would have to find a "shorter radiator".! As in the picture posted from JoeH of his '79, I have the exact same radiator with the huge tank at the top which is too wide to entertain my piping configuration for the air intercooler I have. If I could shorten the radiator by removing the tank altogether, I maybe would have room for the piping on the top. Certainly my radiator does not have provision for an intercooler to bolt to it. It had louvres on it which could be discarded and that would make it a little thinner. My biggest concern is to make it all fit under the hood! Added: Would my hood be the same as a '78 and allow the same setup of the CAC to tuck inside. IT would be great if it would and I could go that route. I would buy another radiator as it is a better system.

I pulled the rear journal cap and it plastigaged at .0035 (tolerance is .002-.005). There were signs of copper on it peakingtowards its back end. I plastigage another connecting rod journal and it was also .0035 (upper tolerance on these is about .0044). As I spun my shells on the #6 connecting rod, it must have been wore? I am at a loss as to why the engine failed. Maybe the last rod journal is always the worst. Oil pressure was always very good and likely at least 75 psi. Startup pressure was more than 90 even with the failed shells. The liners also appeared in good shape. There was very little ridging and was only noticeable on the thrust side. The engine never did use oil apart from parasitical loss. An update on the whiffer valve. It was bypassed on this engine and hence why it rolled a lot of coal on turbo spool up. The lines were in place as someone had put a plug internally in the line and then reattached it!

I would love to see the pictures! Did you have to change the radiator or was your old rad i the 78 compatible? The radiator that is on the 1981 Mack has a huge reservoir tank bolted on the top and also contains water connection to the motor. I tried the CAC I have and the tank is in the way . It is way too tall to go on top and too wide as the necking doesn't extend far enough. My truck uses bracing extending to the horizontal to the large tank which also has the upper water connection to the motor. My donor CAC is necked as well and both outlets come over the top but it has to have a small radiator to fit under the hood. I did see how the bracing was changed on the newer models and the hood roller was different on the cab end as well. I didn't think I could make it fit! Louvres definitely have to be removed from the front. I wondered if the radiator itself needed to be less thick to keep the CAC under the hood or if there was hood changes in the later models?

Thanks for confirming 4valve. Is the valve lash the same for the 2 engines Intake .016, exhaust .024? Both plates on the side of the block say this. The sticker on the top of the valve cover on the 4 valve was unreadable. It was also interesting to note that the E6-300 versus EM6-285 had HP ratings given at 1700 vs 2100 and timing 23 degrees versus 22 degrees BTDC. They both have American Bosch pumps. Initially we were looking at having someone rebuild the injection pump. Parts are no longer available but you can buy a factory reconditioned one for $7800 Cdn! If we put in the donor engine, we will not bother to rip the other pump down. I guess the 1981 was a Maxidyne because it says EM6? And the E6 is an Econodyne as LTK pointed out. I do have a 12 spd transmission so I should be good.

I finally got some pictures of the donor engine. I fit the cooler box onto it. Is it a 4 valve? I have just enough room for the cooler box to go onto the newer engine. I have to make a new angle iron bracket to mount the cooler box onto the top of the cylinder heads. However everything looks to fit. I am going to remove the engine out the front so I don't have to move the big tranny and disturb the dirt!

Well this truck is used on farm only to haul loads out of the field for the most part. It already has been "rolling coal" since we got it a number of years ago! Twice neighbours came over and wondered if we had a fire! The pyrometer comes up rapidly and so you drive it with feel. Lots of power for what we need and eye on the pyrometer always. It looks like the puff limiter is still in place from intake to fuel injection and so I am not sure if it already was overfueling or that item was not working. The only difference in the engine I have to put in is that it was set up for 300 hp (1985 E6-300) while our old one is 1981 E6-285. So I am not sure if that is significant difference in turbochargers although I will be using the old turbo and cooler box setup as the other turbo would not fit in the geometry. I assume they are same series? It isn't like we are putting a 350 in it. I am sure this transmission is a maxitorque (maxitorque 10or12??) as it has triple countershaft and multiple reverse gears. It is a heavy transmission with 5 main gears (hi-low-reverse splitter to give 5 reversing gears) and and a low gear shift on the floor with no air shift on dash). I saw another post that has that style of shift. I guess I would call it a 12 speed! I am leaning to taking the engine out from the front. I never had a chance to clean up the back end well before putting it into the shop and I will get quite a shower working under there. Thoughts?

I will get a picture of the valve cover Terry. I am thinking I can adapt the tip turbine to the 4 valve. I am curious why you would think it is labor intensive fjh? I am going to try fit the tip turbine onto the donor engine. I can understand that it would be hard to adapt if the jake module was on top of the head _ I run out of room! The donor engine I have has no jake. Everything looks very similar space wise. I think I measured 10 inches to the top of the valve cover on the 4valve? while the 2vh is 9ish inches. The difference being the head is thicker (6 1/2 inches?) but the cover is tapered and shorter and has bolts all the way around instead of 2 mid cover bolts (much better system!). The taper should allow me to fit I believe. I am going to make sure I can fit the box onto the other engine before going further. The head itself on that 1985 engine is thicker and has the injectors side mounted while my old one has it vertical mounted on the head LHS. The newer engine has bolts all the way around the cover and looks similar to my E7s from 1998 or 1999. Exhaust manifold looks similar positioned. It looks like the air cooler top bolts will adapt right onto where the air cleaner was mounted for the other truck. They have 3 holes tapped on the cylinder head bolts LHS and in the correct position already to mount it! As long as I am not missing anything, I think I should be okay. The other thing which I see is that the front engine mount on my old engine sits in a saddle apparatus secured with 2 capscrews while the donor engine was flat vertical mounted with a frame. I think that will unbolt and then the saddle will work on the new engine which has the rounded nose on it. Thanks guys for all the tips. I guess in order to get the old engine out I will have to put the transmission on a jack. and pull it back out of the way so I gain clearance on the bell housing to come out vertical? Or would it be easier to open the front plate out and move forward? Any ideas on what is the easier method. That transmission is huge and I sure would not like to have to slide it back with airlines and all and sitting under the cab! This is a first for me and the engine needs to come out to get the crankshaft out of it for repair.

Sorry I was not quite understood. It is the rod bearing journal (crank pin) not the block journal which had the spun shells. I am going to pull the valve cover off my 1985 donor engine to see what the valve train looks like and verify it is a 4VH. I am somewhere's around 2.980 inches on that crank pin. Nominal OD is 2.998 inches. I wished I could fit my 1985 engine into this truck. A lot better design in intercooler! I just need a little space in the front. I don't think the hood was redesigned between years other than markers different? If cab and chassis are the same and the hood is same - why can't I get this other engine to fit? It looks tight. My current engine has louvres on the front of the rad which I would discard. The front intercooler bolts right on to the rad tight. I just don't know....

My initial micrometer measurement shows me about .020 off on the journal from standard. I think I have some taper as when I removed the shell it had climbed each other on an angle and I see some difference one side to the other about .020 to .015. As suggested above, some of shell was crushed onto the journal sureface. I could flake it off though. I am just amazed at how clear the manual is from Mack. It even tells me how many different shell sizes there are to choose from. I was quite excited to see that they could range from .010 to .060 undersize (after initial std .002). I didn't realize you could grind off that much and still use your crank again! I thought I would have to replace because of the damage! My plan is to micrometer the rest and use some plastigage to see how wore out the other journals are. Can anyone tell me if the camshaft lobes are different on a 2 valve hole (2VH) versus a 4 valve hole (4VH) 672 engine? Are there double the push rod lifters or is it all taken care of at the very top end with a fork? My other engine is a 1985 and has the injectors on the same side as my fuel pump RHS. My original engine 1981 has injectors on the top LHS of the cylinder heads. Injector lines cross the head. Would they have the same dish in the piston when running a 4 valve head? Also is the only difference at the top of the head? I can't find any pictures of what the valve train looks like on a 4VH. I only have the 2VH manual. I am still trying to figure out if I should rebuild or use the other engine. I am not sure if I could swap the cooling box over and turbo and remove the newer air to air system. I am not concerned about power as the 285 has more than enough for us. I understand the other system could give 300 but I can't fit the front of rad cooler into our existing truck as the hood nose doesn't come out far enough! The 1985 engine doesn't have a Dynatard or jake brake at all. It looks to me that if a jake were added to that newer engine (1985), They would have added an additional 2 inch (approx.) thick module sitting on top of the cylinders. That is how our E7s look anyway with Jakes.

Great ideas LTK! I removed the oil/water cooler so I can get at the bolts. Sometimes a person looks for the lazy solution! The reality is, just sacrifice a couple of gaskets and get that out of the way. Then I have no problem getting a 12 point wrench on it! It seemed like a big obstacle but really much simpler to get it out of the way with 6 bolts. I found the problem after removing the rear head. The shells rolled on number 6 - that was the tick!!! I can't believe that I wouldn't have seen oil pressure problems - must be a great pump to keep pressure so good even with a rotating shifting shell! I am about 15 to 20 thousands under 3 inch. I assume the crank will have to be replaced. I haven't looked at what standard is and how far down I am. It definitely is not smooth anymore! The rod will have to be replaced too!

I have one of those expensive wrenches (Snap-on) for the Mack rear end peanut section (about 5/8" I believe). If I had one with a 7/16" 12 point and not too long of an extension it might work. The 5/8" I have I had to order from Snap-on and it was super expensive. If I just have a 12 point 7/16" with a short knuckle built into it I will be able to work it. I will have to look around for that. It is so tight in there that you can't use a regular socket and an extension knuckle to extend out! The other ones I removed appear to have some locktite on them too! The ones inside the box they used a 3/8" 12 point. That would of maybe given enough room if they would have used that bolt size under the housing instead of 7/16" heads. I think thread size is 3/8".

The bolts under the box are the pain. I have to move out the entire round oil/water cooler if I want to use any flat wrench. It looks kind of nifty in the picture! I can assure you these bolts are tucked under and only accessible as shown in their nice schematic. Looking at the picture, you would think there is lots of room for a socket and extension. Not the case. Yes the ones in the inside required removing water and air coolers rads.

it is the tip turbine style! I was working on it today - only to find out I don't have a 12 point 7/16 on a short swivel to get at the 12 point bolts under the charge air box. The exhaust side came off really easy and I thought that was going to be the hard side! I sure wished I could mount the other system and swap engines. The 1985 engine has a front mount intercooler! I think I need a few more inches under this hood (which I don't have) in order to make the swap and put that rad in place

Continued the saga yesterday. Adjusted valve lash. The tick didn't go away at idle. It seemed however to not be as prominent at higher rpm. Ran it for a number of minutes. I did change out the broken o-ring on the rear Dynatard Bank at the same time i adjusted valve lash. The engine brake was working as I tested it. That was the only plus! Ran the truck again so that my brother and I could listen to it once more. We revved it up and it was almost like the tick disappeared. We thought then that maybe we had fixed the problem! Maybe we were hearings things all along!!!! We brought back to an idle and continued to play with it but the tick returned. Then, without any warning, the engine seized! No bang, just stopped. Pulled the pan, nothing in the pan. However cylinder 6 was on its way up on exhaust stroke and the valve lobe was in contact with that valve. via lifter and was hard on it. All other lifters were rotating by hand. I backed off the tappet. The rod is not bent but my initial thought now is that the valve ran into the piston and it was likely somehow sticking and made contact. Not sure the mechanics of how that can happen but I guess if a valve sticks open slightly and then piston helps close it, eventually causes a catastrophic failure! Drain the antifreeze and pull the head is my next step! I guess I will eventually get to the bottom of it or maybe the top of it! Gear train looks good. As I said early, cold oil pressure was very good and dropped to 60 or 70 psi when warm. Pan is fairly clean with no metal parts.

I haven't adjusted any valve lash yet. I just recorded the measurements. I definitely was on the compression stroke when I did my measurements. This is a Mack Dynatard system not a Jacob. Like I showed in the picture I took, definitely the rear Dynatard solenoid is leaking oil rather than acting on the valves on that head. I just did a manual push down with my finger. If you look closely, you can see the oil escaping and not being directed to pressure up the hydraulic lock components of the exhaust system to effect the timing. The front 3 cylinders operated correctly and no oil bypassed and it correctly functioned when I pushed or enabled it with my finger. The manual talks about using a special tool to set the exhaust valves. I assume I can do it with just a feeler gauge and pushing down on the tappets and loosening off the nut and turning until I get it where it needs to be. I don't have a special tool but I don't think it is rocket science - maybe just a little more time consuming. If you look near the end of the first video I posted you can see a sign of the mist or smoke? coming from the rear of the valve head looks closer to 5 than 6. I am wondering if I have a leaking head gasket and if that could cause the tick. There looks to be oil drool near 5 and 6. The tick gets more pronounced as the engine heats up and the oil is warmer. It is not so noticeable when you start up the engine and then it gets worse when you rev it to 1000. I don't really see exhaust leaking from an exhaust gasket. Look at the very end of this video (last 5 seconds and you can see a mist from near cylinder 5). https://www.youtube.com/watch?v=QXIPY63tczY Could I have a crack on the valve to the oil jacket on the exhaust and could that cause a tick? I want to check the water and see if there is any oil in there.

I had a good day today on the engine. My 1985 donor engine did have the pointer on it - so I moved it over to this engine! So that is a great help. I located all timer marks and did mark them with a paint marker. It was my first time working with valve lash on a Mack. Mack is different than CASE when it comes to setting valve lash. They must use a different cam cycle. As pointed out above, you have to move the crankshaft to 6 120 degree rotations. to cover firing. My other 6 cylinder engine you can set all lash with 1 rotation and stopping twice on TDC (2 to 1 ratio). Any comments on why Mack is different? Different gear or different lobes. Aside note. I followed Mack instructions and measured accordingly as specified. I noted the exhaust on some others while making the measurement on a particular cylinder did exceed greatly the set gap (on the exhausts anyway). I could even see .100 at times. My engine plate says intake I=.016, exhaust E=.024. My results were: Cylinder 1: I=.019 E=.037 Cylinder 5: I=.019 E=.033 Cylinder 3:I=.021, E-.034, Cylinder 6 : I=.019, E=.036, Cylinder 2: I=.020, E=.040 Cylinder 4: I=.017, E=.033 All the push rods looked good. What I saw as close tolerance on number 1 exhaust was actually the cover gasket laying next to it and so it wasn't bent at all! When I took off gasket, everything looked good and nothing unusual! Although not now to specs, could this be causing problems. There was some gases coming off the rear 5 and 6 cylinders and as well some oil seepage there. Could exhaust gasket create the noise? I also noted that the rear bank Dynatard solenoid is not functioning. I pushed the front solenoid and it jaked and maintained an oil seal. When I pushed the rear solenoid, it leaked oil out (see picture) and did not function. So, we have been running the jake with only one working set. I assume the seal must be gone. Here are 3 videos I have taken of the drive train functioning with the noise. https://www.youtube.com/watch?v=QXIPY63tczY https://youtu.be/kAetRPELwoY https://www.youtube.com/watch?v=GWCbu1FjH7s

The tick does increase frequency with rpm. I will get some pictures today. Maybe a video of the tick! I had the side cover off over the tappets once to put a gasket in. It seems the people that had the truck before had not got it in place (sagged) and it was leaking oil bad. Can I see the cam from there or do I have to look from the pan? It sounds like pan has to come off. The only time I ever saw a bent pushrod it was quite obvious. Number 1 exhaust appears to rotate and I don't see change in clearance during its rotation. If it does, it is subtle or my eyes aren't picking it up. However, as mentioned if it has effected the cam lobe and flaked off on the cam lobe end, I guess that will have to be determined. I just don't feel the tick comes from number 1. I have an old stethoscope, will see if I can pinpoint closer where sound is loudest! I will get picture of front and also finish cleaning off dampener to see if I can find the other 2 positions at 120 degrees. I should check my donor engine as it is out and I could see what it the 1985 engine has for a pointer! Maybe I can even move it if it is attached to the 2 studs. My manual seems to tell me there are 2 versions of pointers. It is a Mack original printed Feb 1981 and I have the 12th printing 1994. E6/672 C.I.D Engine 2VH. Great Ideas. Thanks. I am going to see if I can't video it, before going in to deep. That way I have a reference point. I really don't want to rip everything apart until I have properly diagnosed it with everyone's help! Side cover comes off easy!

We have a 1981 RD Mack with the airbox (series charge air cooling I believe it is called). It has been a great truck and always strong. We recently went through some rough ground on the farm with it loaded and it developed a tick in the engine. We parked it and have now put it into the shop and are trying to figure out the problem and solutions. This engine has the dynatard brake. It has great oil pressure, still starts good and always was a good puller in and off the field. It always leaked a minor amount of oiI but not an oil user. (Oil coming out of interfaces.) Exhaust always looked good and blowby - I would consider normal. I think someone set up the pump before we got it as it will throw a heavy black smoke when you stop on the pedal. As well, pyrometer has to be watched if pulling hard as it can climb rapidly under heavy load. We always suspected the pump was set up but we watch it carefully. Loaded truck on gravel road could be around 58,000 lbs. With a load that heavy, we typically have kept temp down by limiting speed. Try to keep it below 1100 degf and typically 1000 or so degf by not driving as fast. Empty truck is typically 900 degF although if you step on it hard, it will move up! I have taken the tappet covers off and have found all springs in good condition. I ran the engine to see if the tick could be located. I think it is coming more from the rear of the engine - hard to tell. I am wondering if I should use a stethoscope? I did notice the exhaust lifter rod on number 1 runs right on the edge of the cylinder head casting. All the others appear to have at least 1/16" clearance. This one is very close. With tappet covers off and running, the push rod is close enough that the oil wants to seep out over the gasket interface as it is that close. It doesn't appear to be the source of the tick but nonetheless it is a concern I have. Bent push rod or guides wore? The rod is rotating nicely and lubrication is good and have ruled out a bent push rod as the distance should change with rotation. We are wondering if we could be getting a wrist pin slap. The engine is aging and the oil pressure is really good yet. If a problem with a shell, I would expect oil pressure to have deteriorated. One thing I tried (because it was easy) was to crack each injector off the pump to see if there was any change in the tick. There was no difference on any cylinders. I tried checking tappet clearance - chart on engine block says Intake .016, exhaust .024. I have a Mack service manual and it says pointer is in line with the 2 studs on the front cover (approximately?). Degrees are marked on the dampener. I have no pointer. I also have no markings a third of the way around the dampener like they talk about measuring and marking. So I lined this up so I could check 1 and 5. Intake on 5 seems tighter than that, intake on 1 seems good. Dynatard with tappet pushed onto exhaust rod appears to give about .034 on 1 or 5. It wasn't jaking as hard as it used to (and maybe never did). It is working. I was just doing rough checks so far. I would have expected higher clearances with wear and not less on intake 5. It was strange to me that #5 intake is tighter than .016, maybe it is .012. I was just a doing a preliminary quick check! We have another engine. It is out of a 1985 truck we think and it is likely a 4 valve E6-300. It has an air to air intercooler. It also has no jake brake. We are thinking it is out of a superliner (cab over) garbage truck? as it has an extended dipstick tube and a different throttle linkage. I assume if it had a jake, it would have an additional module on the cylinder heads like our E7s have as it is newer. Our 2VH has the dynatard system with brass pickups located on the inside of the tappet cover. We would put this engine in but the additional radiator at the front would not fit under the fibreglass hood of our existing truck as it becomes longer. Thus a dilemma. Can we strip and move and leave internal bottom end? I did note the injectors on this donor engine are on the same side as the injector pump while our E6-285 is on the other side. The turbo is higher up as well. Maybe someone knows if we could move the liquid cooled air cooler from one engine to the other swap turbos and make it work to fit under our hood? We love our Macks and we fondly call this truck Bucky because of its unique multi-reverse gearing, secondary bull low gear and the unique engine characteristics when picking up gears (its Bucky and we can't live without it!) All our other trucks are E7s.

Enjoying everyone's feedback on 2V vs 4V and a great forum of guys! Thanks for all information. I am going to start another thread with my problems and queries.

I have also noticed that the oil pressure is no longer 50 or 60 psi which is likely related to the electronics and jake brake. The oil doesn't look diluted nor above full. I have ordered a oil pressure sending unit. I now have to try locate it as it is NOT on the oil filter housing. I can unplug that sensor and nothing happens and the oil pressure is still reading 35 psi. Should be here Wednesday -had to come out of Chicago. Hope that fixes the problem!

My jake brake was working fine and then stopped working. Trying to figure out why. The Jake brake will come on at about 1200 or 1300 rpm - therefore assume still must have the wiring attached. Does this mean it needs to be adjusted? It always worked good until now and really had no signs of dying and all of sudden went to 1200 or 1300 rpm operation only. One thing I noticed for a long time is that the switch needed to be in on position for a bit before it ever starts to work. I assume that was a normal function. I am quite illiterate as to how the system works and what has happened and what is wrong. Hopefully someone can enlighten me as I love using my jake and it is hard on the brakes and don't know if I have a major issue.......

I have the truck working again. Very nice and quiet and shifts better than ever. I ended up taking the outer cam assembly to the Mack dealer and he installed it. He also installed a cup/cone bearing on the back side because he felt they couldn't get the cone off without damaging it! They used 3" socket and special adapter to break the nut off to remove the outer cam. They charged 2.25 hours labor or $250 to do that! I added in CaseIH limited slip diff oil additive as MACK apparently doesn't sell it anymore. I wasn't able to torque the specialized 12 point bolts to 150 ft-lbs as I am still waiting for the spline torque adapter from Snap-0n to do this with. I tightened them as best as I could with a regular wrench and snipe. No way to get a regular 12 point socket in to break or tighten those bolts. I ended up heating them up and machining down an old 5/8 socket to get them out. They were severally rusted so I replaced all 8 bolts @$14 each. I tried taking them off one without heat and I ended up damaging one bolt so had to struggle a little to get that one off (ended up pounding on a 15 mm wrench! Anyways, thanks for everyone's help - I really appreciate this forum. Allan

Good Advice. That is exactly what I am going to do. Take it off and take it to the Mack dealers so I get it torqued properly. I ended up buying a 1" impact (1600 ft-lb) and having my brother sniping down with a pipe wrench on the socket and I still couldn't get it to budge. Yes, I am using a 1/2" air line and running it at 120 PSI while the tool recommends 1/2" air and 90 PSI! I am scared to use heat because I may wreck the $55 nut and if I reuse it could I trust it? And I don't have the fixture to lock down the cam in order to retorque it properly. I think lock compound breaks down at about 300 degF. Just curious how my costs compare to that of the US. We don't really have true capitalism in Canada as the corporations rule the country and the government basically allows them to keep out or buy out the competition so that the consumer has to pay whatever they desire to charge. Sad but true when the political parties receive their funding from the large corporations they get what they want out of the government in return. Dollar is pretty much at par 1 Cdn dollar = 1 US dollar Peanuts/Wedge Kit $187 Outer Cam $1056 Inner Cam $177 External Power Divider Seal $22 Peanut Cage $789 Power Divider Bearing $99 Just curious to know how badly we get hosed for our parts here in Canada! Allan P.S. My dealer was gracious enough to give me pictures of some of my rear end details so I thought I would share and could maybe help someone else out.

sorry had same message twice. Mack_CRD_203_PDivider_Diagrams.pdf