doubleclutchinweasel

Here is some other discussion from another forum. https://www.smokstak.com/forum/threads/1957-mack-b61-tt-in-tennessee.164568/page-3

Sounds like there have been issues with these before. One post at the bottom of this thread (below) had some interesting comments about the clutch linkage.

There have been several things posted on here with dimensions on them. B-Model_Cabs.pdf

Let me look tonight. There might be some Unishift info in a service manual I have at home.

Sitting here watching “Towing” (1978). Mack C model tow truck. Don’t know if anybody listed this one yet.

It's like I said before. Once you go through that engine, it will be as good as the rest of that ride. I'm glad it wasn't worse. I look forward to seeing it back together.

Different people have different things which are important to them. Me, I don't like to do body work. So, I would pay extra for good sheet metal. Some of these (like Matt) are absolute artists with sheet metal. So, sheet metal work doesn't bother them. If you feel good after buying it, then you got a good deal.

Glad to hear you found the switch and got it sorted out. Plus, you were able to keep it original. Thanks for posting the follow-up. Follow-ups are very helpful for the next guy with a similar issue.

Sounds backward. Red line (IIRC) should pressurize when trailer air system is charged. Blue line should be "service" brakes, pressurized when service brakes are applied (foot valve or hand valve). I'm sure the brain trust will chime in here before long.

Did I ever mention that these guys are amazing?!

The 1750 engine (I think) were referred to as "limited" or "governed" engines. But, yes, the PERCENTAGE drop or rise would be the same. So, if you can go to 2100, then you would drop to about 1200 instead of 1000. And, you could downshift at 1200, double-clutching up to 2100 to catch the next lower gear. Sounds a lot like your "1200-2100 operating range", doesn't it?!

Actually, it looks like the ratios in that box ARE pretty widely spaced, requiring lots of "drop" while upshifting and lots of "rise" while downshifting. The MPH on this chart will vary with rear gears and tire sizes. But, the top and bottom of the RPM changes should be constant. Looks like shifting around 1750 will drop you to about 1000 on most shifts (43% RPM drop). Conversely, you would have to pull down to about 1000 before downshifting, double-clutching up to about 1750 to match the gears and let them slide in (75% RPM rise).

Welcome aboard. Shifting a non-synchro box (up or down) is a matter of matching the speed of the engine/input shaft to the rest of the transmission. "Double-Clutching" takes just a little practice to master. You have to listen to and feel of the beast to know what it wants. As far as smooth engagement of the gears, there is no magic RPM. If it matches, it goes in. If it doesn't match, it grinds. That simple. Now, some engines do have a preferred RPM range to work within. So, for any combination, there will be a sweet spot where the power delivery is optimized. But, work out you shifting to get it smooth first. You may be correct on those downshifts. That may be the speed you have to be at before you have enough RPM available to make the downshift smoothly. Or, it may need a different RPM before going into the lower gear. Each box (and each shift, for that matter) has a specific RPM percentage change. For instance, your comment about coming out at 1800 and going in at 1200 represents a 33% drop in RPM for that shift. I don't know the ratios for that box, but that may be a little too much RPM drop. If, for instance, the drop is really 25%, then it might go in better at 1350 instead. Your 2000-1600 is a 20% drop, so that might be the percentage you need right there. And, remember that downshifting is the inverse of the upshift. That is, if the RPM after an upshift is 3/4 of what it was before the shift, then downshifting that same gear change will give RPM that is 4/3 of what it was before the downshift. It just takes a little time to get used to it. And, each transmission will have its own drops. Obviously, a 5-speed will have far greater drops between gears than an 18-speed. Pretty soon, you will learn how to unload the drivetrain with careful throttle work, and you will be shifting without even using the clutch..."floating" the gears. When floating, the correctness or incorrectness of the RPM matching is even more evident! Just be patient and give yourself time to learn it.

Way back when, Mack sold a conversion kit. Had one on the truck I drove in ‘80. Still had the oil bath bottom on the can. But, when you dropped it, there was a dry filter up in the main body. I couldn’t find anything like that for my later truck. I’ve seen a lot with the Donaldson can swapped onto them.

Yep. It loaded. That's what I thought...or was afraid of! LMAO!

I couldn't get it to load right now. Is that Raymond J. Johnson, Junior?

By the way, we also called the forward/reverse lever in a direct-drive Caterpillar tractor the "Johnson Bar". I think the steam locomotive engineers used that term for other things, too. EDIT: A quick search shows that steam locomotives' "reversing bar" was sometimes called the Johnson Bar. I guess that's where the Cat guys got it.

"Missed it by thaat much."

Did you ever get this sorted out?

Just saw this post. Wow! That is one beautiful truck!

You definitely have some serious stones, Swishy.

That's what I was thinking. All the ones I grew up around had the larger air cleaner, and the RH door was definitely harder to get into. Same deal on the R-models. Less swing on that door.

With the "clutchless" Liberty (twin countershaft, by the way) and Jerico boxes, it is now WAY easier to make a good pass. The sliders in those are a great idea, and I wish I had thought up the design! What I like about all those old-school guys is that they were still using a "conventional" box. Takes a lot of skill to hit those just right.

There is a LOT of renewed interest in straight-shift drag cars. It is getting popular again. I guess some of the guys wanted to put the driver back into the equation!

That’s awesome!