Dan107 Posted January 8, 2020 Share Posted January 8, 2020 (edited) Quick question about piston design. The truck dropped an exhaust valve and cracked a cylinder. I couldn’t afford a Mack cylinder kit, so I purchased an aftermarket one. Not mentioning the brand, but on the original piston there are two small reliefs molded in alignment with the exhaust valves. The new piston is flat with no reliefs. Every other dimension on the new piston is the same. My question is will it work, or will the exhaust valves strike the new piston? Edited January 8, 2020 by Dan107 Forgot engine Quote Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/ Share on other sites More sharing options...
Rob Posted January 8, 2020 Share Posted January 8, 2020 Myself, I can't say I had good luck with a mismatched piston. Only did that once in a Detroit Diesel with a customer supplied piston and sleeve assembly but the engine always ran less than 100% when put back together. Not really a miss and compression pressure was very close to the other seven cylinders, (analog gauge) but there was always a slight "stumble" in that cylinder I felt. I'm thinking the brand on the piston and liner was "Speed Pro", but that was many years ago. Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/#findComment-439781 Share on other sites More sharing options...
theakerstwo Posted January 8, 2020 Share Posted January 8, 2020 When you mismatch pistons you may have a balance problem.Only way to know is have them weighted and not by your bath room scales. Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/#findComment-439791 Share on other sites More sharing options...
davehummell Posted January 8, 2020 Share Posted January 8, 2020 You could have relief's cut on a mill and then weighed other wise you would have to figure out how far the valves open down in the chamber. This is standard stuff when you're building a racing engine. Quote Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/#findComment-439793 Share on other sites More sharing options...
Rob Posted January 8, 2020 Share Posted January 8, 2020 19 minutes ago, davehummell said: You could have relief's cut on a mill and then weighed other wise you would have to figure out how far the valves open down in the chamber. This is standard stuff when you're building a racing engine. Remember the old modeling clay trick and playing with head gasket compressed thickness(s) to keep the valves and pistons from "Kissing" each other? Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/#findComment-439794 Share on other sites More sharing options...
fjh Posted January 8, 2020 Share Posted January 8, 2020 (edited) 9 hours ago, Dan107 said: Quick question about piston design. The truck dropped an exhaust valve and cracked a cylinder. I couldn’t afford a Mack cylinder kit, so I purchased an aftermarket one. Not mentioning the brand, but on the original piston there are two small reliefs molded in alignment with the exhaust valves. The new piston is flat with no reliefs. Every other dimension on the new piston is the same. My question is will it work, or will the exhaust valves strike the new piston? Not Worth taking a chance on in my mind! don't care how much you saved!Money better spent buying the proper part! The reliefs are there for a reason! Edited January 8, 2020 by fjh Quote Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/#findComment-439796 Share on other sites More sharing options...
davehummell Posted January 8, 2020 Share Posted January 8, 2020 I was restoring a velocette Truxton and I bought a new head it was semi finished valve holes and a ruff hole for the valve guide and chamber was not finished I had at least 30 hours in that head and setting up squish band for the right compression and I had to machine the piston crown for the hemi head. That is the last British bike I wanted to own. Quote Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/#findComment-439805 Share on other sites More sharing options...
Phase 1 Posted January 8, 2020 Share Posted January 8, 2020 Check with the supplier. You may have gotten the wrong piston and they have the right one. Quote Link to comment https://www.bigmacktrucks.com/topic/60457-2012-mack-cxu613-415-c-mp8/#findComment-439811 Share on other sites More sharing options...
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