I want to get everyone's opinion. I am helping assembly what i believe is a 270 CI GMC inline 6. It is a little built up with a custom Howard's cam grind, head work and domed pistons with a standard bore. When sizing the rings, the factory assembly manual says .005 to .015 ring gap. The new set of rings are all at .037 and the rings on the original standard bore pistons are .065 ! The guy whom the engine belongs to said it ran before he took it apart but I wonder did it smoke a coal fed locomotive or am I missing something ? I have done more small block Chevy's so I don't know any quirks with this engine. the .005 to .015 gap seems correct for an engine build of this type so I'd just like to hear what anybody can lend. I hate to put this together with this ring gap and the engine not perform properly. I don't put together junk, I try to do it the right way !
I was in that situation years ago. At about .045 end gap with new rings in a used bore. Talked to a rep from Hastings and they said it would be fine. I was expecting around .015 end gap, I put it together at .045 and it was fine, ran well and no smoke or excess oil consumption..
All engines are air pumps. I assembled more GMC6’s in 45 years than I can count. There are a few assembly quirks but clearances are the same as any typical GM engine. We need to know your exact bore. 270’s are 3-25/32”or 3.78125…which is a bastard size today. If your pistons are domed Venolia for the first style head which were very common in the past. The rings are right near .100” thick sock and Venolia used that size except for the oil ring. Stock pistons had 4 rings and aftermarket 3. Side clearance at the pin was .003” for the piston to bore. You need to know the bore of the block and the size of your pistons at the bottom. If your lucky places like Egge machine in So Cal may be able to come up with .rings. I had forged flat top pistons made once for an over sized 270 made to a ring pack available at the time. I was going to use the last 270 small port head which had the smallest chamber they ever made. Do your pistons look like this. It’s a std bore Venolia for a 4” 302. Good luck.
We did, they are pretty consistent. It just concerned me that the information I was reading in the manual was no where close to what I was measuring.
I will have to get pics of the pistons. They don't have a dome like that. The owner of the engine is an older hot rodder and he collected up the parts so I am assuming he knows his stuff based on some of the other car work I've seen that he has done. We did not measure the bores yet, I didn't have my inside mic's at the time because I was basically going on the "assumption" of someone else's info. and I should no better ! I should have just started from scratch with a clean slate so I am using my own info. and measurements. These are cast pistons and I am willing to bet they were purchased from Egge or a similar vendor because they look very new. I will get pics (of the reworked head too) along with casting numbers and good measurements so I'm not just throwing out random crap and I am sure you and all the others can point us in the right direction. Over the weekend I will get the info. and post it. Thanks for your help !
Some of the stock GMC pistons had domes, since the combustion chamber of the head was large enough that, without domes, the compression ratio would have been 4:1.
I can give suggestions when you know #’s and am glad to help with assembling techniques that will make them live. They are a fun but heavy engine. Valves are simple today not like the 50’s. I’m very familiar with most of the pistons used in the past and also current ones since I’ve raced these seriously since the early 70’s and still do. A good street engine is very similar and it sounds like you have a lot of good parts. Stock compression was 7-1..
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