Zardilla Posted March 5, 2009 Share Posted March 5, 2009 When my builder started the motor today he ran into a bit of a problem with the pistons I am using for my rebuilt RB26. It appears the pistons are lightly touching the squish pads. I am running a 1.1 mm Cosworth metal headgasket on an unmodified block and head. Should have kept the CR at stock 8.5 and no clearance issues. Anyone else experience this with CP pistons? I will be putting in a call to CP tomorrow but wanted to hear if anyone else had issues with aftermarket pistons like this. Thanks Matt Two potential solutions are to increase the gasket thickness (lose some compression and performance) or remove material on the squish pads. Not sure if either is ideal. Quote Link to comment Share on other sites More sharing options...
strokerzedd Posted March 5, 2009 Share Posted March 5, 2009 I put 86mm CP pistons in my RB26 and ran into the same problem. The pistons stood a couple of thou proud of the block but weren't identical across the board. The pistons and rods were all identical. I called CP as well and they were perplexed. Meanwhile my mechanic moved the pistons around and kept getting the same measurements. He measured the connecting rods in the bores and got the same weird measurements. His explanation was that the crank journals were not identical. I had already purchased a Cosworth 1.2 mm head gasket but had to buy another one, 1.8mm thick this time. That solved the problem and I'm not sure that the small decrease in compression is really going to make that much difference in my tweaked motor. When I'm finished my build I'll have a garage sale with all of the extra parts I picked up! Quote Link to comment Share on other sites More sharing options...
Almighty Wez Posted March 5, 2009 Share Posted March 5, 2009 Less compression just mean more boost Quote Link to comment Share on other sites More sharing options...
cheftrd Posted March 5, 2009 Share Posted March 5, 2009 Sounds like You got RB25 pistons. 25 and 26 engines use the same length rod, but the pin height on the 25 piston is 1.5mm taller for the shorter crank, which would make it stick out 0.75mm more. Quote Link to comment Share on other sites More sharing options...
Zardilla Posted March 5, 2009 Author Share Posted March 5, 2009 Matt - I don't think the pistons are RB25, unless they were mislabeled - the box showed the correct p/n (sc7311). Of course I didnt buy directly from CP and instead used an Ebay vendor (Eric's performance parts). Nevertheless we'll take another look at how much the pistons protrude. StrokerZedd - I'm considering the 1.8mm gasket as a fix, but we'll be taking some additional measurements today to verify if that will address the issue. Thanks for the helpful comments guys, much appreciated. Quote Link to comment Share on other sites More sharing options...
Careless Posted March 5, 2009 Share Posted March 5, 2009 you're looking at nearly half a compression point with that different in gasket height. negligible to most big-boost engines. piston mishaps DO occur. it could be that the offset for the gudgeon pin or the pin height was located improperly. in any case, if you have to... eric should be able to send you a second set of pistons. he does stock manley, pauter, and je stuff too... but I recall quentin (fentin_fury) using CP's too... i think without any issues, so maybe discussing what he found out with him would be a good idea too. this is weird. Quote Link to comment Share on other sites More sharing options...
strokerzedd Posted March 6, 2009 Share Posted March 6, 2009 I asked my mechanic about our experience with the CP pistons: Different numbers are due to factory tolerances. A bit more variation than I expected but still livable. If they were RB25 pistons, they would have stuck out .060 more. That is way more than can be compensated for with head gasket thickness. Stock pistons were close to the same protrusion. Remember the block was surfaced because the top was badly etched so we gained a bit of protrusion that way. The other guys should have measured the piston protrusion before final assembly. Have to allow enough squish for clearance. .035" is minimum recommended piston to head clearance to allow for stretch. 1.1 head gasket is .044 thick, leaves maximum protrusion of .009" Not very much. Hope this helps. Quote Link to comment Share on other sites More sharing options...
Zardilla Posted March 6, 2009 Author Share Posted March 6, 2009 Cool, thanks for the additional perspective guys. The current plan is to remove some material off the squish pads (on the head) to provide additional clearance. Hopefully this will do the trick. I may still run a thicker gasket for additional insurance - Cosworth has a 1.5mm version. I'm running GTSS turbos, so we can probably run the boost up to 1.3 bar just fine to make back some of the horsepower loss. Quote Link to comment Share on other sites More sharing options...
TurboSE Posted March 6, 2009 Share Posted March 6, 2009 so this isn't just a CP issue? It can happen with any pistons? Quote Link to comment Share on other sites More sharing options...
Stealth-Z Posted March 6, 2009 Share Posted March 6, 2009 so this isn't just a CP issue? It can happen with any pistons? Of course it can happen with any piston. Just because the box off the shelf says 8.5:1CR does not mean it will be that way in your setup. With the piston fiasco I have gone through lately, I would look very closely at what cheftrd has suggested. And there is a third possible solution instead of a thicker head gasket or cutting the quench pads. You can have a machinist cut the piston down some. Quote Link to comment Share on other sites More sharing options...
Zardilla Posted March 6, 2009 Author Share Posted March 6, 2009 Thanks Stealth. i'll take another look at the pistons but they appear to be at/below deck level as they should be (CP tech confirms these are zero clearance pistons). So I don't think I have the wrong pistons but its worth a triple check. Only the dome of the piston protrudes (as it should) and the point of impact is just on the inner edge of the squish pad. Altering the pistons is certainly another solution, but I'm not sure I want to mess with CP's design and it means pulling the engine out again and the time involved with that. I'll let everyone know what happens with this in the next few weeks. Quote Link to comment Share on other sites More sharing options...
cheftrd Posted March 7, 2009 Share Posted March 7, 2009 Only the dome of the piston protrudes (as it should) and the point of impact is just on the inner edge of the squish pad. That makes a lot more sense. The way I was reading it was that you have a 1.1mm gasket and the pistons are "kissing" the pads. It sounded like the pistons were 1.1mm out of the bore... Quote Link to comment Share on other sites More sharing options...
Zardilla Posted March 7, 2009 Author Share Posted March 7, 2009 Yep, poor description my part. The wierd thing is that if the pistons are correct and the block/head is undecked/milled what's causing the dome of the piston to touch the inner edge of the squish pad? One suggestion I've heard is that when I had the block bored out to 87mm the machine shop didn't get the cylinders exactly round and so there is a bit of movement when the pistons are cold and so they are slapping on the squish pad. anyway - thanks again for all the help and suggestions guys. Quote Link to comment Share on other sites More sharing options...
Careless Posted March 7, 2009 Share Posted March 7, 2009 had me confused too. LOL. I was reading your post about grinding SOME of the pad out, and thought to myself "how is that going to help a piston get further away from the entire pad... it's flat!" lol. I guess it's bore straightness then. what is your compression now? and what would it be if you stuck with the original 1.1mm headgasket and rounded out the inside diameter of the pads/combustion chamber? Quote Link to comment Share on other sites More sharing options...
fentin_fury Posted March 7, 2009 Share Posted March 7, 2009 i did/do not have any clearance issues with the cp pistons. 1mm gasket - however in porting the head I removed some material from the pads - but not all of them. Calculated compression is 9:1. Forget what my final chamber cc measurements were.... but the block was scimmed a couple of times as was the head. Q Quote Link to comment Share on other sites More sharing options...
fentin_fury Posted March 8, 2009 Share Posted March 8, 2009 http://forums.hybridz.org/showthread.php?t=132156 there is a picture in the above thread showing how proud my CP pistons are. Quote Link to comment Share on other sites More sharing options...
Zardilla Posted March 8, 2009 Author Share Posted March 8, 2009 Thanks Q - I'm going to take another look at mine tomorrow, but I don't think they sit "proud" like that. http://forums.hybridz.org/showthread.php?t=132156 there is a picture in the above thread showing how proud my CP pistons are. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.