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THE BIRDMAN
02-29-2008, 09:28 PM
Do our pistons sit above the deck on our engines stock?I'm curious cause I did a refresh on my engine new rings,bearings etc.And while I had it apart I had the block decked to ensure against any headgasket issues and after I reassembled it I noticed my pistons at tdc sit above the deck.I didn't notice if they did this prior to decking or not so does anyone have the specs of the piston to deck clearance on a stock engine.Are they installed at zero deck stock.




Jay

Flex
02-29-2008, 09:36 PM
Ask Dave Dalke to be sure but I'm sure the answer is no. I don't remember either of the blocks I pulled apart having the pistons sitting above the bore. How much did they hack off?

THE BIRDMAN
02-29-2008, 09:44 PM
They said they didn't shave off very much.I just had a few questionable spots on the deck so I told them to go ahead and shave it.The pistons seem to sit about 15 thousandths abouve the deck.


Jay

XR7 Dave
02-29-2008, 11:20 PM
Stock is flush. Center the piston carefully because they will rock back and forth. However, if you are using stock pistons, then it's probably not going to work. Most times the stock deck is very uneven and so it is not uncommon to have to take .015" off to get it to clean up. For this reason the aftermarket sealed power pistons are .020" shorter than stock.

You can be +.005 with no problem and even +.010 and be ok but at +.015 I think you are asking for possible contact.

Mike8675309
03-01-2008, 12:14 AM
If it's greater than .010, I'd be concerned. It may be possible for them to mill the tops of the pistons to take .005 off crown. Just make sure they understand these are high silicone content pistons. They'll be tough on their tooling.

when I did mine, I specked the pistons for slightly above a reasonable deck height, then had the machine shop cut the block to that deck height. Just having them cut it without being certain of the piston assembly heights isn't a good idea.

Also note that you may need to do some machining on the intake manifold now, as the angle will change slightly from the shaved deck, and it may not seal very well.

THE BIRDMAN
03-01-2008, 08:32 AM
Stock is flush. Center the piston carefully because they will rock back and forth. However, if you are using stock pistons, then it's probably not going to work. Most times the stock deck is very uneven and so it is not uncommon to have to take .015" off to get it to clean up. For this reason the aftermarket sealed power pistons are .020" shorter than stock.

You can be +.005 with no problem and even +.010 and be ok but at +.015 I think you are asking for possible contact.




I am using the stock pistons.When I measured I didn't center the pistons I just left them in whatever position they were in when that cylinder was at tdc and then measured it there.I assumed this would be kind of the "worst case senerio" which the farthest out measured was the .015 with the opposite side of the piston being .007 out of the hole.So does this info help my chances of possible contact or does this decrease my chances of possible contact?Thanks



Jay

XR7 Dave
03-01-2008, 09:24 AM
I am using the stock pistons.When I measured I didn't center the pistons I just left them in whatever position they were in when that cylinder was at tdc and then measured it there.I assumed this would be kind of the "worst case senerio" which the farthest out measured was the .015 with the opposite side of the piston being .007 out of the hole.So does this info help my chances of possible contact or does this decrease my chances of possible contact?Thanks



Jay
You need to average the readings from the two sides. Looks like you have about .011" out of the hole average. The "golden rule" is .035" clearance for 9000rpm race engines. Your gasket is .040" so you've got about .029" right now which should be ok being that your engine doesn't rev that high.

On the other hand, 9000rpm race engines use forged pistons which won't die a horrible death if they make light contact. Hyperutectic on the other hand.... It's up to you. Getting the pistons shaved .010" would remove all doubt but for that expense you might as well get new ones.

Keep in mind also that if you do use those pistons you'll have slightly higher compression even if you shave the tops.

cbt
03-01-2008, 02:21 PM
you could take some soft clay and put it on / across the piston, use a decent used hg, if possible put the head on and the valve gear, roll the motor over by hand
take the head off check the clay

compression will be higher i think, as XR7 Dave says

Flex
03-01-2008, 06:45 PM
Most definetly it will be higher. The overall "combustion chamber" space has been decreased by a fair amount. As Dave said Jay, probably cheaper just to buy the Sealed Power replacements.

S_Mazza
03-01-2008, 07:37 PM
Aha, I did not know that about the Sealed Power pistons. Thanks, Dave, for bringing that up.

By the way, do you like those pistons overall?

THE BIRDMAN
03-01-2008, 07:55 PM
Man this sucks.Well I guess it time to spring for a forged bottom end.I don't see a reason to spend the money getting the block all bored and honed and then using cheap pistons.If I'm going to go that far with it I might as well spend the money and put a good set of forged pistons and better rods in it.That way I don't have to worry about it when I do more upgrades later.Cause I'll already have a built bottom end.




Jay

Mike8675309
03-01-2008, 08:08 PM
The sealed power pistons are gonna be stock factory compression height so I don't think it's gonna fix your deck height issue with them.

The only way to fix such an issue, if you feel it is an issue, is to cut some off the top of the pistons, or get some new pistons made with the rod pin placement slightly higher in the piston to bring the piston back down into the bore a bit.

THE BIRDMAN
03-01-2008, 08:11 PM
Mike Dave said the sealed power pistons are .020 shorter than the factory pistons.So if someone had the same problem as me they could use them and be good to go.




Dave you have an e-mail.




Jay

Mike8675309
03-01-2008, 10:43 PM
Missed that one. I guess I wouldn't be happy that they did that personally unless they adjusted the dish to maintain the CR.