Has Anybody machined domes off pistons to Lower Compresion?

Can you buy a thick copper head gasket to do what you want? I know they make all kinds of thicknesses for a late model hemi.

 

Or running E-85 fuel? I understand you can run race car compression on the street with that stuff.

And just for the sake of argument, you could balance just the pistons, since the rest has already been done, but the pistons being lighter might change the bob weight value. Anybody know about that formula?

 

Is the shortblock still in really good shape? Are the heads something special?

I'd be tempted to look for larger chamber heads. Just think about it from a cost standpoint, if you buy an equivalent but larger chamber head, your existing heads ought to be worth the same. Yeah, there's more money thrown out initally, but when recoup/resale is figured in I don't see how changing pistons could cost less.

good luck

 

Thanks Dolmetsch for providing numbers to back this procedure up. It's easyier for me to do something than it is to find the words to explaine it! Something else, flat top pistons will give you better flame travel and more complete burn which will be more practical for street use than the domed............

 

I don't think the SBCs would be badly affected vis a vis airflow as compared to the Big Buicks (BBB).

Factory heads for the BBBs come in 7.9, 8.7 and 10/1 Compression Ratio (CR).
Although the 10/1 CR head could be the same as the 8.7 CR head and the CR is different due to the smaller dish in the flat top piston.
The Buicks have a fairly large dish-head piston and I understand that installing flat-top pistons on the 8.7 CR engines will give you a 12/1 CR.

I'm aware of the Stage 1 and 2 heads, but they can be tough to find and expensive when you do.

And . . . the guys over on the Buick V8 board don't think much of the flow capacities of the 7.9/1 CR heads and I'm lucky enough to have a pair of them on the 31's engine.

Maybe I should be thinking about un-natural aspiration for this engine....

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Not tryin' to highjack the thread, but knowing whether or not I could cut a small dish into a set of flat-tops could be useful.

Interesting discussion.

 

Be a lot cheaper in the long run to just swap heads for some with much larger combustion chambers, and use a thick head gasket (provided it does'nt mess up your quench too much). The large domes will still delay the flame front with combustion, but you won't have to rebalance your rotaing ***embly. You may also be able to get by with cutting a flame trough across the dome to lower C.R. even more; should'nt make a lot of difference in the balance to go slightly "light" with a flame trough, better to go lighter than heavier when it comes to balance. Butch/56sedandelivery

 

T/a performance heads would do Ya right. they make a scary difference in performance. like WOW.

 

this is going to set hard but for information sake uhhhh errrrrhhh simpli put a 307 crank has less stroke you could leave the pistons alone and stick a 307 crank into the bottom end also i think gmc 307s came in odd stroke lengths i was mentioning chevy variety
such a change could be considered reliable

 

http://cgi.ebay.com/ebaymotors/SBC-...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

 

I could be wrong , but I thought that the 307 and 327 were the same except for a slightly smaller bore on the 307, like 1/8 inch???????????

 

The 327 and 307 both have a 3.25" stroke crank.

The 307 has a 283 bore size, 3.875.

The 327 has a 4" bore.

Even Pontiac guys know that.

 

Thanks to all who posted for sharing your experience.
As shown in an earlier post a guy can pic up some pistons for under $200. Best to save the domes for some open chamber heads. Ten years downd the road I might need them. Its easy to remove metal but putting it back on is a real *****. Anybody who has spent 40 hours machining a part and
removed to much material on the last operation can tell ya that.

The new pistons will probably weigh less than the old ones. But if the where close could a guy cheat and balance all the pistons if the weight is close?

 

That's what them lathe files are for.

I thought for a while the pro's didn't use them and they were mainly for amateurs like myself.

Then I saw a couple machinists using them to get that last .0005 or so.

Haven't done it for a while, but my best trick so far was cranking in the cross-slide .010 to remove that last .010.

Many bad words were spoken....

 

Haven't done it for a while, but my best trick so far was cranking in the cross-slide .010 to remove that last .010.

OH THats Painful!!!!!!!! Been there too. Most embarrasing. Worked all day on a set of pistons and was taking the last 002 off the skirt for a 002 tapered skirt. Took of 002 with the toolpost crosslide adjustment which is of course 004" in diameter since the 002 comes off all the way around. Felt stupid but I must have learned something from the pain since I havent done that since.