Hey guys, Im in a bit of a dilemma. What started out as a simple chevy 350 build for my old car has now got me confused. I dropped my block off at the machine shop to be built. I told them just to install some cast flat tops in it. Well, i had no idea about the different pin heights etc on some of the aftermarket "rebuilder" pistons. Well, this is what I got in my engine without any deck work being done. These rebuilder pistons are .020 shorter than the stock pistons to allow for block decking. I got the shortblock home and checked the piston to deck height with my dial indicator. Its about .045+ in the hole. The engine is bored out .040. I figure with even a "thinner" head gasket say like a .025 and 64cc heads, my quench is gonna be way too high. I just learned all of this here recently after a few hours of reading. The problem is I have some family who work at the machine shop, all they basically do is stock rebuilds and turn out probably 15 engines a day or more. Im sure they took the time to do a good job for me on all the bearing clearances etc, but probably didnt take into account this measurement due to the engines they turn out it really doesnt matter. (plain jain bone stock replacements). My question, should I just order some new pistons with the right pin height and install them? I will take a small hit by buying pistons twice, but i know it would be right. I dont know how much of a difference it will make if i leave them in there vs swapping them out. The other option would be to pull it all apart and have it decked, im sure that would be more money. This is jus going to be a street motor with a decent size cam. I just dont know HOW much this quench is going to affect it. Like I said, Im still learning.

thanks alot
mg

 

anyone? I need to know if its worth it to get some new pistons or just roll with it like it is. FWIW, im going to be running vortec heads and a "decent" sized roller cam.

 

i would think you could also go with a longer rod to fix the problem mybe. this would also put less of an angle between the piston and rod and reduce some of the force pushing the piston against the side of the piston wall. Dont forget about balancing every thing. correct me if im wrong.

 

i cant correct anyone because i have no clue. It makes sense a longer rod would do the trick as well, but im trying to get out as cheap as possible. Another set of cast pistons would only be about 90 bucks, plus the labor to press them on. I just wonder if its even worth redoing it. I dont know how much having this much quench will affect the engines performance and driveability. The measurement I took for the piston to deck height was ~.043". I know gm sells a .28 compressed gasket. I have been told this would put me approx at .069" quench. I am aware of the .015 shim gaskets but i dont feel comfortable running those.

 

I'd tighten the quench. I ran into a similiar situation with a big block and had to beg a shop to deck .040 off it. They didn't want to do it because regular pistons would not work if future plans required them.
I did it this way because the bottom end was already together (pistons pressed on) and balanced. It turned out great.

Do what evers cheaper. New pistons or have it decked, but get the quench right.

 

do most "standard" machine shops not take this into consideration? I mean the shop is a pretty reputable shop, I cant understand why they didnt catch/care about this. The only thing i can think of is like i said, they manufacture replacement engines. Thats pretty much all they do, im sure no one is ever going to check something like that.

 

The thing is you can run it with huge quench. On a replacement engine I'm guessing they cut costs somewhere. Resurfacing the deck is way easier on the tooling than cutting it down. Plus you need to measure for specific amount to cut. Thats not the point so nevermind that.

So your quench is too big, you could run it, but I guarantee it'll detonate way worse than one with tight quench. So you can back off timing and lose power that way or use 93 octane and lose money. Octane isn't even the answer because big quench makes an engine run inefficient.

Your piston quench area to cylinder head needs to be .035 to .045, the tighter the better. It's up to you to get it there.

 

I wonder what your compression ratio is? Another .020" in the hole is quite abit.

 

Good thought. I've run into the problem on sbc's a few times and even though being that far down reduces compression it's still more prone to detonate.

 

well, it doesnt matter now. The thought of it being all screwed up was killing me. Even though i knew it would run, i had to fix it. I ordered some keith black hyper pistons that are the right height for a non decked block. It says 10.2 compression with 64cc heads.

 

let me know what you think on this. The chart says 10.2 cr with 64cc heads. The vortecs are obv iron heads. I am going with a comp xr276hr cam. Do i need to run a thicker head gasket and raise the comperssion a tad now that i got the "right" size pistons. If needed i guess i could go with a bigger cc head, but i would have to get something other than the vortecs. maybee some iron eagles or something.
link to pistons
http://kb-silvolite.com/performance...etails&P_id=157
link to cam
http://www.compcams.com/Technical/Dy...6HR-10_001.asp

 

I bet your machine shop doesn't really take that kind of stuff into consideration for rebuild stuff. .040" in the hole should have been PAINFULLY obvious. What you could have done is cut the block down .030", leaving .015" or so to clean up for a rebuild and putting you at a MUCH better .043" quench. And if you ever do need a rebuild, you have already found pistons with the correct compression height!

 

yes, i could have had it decked, but i figured with the boring already done, the crank and bearings already in, it would be easier just to replace the pistons. Plus, i even went with some better pistons so i guess its for the better. Now if i can just verify my compression will be ok with those heads i will be golden. That and convince myself to go ahead and order that cam vs the next bigger one. the xr282hr.

 

Not to mention, make sure that the new pistons are close to the same weight as the old ones or the balance will be off.

 

http://www.chevyhiperformance.com/te...ide/index.html

http://books.google.com/books?id=GUC...esult#PPA48,M1
page 48

http://www.babcox.com/editorial/ar/eb80224.htm

 

well, i need you guys help on this. I get lost when it comes to static compression and dynamic compression. My question is im trying to figure out if my 357 will be ok with iron heads and pump gas. Ive come up with a static ratio of about 9.9.1. Ive got a deck height of .024, running keith black kb120 pistons. Im wanting to use the 64cc vortec iron heads with a comp cams xr276hr. The motor is bored .040 and I will be using a .028 gasket to keep the quench in the recommended .040-.055 range. I would like to be able to use teh iron heads to keep the cost down. My next option would be a set of 76 or 72cc iron heads, but they arent as cheap.

 

Well, here's what I came up with.

-KB120040 (.040 over KB120) piston put you .024 in the hole.

-I didn't see any .028" gaskets on summit for a 4.040" bore, but I did find a .016" x 4.1" which would get you right at .040" quench (FEL-7733SH1 $13 each).

With the 64cc heads, you're looking at about 10.2:1 by my calculations. (810.66cc/79.5cc)

 

i think the .028" are gm gaskets. I havent yet searched to see where i can get some.

 

I saw the .028" but it looked like they were just in a 4.00" bore.

 

really? damn it. I hadn't looked yet, just going off what someone else had said. I just assumed they had it for a bored block. What bs. I have to find something that is no more than .030 to keep my quench below or at .055. I really dont want to use the .015 shims because im not positive my deck is 100 percent straight and .015 doesn't seem too forgiving.....

while i got you guys attention, assuming i can find a ~.030 head gasket, my static cr would be ~9.9.1. Im assuming there is no way in hell i can run this compression with iron vortec heads? im guessing Ill have to switch to aluminum?