Engine rebuild

[geoffinbc]

Well I am getting a 3.1L tomorrow and after I tear it down I will send it in to the machine shop. The whole engine kit from Clevite is $750 including pistons.

All the machine shop charges including blueprinting, 0.040 overbore, head rebuild and assembly of the short block will cost $1200

Everything is good except for the measly stock compression of 8.8:1. The guy looked and phoned 3 places for me and that is all he could find.

Both him and I agree we don't want to mill the block or heads if we can avoid it as that weakens the deck and/or heads and also can create intake manifold problems. Which would require machining and more cost on my part. So if I can I would like to be in the 9.5:1 range any advice?

[Ray Hamilton]

I wish mine had a little higher comp but it works very well as is in my Blazer. I got most of my tech advice from you.

[RedBird]

using the copression ratio calc, with a .040 overbore, and .040 thickness headgaskets(camaro 3.1) it came out to 9.55:1. the quench is a bit much, .057, but your compression is there. it's up to you.

EDIT: since i did that, and i've always heard people talking about quench, i looked up some info on it. turns out that .057 isn't nearly as bad as i thought it was. you'd have something more like .077 with stock headgaskets and that's way too much. so you're actually much better off with the camaro hedgaskets.

[geoffinbc]

What heads and pistons cc's did you use? This is an iron head motor

[RedBird]

bah. ok then i'm dumb. i'm a FWD guy, and i'm so used to hearing 3.1 refering to the alum head motor, that that's what i used. damn lol. let's try again then lol

[Superdave]

what about using 2.8 pistons with a bit machined off the tops?

i forget the compression height difference.. might not even be possible.. lol

[geoffinbc]

The difference was about 4MM I am not sure if it is possible either. Even if you did not it the upper ring land you might compromise its strength.

[gectek]

you are worrying WAY too much about block/head/piston strength and integrity.

it isnt made out of plastic. flycutting is normal place on stock bottom end big cam cars. machining of the pistons to get the right shape dome is as well. those pistons are thick on top, and the first ring land on the 2.8l are very thick. id look at very good shop that could machine the pistons and go that way for increased CR

[project89]

ive been looking into this as well.what is the distance the 2.8 piston will pop out the top of the bore on a 3.1 crank?

i have a set of pistions i can throw int he bridgeport/lathe whatever to cut them down then cut in half to see how thick the top of the piston is after the machine work

[geoffinbc]

you are worrying WAY too much about block/head/piston strength and integrity.

it isnt made out of plastic. flycutting is normal place on stock bottom end big cam cars. machining of the pistons to get the right shape dome is as well. those pistons are thick on top, and the first ring land on the 2.8l are very thick. id look at very good shop that could machine the pistons and go that way for increased CR

Yea but 4mm is a big cut. But I suppose the dish is even deeper than that so it should be ok. But what happens if the required cut to zero deck the pistons sends the compression through the roof? Then I suppose I have the option of a thicker aluminum head gasket.

[project89]

4mm is .157 inches

i will grab one of my pistons now and measure. if i have to i will cut the piston in half and measure the top thickness

[project89]

ok from the top of the ringland to the top of the pistion measures

.330

so if we take off 157 that leaves us with

.176 left for the top ring land.more then enough material

now for the thickness of the piston before machining
.350

and after machining .193 thick at the thinest point

so it is possible now u just need to figure out base cr with 0 deck fith a flattop piston.

from there we can see how much if any more material u would have to remove to get desired cr

[torq455]

nice to see actual numbers on this subject. this was discussed earlier and was assumed the ring land would be way too thin to make this work.

[project89]

nice to see actual numbers on this subject. this was discussed earlier and was assumed the ring land would be way too thin to make this work.

i was interested in this myself for a cheap nitrous build,i had a set of stock 2.8 pistons and i was at work so i grabe done and pressed it off a rod and took the measurements.

then i put it in the lathe and made the cut off the top.measured for the ringland
then i cut the piston in half in the bandsaw to see how thick it was at the thinest point.

now this wont work for my build the piston top would be to thin for a healthy shot of nitrous. but in an n/a application there is no reason this wont work.

somone should figure out the cr with the 0 deck flatop and see what it comes out to

if need be i can chuck up another piston and machine an .025 or so recess into the piston top and cc it to see how much it will bring it down

or u could go the other way and machine these in a mill for a popup style dome for even higher compression

[project89]

got some solid numbers for u guys

ok heres what i come up with

3.1 with cutdown 2.8 piston as above
this is with a .040 gasket

and this one
same as above but with .060 gasket


now the same as above except with a minus 6 cc dish milled into the piston and .060 gasket


and now the same with the standard .040 gasket

im not sure just how much material u would have to remove to get a -6 cc dish but i have 5 more 2.8 pistons at work i can throw in the milling machine or lather and cut some dishes into and try some diff configurations, afterwards i could cc each piston to come up with some solid numbers and combos to calculate compressions with domes and dishes on the pistons



just to make u guys aware were it says dome in the calculator actually reflects dish volume
a positive cc number means dish
and a negative (i.e -6 would mean 6cc dome)