NowhereFast

I'm not sure which head gaskets I should be ordering for my car. I've heard things about rubber coated shims, different thicknesses, as well as 350 gaskets being right for certain applications.

I'd like to have the heads milled too for a little bump in compression. How much should they be shaved down?

NowhereFast

Bump. Would like to start taking the car apart soon. Want to have the parts ready.

BASSETT IROC 85

iTrader: (2)

Fel-pro 8510PT

Night rider327

iTrader: (13)

Milling the heads.. .006" to .008" = 1cc chamber volume

What block do you have? Is it stock or been rebuilt? Has it been milled/decked?

If stock 350 block un-decked I recommed fel pro 1094 head gaskets. 4.100" bore, .015" thick. This helps to bump up compression and gets your quench height down to a good useable range

NowhereFast

I have an original 305 in my car. Completely untouched.

Night rider327

iTrader: (13)

Yeah I would still use the fel pro 1094 head gaskets. About $20 each. I like to spray both sides of these gaskets with copper spray a gasket type sealer before installing.

NowhereFast

Do you happen to know the quench height of a stock 305 with original flat top pistons? I know it's best to have it somewhere between .035"-.040" right?

Night rider327

iTrader: (13)

The target for quench height is .040", .035" to .060" is an ok range. .035 is getting a little too tight for some and after .060" all benefits from quench are gone.

The factory deck height on the gen 1 sbc has always been 9.025" and spec for piston in hole at TDC has been .025" but with GM over the years I have seen the pistons on factory blocks at anything from .020" to .035" in hole.

Factory spec though is listed/set at .025"

IIRC GM used a .018" thick steel shim head gasket on the 305's

So stock .025 + .018 = .043" quench height.
With the 1094 .015" gaskets that will put you at .040"

NowhereFast

Milling the heads doesn't have any effect on quench height right? That will just raise the compression? And I could check piston in hole depth. I'd imagine that would be done with a straightedge and a feeler gauge.

Also, do you think the block will need to be re-surfaced? How straight is straight enough? Dead on, or is there a tolerance? It has never overheated to the point of blowing anything, but it has been hot enough to turn the CEL on. The head gasket is just leaking compression into the cooling system.

Night rider327

iTrader: (13)

Correct, milling heads does not change the quench height, just chamber volume, which it lowers the volume and that raises your compression.

Yes you can measure piston in hole by bringing cyl. to TDC, laying a true straight edge across bore and measure down from bottom of straight edge to top of piston (at it's tallest point, some pistons will have a whole crown dish with a raised ridge or lip sticking up around the whole side top of piston, if like that you measure to the lip/ridge of piston.

You can use feeler gauges, dial caliper or dial indicator.

With the block, yes there is a range. Up too .003" across a 6" span lengthwise of head, and up to .002" width wise over a 6" span on head.

With no more diff. than .004" across whole head lengthwise.

Again you can measure it with a machined straight edge and a feeler gauge.

NowhereFast

Ok great. Thanks, I appreciate all the help.

8Mike9

?

What happens when the top of the chamber gets closer or further way from the piston?

I/e milling the head, top of the chamber gets closer...etc.

If milling the head won't change quench area, I guess I could run a 1 inch head gasket, and only affect compression?

MoJoe

iTrader: (5)

The chamber volume is smaller, raising compression.

Not quite.. You would be increasing quench from 0.0xx" to 1.xxx" Useful quench is at or less than 0.060", with 0.040" being optimal. Less distance risks piston to head contact at heat and RPM. More distance is not as effective.

Quench is the distance between the piston to head surface... It is not the TOP of the chamber that causes quench, but the surface of the head that is closest to the piston.


You are correct that milling the head moves the TOP of the chamber closer to the piston, but the deck surface that is responsible for quench always meets the head gasket surface.

You adjust quench with: head gasket thickness, decking (removing material from) the block, and piston "compression height. While crank stroke and rod length build into the stack, they are not sold in 0.010" increments.

8Mike9

I gotcha!