steve8586iroc

What is the thickness of a stock head gasket on a 85 305tpi engine? Will going to a .015" gasket cause any problems other than raising compression a little?

Steve

steve8586iroc

Anybody have a clue?

Guest

Not positive, but I think the stock gasket is around .035-.040"
I dont see a problem with using a thin gasket, people do it all the time.

ws6transam

WRONG!!

The stock 305TPI engine is equipped the same as the L69 engine. Both have 9.5:1 compression and use a .015" steel head gasket with 3.2cc volume.

If you use anything else BUT the .015" gasket, you will lose compression.

I know first hand as I just popped off the cylinder heads on my '84 L69 just this winter. I used a Felpro steel head gasket, but would use the GM head gasket next time. The GM 305TPI gasket is cut to match the 3.736 bore whereas the Felpro is cut to 4.125 inches on the bore.

See my 305 cylinder head project at:
http://www.isthq.com/~dan/ported.html


------------------
Daniel Burk
http://www.isthq.com/~dan/fcar.html
'84 Trans Am WS6/L69
KB SFC, Moser axles, Torsen Diff. PST suspension, Braided stainless brake lines,
5-point racing harness, Koni struts, 11-inch rear disks,Spohn Adj. torque arm,
Ported 305 heads w/1.94"intake valves, Comp Cams XE262H, Griffen alum. radiator,
Speed-sensitive steering conversion, underdrive pulleys, Crane Hi-6 & more. 1.05g skidpad verified.

Guest

Well excuse me.

I dont agree with you though. I pulled off the stock gaskets on my 86 TPI TA (still had GM stamped on them) and they were not steel shim gaskets, but rather a composite gasket that was definitely alot thicker than .015.

94-6spd

.039 I think.

ws6transam

I wonder how they make up the 9.5:1 compression on the TPI? Smaller combustion chamber in the TPI heads versus the L69? They both use a cast flat-top piston, and a .025 to .035" deck height. Assuming 58cc heads, practically 60 cc, it comes in as 9.5:1 compression with a 3.2cc volume head gasket.
A .039 gasket (with nearly 9 cc of volume) would make the c.r. drop below 9:1, Is this the case?

Guest

I'd say the actual figures on the volumes and such vary quite a bit from actual. They didnt use just one piston in the different 305's, there were actually a few. Maybe the reliefs were smaller or something. Take a look at what .015 is and see if you think your gasket was that thin. A .015 gasket is one of those steel shims, its like a wafer. My 85 TA did not have a thin gasket like that either.

ws6transam

I measured my old gaskets with a micrometer. They were .017" in thickness and made of steel. The engine has never been opened up as I have owned it since 1990. The original owner sold it to me at 39,000 miles, and he drove it only to go golfing.

I used a .015" Felpro steel gasket as a replacement. Maybe you are right on gaskets, but I could have *sworn* that the 305TPI gaskets were steel and 3.2cc!! The piston reliefs only make up 5 cc or so. Either the TPI uses a different head with less chamber volume than the L69, which I doubt, or else the c.r. is not 9.5:1 in that motor of yours. Do you remember the specs? Deck height, etcetera? I have the formulas posted on checking compression ratio on my site. A .039" gasket for a 305 will displace 7.1 cc.
http://www.isthq.com/~dan/305HO.html

A 305 with 60 cc chambers, production deck height, flat-top pistons with 5 cc volume, and a 7.1 cc gasket will sport a 9.1 compression ratio.

Guest

No, but I can find out in the next few months. I am picking up another engine, and the one in my car is going to become an oversize paperweight.