I was measuring up my heads for thickness after they were skimmed at the shop...I took a number of measurements, and ended up with figures that fell between 146.83mm to 146.85mm. If you look at the WSM, it shows two head gasket sizes: a 1.1mm gasket for heads that measure 147mm +/- 0.1mm (147.1mm - 146.90mm), and a second gasket that is 1.4mm for heads that measure 146.6mm - 146.8mm.

My question is which gasket do I use here, since the figures I'm getting are falling into a range not covered in the WSM. Not sure how careful you need to be with gasket thickness, but since it involves compression and clearance, I'd rather get a knowledgeable view / opinion before trying to put things back together.

Thanks,
Dave

 

Just get the standard gasket. Don't worry about it!

 

I went with the thicker one and haven't had issues. Unless you're running boost it really doesn't make much of a difference, but does affect compression ratio slightly. If you use the 1.1 gasket your CR will be ever so slightly higher (good if you're Naturally Aspirated) and if you use the 1.4 then CR will be slightly lower which will lower HP in a NA motor, theoretically. I doubt the differences would be noticeable even on a dyno, so certainly won't be noticed by your butt.

 

Your 87 has the lower compression engine anyway (9.5), later they changed it to 10 by using different pistons in 88. Use the standard 1.1 mm size, you will get a little bump in your compression...

 

The '87 manual in AOs sig says CompRat of 10:1. .

Assuming 10:1 with a 1.1 gasket, according to my calculations, a 1.4 gasket would yield a CompRat of about 9.6:1. .

The 1.4 gasket adds 11.8 thousands clearance over the 1.1 gasket.


Merry Christmas,

DPM .

 

Wise of you to research it. Use the standard gasket. Just out of curiosity, why did you pull the heads? I am assuming an internal water leak.

 

just bolt it together with a standard gasket. dont worry about clearance as well. it all should be just fine, and actually better than fine, as you will bump the compression near .5 point.

 

The S4 spec says compression ratio 9.4-10.0. Search for S4 compression ratio and piston dish size cc, you'll see that '87 and early '88 cars have 9.4 because of a bigger piston dish and later have 10.0 because of the smaller dish. I want to guess -25cc for early pistons and -20cc for the late pistons, but don't rely on that. The most reliable data is from Vilhuer as always, so look for his posts, like this one: https://rennlist.com/forums/928-foru...c-numbers.html

The 9.4 compression ratio pistons also have thicker tops, which is useful in many settings. My low compression engine has an even bigger dish machined into the piston for an even lower compression ratio. The dish was designed by Jim to keep the piston as strong as possible and Rebco machined the pistons for a price that would normally in the 928 circles be associated with postage... Some piston photos here, including some copied from Tony who to my knowledge was the first to dish these early '87 pistons: https://picasaweb.google.com/1059510...eat=directlink

Also, I was under the impression that the available factory spare gaskets are all of the thicker variety, but this might be my memory being faulty, last looked at this two years ago.

 

Both the standard head gasket and the one for resurfaced heads are readily available

 

there it is

However I would use the thinner gasket.... a .5 compression bump is around 1.5-2% power gain

 

Happy holidays, and thank you to everyone for all the feedback on which gasket to use...sounds like the 1.1mm is the way to go, which is just as well because I have those in hand. Plus, 2% gain is not bad for just putting things back together.

Austin, I pulled the heads as part of a general rebuild / refresh of a car with low miles and a limited service history. Pulling the heads turned out to be the right move as there was some deep pitting / corrosion on the head surface - corrected with welding prior to skimming the mating surface. In addition the gasket was beginning to fall apart around the sealing rings. I never saw a leak, and the car ran cool, but eventually it was going to be an issue.

 

Were the heads warped? Will the cams be ok?

 

Heads / cams were ok. Do they often warp under similar circumstances?

Edit: maybe not....

 

A really bad welder can warp them.

 

I don't know how prone to warping 928 heads are but when a warped head is surfaced, cam bearing alignment may need attention.