Head Gasket Thickness and Timing
07-06-2016, 02:02 AM
#1
Pro
Thread Starter
Head Gasket Thickness and Timing
Hey guys, I have had 0.005" shaved off my head, and 0.004" shaved off my block. I was planning on using a 0.045" cometic headgasket but all the Australian suppliers are out of stock and I don't want to pay extra for shipping from the US.
So my question is, can I still use the stock thickness 0.040" cometic gasket?
If so, how much will my compression be affected, and by how much will my timing be retarded?
Should I get an offset cam key? And if so, is 928 motorsports the only supplier? Does anyone have an offset key they want to sell?
Cheers, Timm.
So my question is, can I still use the stock thickness 0.040" cometic gasket?
If so, how much will my compression be affected, and by how much will my timing be retarded?
Should I get an offset cam key? And if so, is 928 motorsports the only supplier? Does anyone have an offset key they want to sell?
Cheers, Timm.
07-06-2016, 09:39 AM
#2
Burning Brakes
You will want to know the volume of the head and the best way to determine that is by using a CC kit. This way you can be sure of the value.
In addition you'll want to measure the piston to deck clearance. If those values come back positive (ie: @ TDC the piston is actually above the deck) then compensate the head gasket by the maximum value measured.
I wrote a blog post about this because I got so annoyed by the amount of misinformation I was being served.
In addition you'll want to measure the piston to deck clearance. If those values come back positive (ie: @ TDC the piston is actually above the deck) then compensate the head gasket by the maximum value measured.
I wrote a blog post about this because I got so annoyed by the amount of misinformation I was being served.
07-06-2016, 09:02 PM
#3
Another thing to consider is how much the block will grow when up to temp (0.0014") . It is small, but affected my decision on the thickness of the gasket. That blog post is great!
It would be wise to actually calculate the compression ratio and then come to your decision. It was my goal to keep the compression ratio as close to stock (8.0:1) as possible. I had a machinist do a terrible job at the deck, so I had it redone. The head was already machined, but the surface wasn't quite what I needed for a cometic so it was machined again. While the head was at the shop, I had them CC the head. I measured the piston to deck height (mine were protruding above the deck). With that knowledge, you can calculate the CR and then adjust it by changing your HG thickness.
With the timing issue, as long as your HG thickness is enough to compensate for the shaved material, the timing would remain unchanged, but some people swear by retarding the timing 4 deg regardless
It would be wise to actually calculate the compression ratio and then come to your decision. It was my goal to keep the compression ratio as close to stock (8.0:1) as possible. I had a machinist do a terrible job at the deck, so I had it redone. The head was already machined, but the surface wasn't quite what I needed for a cometic so it was machined again. While the head was at the shop, I had them CC the head. I measured the piston to deck height (mine were protruding above the deck). With that knowledge, you can calculate the CR and then adjust it by changing your HG thickness.
With the timing issue, as long as your HG thickness is enough to compensate for the shaved material, the timing would remain unchanged, but some people swear by retarding the timing 4 deg regardless
07-07-2016, 01:36 AM
#4
Pro
Thread Starter
I am just going to go with the 0.040" gasket. My head measured out at 23.9mm, and that combined with the -0.1mm block surfacing is still 0.2mm within factory limits before the 1.4mm head gasket is required. I need to stop pissfarting around with this build because the engine needs to be out of my living room in 3 weeks when the next rental inspection is due lol.
One extra query though: The workshop manual states that the stock head gasket is 1.1mm thick...which converts to roughly to 0.043", however everywhere I look the stock head gasket is sopposed to be 0.040" thick?? Am I correct in assuming that the 0.040" is the compressed thickness, and the 1.1mm in the manual is uncompressed thickness of the paper head gasket? And since a cometic mls doesn't compress, it is 0.040" thick to begin with?
One extra query though: The workshop manual states that the stock head gasket is 1.1mm thick...which converts to roughly to 0.043", however everywhere I look the stock head gasket is sopposed to be 0.040" thick?? Am I correct in assuming that the 0.040" is the compressed thickness, and the 1.1mm in the manual is uncompressed thickness of the paper head gasket? And since a cometic mls doesn't compress, it is 0.040" thick to begin with?