Have two HG's. Which one do I use??
01-12-2012, 03:59 PM
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I use superglue, 3 layers of double sided sticky tape and then duct tape the edges. 
Surface finish is important - the ‘old school’ machinists used to leave a texture on the new finish to provide ‘bite’ for the head gasket. That won’t work well with the Cometic – you want the surface finish as smooth as possible. Make sure that the shop is using a good surface, if the bits are worn it will leave grooves in the finish (BTW – this is a real pain if you are surfacing a sleeved block – the steel shavings from the sleeves can make grooves in the aluminum if you don’t use a very sharp cutter and are conservative with the speeds).
The only ‘trick’ that I use with the Cometics is adding a thin coating of RTV on all the layers at the coolant ‘ear’. 90% of it gets squished out so make sure you make it a very thin coating. The other step I take is to torque it to 40 ft lbs and leave it overnight and then loosen the nuts and use the standard torque method.
Some of the early Cometic gaskets were not really well designed for the 944….but that was sorted out a long time ago. On the plus side you can get them in many thicknesses so you can make up for the head / block surfacing if you had to take a lot off.
Surface finish is important - the ‘old school’ machinists used to leave a texture on the new finish to provide ‘bite’ for the head gasket. That won’t work well with the Cometic – you want the surface finish as smooth as possible. Make sure that the shop is using a good surface, if the bits are worn it will leave grooves in the finish (BTW – this is a real pain if you are surfacing a sleeved block – the steel shavings from the sleeves can make grooves in the aluminum if you don’t use a very sharp cutter and are conservative with the speeds).
The only ‘trick’ that I use with the Cometics is adding a thin coating of RTV on all the layers at the coolant ‘ear’. 90% of it gets squished out so make sure you make it a very thin coating. The other step I take is to torque it to 40 ft lbs and leave it overnight and then loosen the nuts and use the standard torque method.
Some of the early Cometic gaskets were not really well designed for the 944….but that was sorted out a long time ago. On the plus side you can get them in many thicknesses so you can make up for the head / block surfacing if you had to take a lot off.
Is that ur process with stock head studs and cometic or with ARP's and cometic?
What torque i have to use with ARP's and cometic HG ??
urs
01-12-2012, 08:12 PM
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Great info Chris! Doing the search was not very helpful but a kot of myths...
Due to head and block resurfacing, how much do you think the stock size cometic can get away with? I installed the pistons i noticed the pistons are slightly higher (fingernail feeling) than the block. I'd say .01" is what you can feel. I'll put a straightedge and measure the gap.
Due to head and block resurfacing, how much do you think the stock size cometic can get away with? I installed the pistons i noticed the pistons are slightly higher (fingernail feeling) than the block. I'd say .01" is what you can feel. I'll put a straightedge and measure the gap.
01-14-2012, 10:56 AM
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Great info Chris! Doing the search was not very helpful but a kot of myths...
Due to head and block resurfacing, how much do you think the stock size cometic can get away with? I installed the pistons i noticed the pistons are slightly higher (fingernail feeling) than the block. I'd say .01" is what you can feel. I'll put a straightedge and measure the gap.
Due to head and block resurfacing, how much do you think the stock size cometic can get away with? I installed the pistons i noticed the pistons are slightly higher (fingernail feeling) than the block. I'd say .01" is what you can feel. I'll put a straightedge and measure the gap.
You can get the Cometic in .040” or even thicker. You can also find a 1.4mm (.035”) stock gasket – they make one that is thicker for shaved heads.
01-14-2012, 11:07 AM
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Lots of possibly answers to that!
APR and Cometic say to use stock torque. Some folks run the torque way up. The theoretical failure point of the block threads (when new) is around 110 ft lbs.
I believe that the head will start to deform if you increase the torque a lot. The reason (IMHO) for the common #4 head gasket failure is head deformation – there is not much support to the casting in that area. If the head is not very stable then increasing torque will make the mounting surface warp.
I use a little more than stock and not all the same on every bolt. The details are secret (and may change at any moment!)
02-12-2012, 10:40 PM
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Chris, I looked at my Cometic receipt and apparently it is a .040 HG. I measured the piston pertrusion over the block surface with a straightedge and feeler gauge and the highest point is .012.
Do I have green light??
Do I have green light??
02-13-2012, 07:55 AM
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If you are running a stock type block/piston (Alusil bore and 4000 series alloy piston) then you are OK with the .028” – with the tighter side clearances you get less piston rocking which helps with the piston to head clearance issue.
02-13-2012, 06:30 PM
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02-14-2012, 10:47 AM
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