9 replies to this topic

[robert.hedges]

Struggling to fine one. Does anyone have an idea where I can purchase one for a 998? I have thought about a overthick (shimmed) cosmetics gasket but they only appear to come in 1275....

I want a decompression player so that I can bring it back to current CR if things don't work out. Please don't tell me to increase chamber size, piston dish etc.

Edited by robert.hedges, 24 February 2024 - 08:26 PM.

[Shooter63]

Cometic do small bore gaskets from 32thou to 80 thou thick, one should be OK for you.

Shooter

[r.tec]

I have a Cometic copper gasket on a high compression 998 engine. Copper means here full copper, nothing else. My verdict: completely useless.

 

I had 998 shape stainless steel compression plates manufactured (I have to measure but I think they are 1mm). And I have left some of them.

 

But notice that you have to fit two gaskets below and on top of the plate.

 

If you cannot find a plate in Brittain we could perhaps arrange that I send you one although I do not know how to send something from a private person to a private person. That was simply forgotten in the Brexit agreement. What a shame!

 

Helmut

[timmy850]

I guess you have seen Vmax make a 2mm decompression plate?

Another option is to use two head gaskets and no decompression plate

Or with some slight mods you can use a 1275 gasket

[lsto]

What cc does a 1mm decomp plate work out as? I have a 998 turbo that I use as a daily. A copper HG works out about 3cc and I used 2 copper gaskets to drop my CR. Running a slightly home polished cam4810 head and standard pistons at 7psi.
Car is used as a commuter daily with no HG issues.

Edited by lsto, 29 February 2024 - 04:00 PM.

[r.tec]

According to my calculations (or better that of FreeCAD FCInfo Macro) the volume for a 1mm plate and 998 gasket is 3.7 cm³ per cylinder.

 

 

Attached Files

•  Cometic small bore head gasket 004.jpg   6.87K   0 downloads

[DeadSquare]

To cut four 64.5mm holes in a plate with a milling machine, would cost about £100-00.

 

 

Using a head as a jig for the cylinder studs, I used to drill through a decompression plate, into a piece of wood. 

 

Using a 2" QMax cutter, I'd mark out the bores.

 

By juggling bolts to hold the plate in place, it is possible cut the plate as the block it being rebored.

[mini13]

Well... I dont like decomp plates so my advise is.... change the chamber volumes....  as decomp plates screw up the squish band

I would help if we know the background behind this, I assume some kind of forced induction?

 

However, I have just stacked 2 head gaskets before as a temporary measure to get CR down to run a turbo.

[robert.hedges]

For anyone interested I managed to source a very thick copper gasket from Cometic. It was about 0.094" which is the thickets they do in copper. It's not a stock item and has taken 8 weeks to arrive. It cost around £90 + VAT and delivery but to my mind its better then a 1mm decompression plate and two gaskets. Really happy with this approach. Now to progress with the supercharging.

Edited by robert.hedges, 13 April 2024 - 07:26 AM.

[DeadSquare]

For anyone interested I managed to source a very thick copper gasket from Cometic. It was about 0.094" which is the thickets they do in copper. It's not a stock item and has taken 8 weeks to arrive. It cost around £90 + VAT and delivery but to my mind its better then a 1mm decompression plate and two gaskets. Really happy with this approach. Now to progress with the supercharging.

Well done, finding that.  Thank you for telling us, that is useful for other supercharging ventures.

 

I think that was a fair price.