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Vmaxscart Supercharger On A 998 - Head Gasket Issues

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#1 RowlsMini


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Posted 19 July 2022 - 11:59 PM

I installed a Vmaxscart supercharger on my 1989 JDM 998.  After 700 km of driving the head gaskets on either side of the decompression plate blew between pistons 3 and 4.  I replaced my distributor with one supplied by Vmaxscart, replaced the head studs with ARP high-strength studs and eliminated the decompression plate.  (it appeared the cylinder openings in the decompression plate were too big to properly sandwich the gaskets).  I also had a valve job done and replaced the piston rings.  I ran the car in for 800 km and then 300 km later, while doing a rolling-road tune-up, the head gasket blew again, between cylinders 3 and 4.  Based on the condition of the spark plugs pistons 3 and 4 are running leaner than 1 and 2.

Any advice and/or similar experiences?

#2 DeadSquare


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Posted 20 July 2022 - 09:39 AM

Surface grind the minimum off the block and the head.

#3 cmonson


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Posted 26 July 2022 - 11:17 PM

Using a 1275 head (12g940) helps too.  You will need to pocket the cylinders a little so the valves clear.  The decompression plate is there for a reason.  Get with Stuart at Vmaxscart and get one that fits better.



I have a decomp plate on my 1293 (9 stud). 

I use the stock Rover head gaskets. 

First clean all surfaces with acetone. 

then Follow the manual for order to tighten the studs.  Per the manual you go around twice. 

first round is 25Lbs on the torque wrench.  With ARP the second round is 42Lbs (also use the a little of the ARP lube on both ends of the studs. 

Once tight remember to run the engine with NO water for 30-45 seconds shut it down AND LET IT COMPLETLY COOL this will activate the coating on the head gasket and act as a sealant. 

I let mine cool over night.  


I did have my head rebuild and skimmed.  the block itself is stock.


drives like a beast.  I have had no head gasket issues with this method.

#4 Northernpower


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Posted 27 July 2022 - 09:06 AM

You need to put a straight edge across block between cylinder 3 and 4. It sounds like the block may need the thinnest skim possible.

#5 RowlsMini


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Posted 11 November 2023 - 09:27 PM

For the record I found no issues with the surfaces of the head or block.  I reassembled it, again without the spacer, this time spraying the head gasket with Permatex Copper Coat.  To-date, admittedly with only 1,900 km on it, everything is holding together.

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