Strange Bore MArks

[970s]

Help!
Anyone had the same issue on bores having run in for low milage, its a 998 Cooper block with 12G940 head conversion without valve pockets machined, the head gasket is blown between no. 2 & 3 cylinder, rings and compression seems OK after gasket replaced.







Al

[contour51]

viewtopic.php?f=3&t=5419

[814CWE]

pre-ignition!!!

[Smiffy]

pre-ignition!!!

& it's taken out the ring lands by the looks of it.

[970s]

Yes, That is what I suspected, cause the engine had run at a retarded timing for a while! But fool around with the SU mixture under a certain period!



Al

[Chalkie]

How are you running a 940 head without pockets? I have a tech drawing for doing the pockets if you want a copy just drop a PM

and pre ignition looks pretty likely

unlike on modern injection systems where it can compensate for stuff like this old engines can't, alot of time bad timing is down to alot of it really it should be timed up with a DTI & timing disc get it spot on to run at its best.

[mini13]

the 940 head can be stuck on the small bore as l long as the valve lift is less than the chamber depth + head gasket thickness, ie either use a small cam, or have the head chambers sunk deeper.

[Chalkie]

The cost of sinking the valves into the head is a bit mad cheaper to mod the block.

[mini13]

agree'd, plus the moded block will flow better too aparently.

I'd still rather use a 295 though.

[Chalkie]

Not heard it flowing better to be fair! but if they are done properly you can run 1.5.1 Roller rockers and better cam's used I'm currently building a Turbo 998 using 940 head with pockets and Bk450 Head gasket.