850 pistons with JS2 cam

Hi, here is my question, Yves Seeley, had sold me his old (but nice !!) JS2 cam, which will be fit in a 850 engine , the alloy cylinder will sleeved by Pete Lovell,and I will put some Emgo std pistons , should I cut some notches in the pistons due to the increased valve lift (0.040 ish more than stock), in the same way , I will have only the std CR which is pretty low (8.4 to 8.7 according to the commonly found value), so must I increase it ( by reducing cylinder height by 0.040 thou, quite easy when Pete will machine the jug!!), or just put a thinner head gasket to gain 1/2 point (minus 0.020 gasket)............????? your clever thoughts will be welcome!

My thoughts would be to remove the cylinder base gasket and seal with something like yamabond or Moto seal. Just make sure you keep the oil dran hole clear, I used a pipe cleaner as I installed it, after install, remove the pipe cleaner and blow out with compressed air, that should give you about 20 thou. Then use a thinner head gasket.

Craig

Common practice is to roll up a bit of kids' plasticine and put it into the valve cutaways in the piston, then turn the motor over. If you then section the plasticine with a razor blade, you can see how much clearance you have in the cutaways at full lift. You also need to do this if you alter the valve timing to suit different exhaust systems. If you simply start changing the barrel height by using different thickness head gaskets, you won't know where you are. Sometimes you can make a cutter out of an old valve and use it to deepen the cutaways.

If you use the higher lift cam and also shorten the cylinder (and/or mill the head, or use a thin head gasket), you definitely need to check the piston-to-valve clearance. If you do the clay method acotrel mentioned, you should probably add an extra head gasket or two for the measurement, and then correct the measured values for the extra gaskets. Without them, it's likely the valve will hit the piston. In any case, be careful when turning the engine over the first time. It is very easy to bend a valve if you try to force things.

FWIW, the factory service release for high performance improvements for the '73 850 tell you that with the "SS" cam and .075" milling of the head, the pistons will need valve notch cuts. The intake valve notches they specify are .193" deep, and they say that will give you .030/.040" clearance. If you are removing .040" off the height of the cylinder, not .075", then the same specs would indicate that you will need notches for the intakes that are something like .158" deep. The js2 cam is not the same as the "SS" cam, so your actual clearances will be different. But it does look like you will need to cut the notches in your pistons.

I only have the service releases in .pdf format, but I don't think I can attach them here. I'll see if I can find a way to do so.

Ken

These are the pages from the service release. Sorry about the quality. Hope they are of some help.



Ken

Hi Ken , Thanks for your help!

I've got the same info, but in slightly better condition. I'm attaching it so you can download it.

Charlie