Valve Adjustment problem

[jashropshire]

I’m in the process of putting my ’68 912 engine back together after new big bore pistons, cylinders, and rings (and various minor repair/clean-up jobs that needed doing)

I have a deck height of 1.65mm on the left side, and 1.52mm on the right. After several attempts and various combinations of shims, that’s as close as I can get it without more drastic measures.

The left side head measures approximately 61cc’s in both chambers. The right measures 58cc’s in both chambers. Hopefully that difference is minimized now with the cylinder shims.

When removing the heads for the first time, 3 shims came off when I unbolted the rocker arm stands. (I only noticed 1 shim on the right side at the #3 exhaust valve. I found the other two when I was cleaning up the head bolts from both sides) These go on top of the head bolts. After reinstalling the right rocker stand with rockers, and adjusting the valves (without replacing the shims), I didn’t like the appearance of the #3 exhaust rocker in the way it was hitting the valve stem. It was way off (hitting at an angle on the extreme edge of the valve)

This evening I removed the rockers and rocker stand, and installed the 3 original shims all on the right side. #3 Exhaust now looks much better. I was able to easily adjust the intake and exhaust valve on #3.

The problem now is #4 rocker arms are too tight; both intake and exhaust with the piston at TDC. I can’t turn the screw out any further. I am also not certain these shims came off that side anyway. The shims, if I recall correctly, measure about .70mm.

My question is, must I have all 3 shims installed on one side? Would leaving one or two out so that I can get proper adjustment cause stress on the rocker stand when I tighten it down? Or, do I have another problem?

I am hoping to stuff this engine back in this weekend. Your usual good advice is much appreciated.

Thanks

Jeff

[bigbrotherjohn]

Hey Jeff, sorry I didn't write sooner. My engine is pretty much ready to go. I used the shims from aircooled and my clearances measured out OK. My deck height is of course a little bigger and I will have lower compression but that's ok with me.
About your engine, I don't think that the valve to piston clearance is supposed to be measured at TDC if you are going to do it the way that you said. This is because tdc is not when the piston and valve are actually closest to each other. The piston moves toward the closing exhaust valve and comes closest to it while the valve is still partially opened. I did not use this method of measurment. I used a peice of silly putty on top of the piston and turned the engine over. I then carefully took the heads off and measured the thinnest part of the silly putty(don't use playdo, it is to soft). This way was reccomended to me by many people from the pelican forums and from the 912 guide book. Anyway, the valve screw adjustment method is probably the best way but is more involved and must take a long time. To do it correctly you must measure the clearance at many points during the upstroke of the piston on the exhaust stroke. Some guys on the 911 engine rebuilding forums say that they make graphs of the clearance during the entire exhaust stroke. The exhaust stroke takes 180 degrees of rotation so i would think that plotting a point every ~10 degrees would give you a nice graph. then you could interpolate between the points and find the smallest clearance. It is hard to be exact with this method (because you can only plot a certain number of points) but it is at least as good as silly putty.
Now, about your deck height, chamber cc's and compression ratio. the deck heights are pretty close but the chambers cc's ar not so good. From the factory the chamber's were supposedly balanced to 1cc. During a rebuild it is fairly easy to do better than this. Grind away material from the smaller chambers to make them closer to the big ones. I used an electric drill with some kind of grinding attachment from the hardware store. In your case you might want to look for a different head because to get from 58cc to 61 cc will take alot of material off. I would guess that you have one head that was flycut and one that was not. Also smaller chambers make for higher compression ratios if you don't have to shim under the cylinders to much in order to get proper clearances. If you really don't want to bother with this at all then you should at least put the small cc head on the side with the large deck height. If you do the math you could see if the compression ratios are close on both sides. The closer the smoother your engine will run I suppose.
Now the valve train problem. I would not be happy with shimming under the rocker assembly. Their should be one (exactly one, not 2 not none) steel washer under each of the head bolts. this is important because it insures that the o-ring on the head stud will seal on the head. But it seems that you are talking about shims or washers on top of the head bolts. I don't beleive that these are supposed to be there but I have heard of it. There could be a number of reasons why these were needed. It is possible that the bottom of the rocker carrier (the part that contacts the head bolts) are not straight with each other. It is also possible that the wrong number of washers were used under the head bolts(there should be just one remember). Also possible is that the wrong head bolt was used. When porsche switched from the iron rocker arm carriers to the aluminum carriers they changed the head bolts used. The one head bolt that is inside of the part of the head that is covered by the valve cover that does not contact the rocker arm carrier was the only one changed(as far as I know). The one that should be used for a 912 engine has a reccesion milled around the center of it. The older ones from the 356 era do not have this and they are longer. If you are using a 356 head bolt it's possible that it could be contacting the rocker carrier assembly making it impossible to tighten the assembly onto all three of the other head bolts that it's supposed to go onto (also since it is longer the o-ring won't seal and you'll leak oil from you head). You want to make sure you use the right bolt because it probably is not good to bolt down the rocker carrier if it is not going to be straight. Thats all that I can think of right now. If you find the problem let me know. If you need a head bolt I might have an extra, if you find that your rocker carrier assembly is not straight I have an extra pair that I could sell you.
If fixing this doesn't make valve adjustment easy, the best thing to do would be to get your pushrods cut to size at a machine shop. You could also get chromoly pushrods and get them cut. The valve adjustment screw should not have to be put all the way in or all the way out in order to adjust the valve correctly. This could block the oil passage in the screw and deprive the rocker arm of oil.
Good luck with the engine, it seems like your engine has it's fair share of mismatched parts so you will probably have to fight every step of the way to assemble it correctly.