Hi guys, I have a question/suggestion regarding the valve sizes on the 1986 M103 cylinder head. I am finding contradictory information on the size of the valves in this engine. But, the consensus seems to be 43mm (1.7 in) intake, 39mm (1.53 in) exhaust. Which, apparently is the same as the M102?? If I pull the head, I am inclined to have 1.94/1.60 (51mm/41mm) Chevy valves installed along with doing a little judicious porting. That is an 14% increase in the intake and a 5% increase on the exhaust. Those are inexpensive valves, due to their popularity and come in varied valve stem lengths. My only hesitation is, that is a pretty large increase on the intake, so I might have to go with a smaller size on that one. It shouldn't really affect the price of the head overhaul, since they wouldn't have to turn the current valves and you could have new seat inserts installed, so just new valve stem seals and truing the head surfaces and camshaft bores. I would probably keep the camshaft size the same, since it is set up for high torque at low rpm. I can't imagine there would be a major drop in performance, since we're not talking about a huge upgrade in size. It's also a small enough change, that the intake manifold and exhaust system wouldn't need to be modified to realize those gains. But, I do believe that it would produce an average of up to 10% more horsepower at higher RPM while not impacting low end torque significantly. NOW, I also believe that a performance header (if there is such a thing for this engine) would be the perfect complement to the valve upgrade to really bring the engine to life.

SO, has anyone upgraded their valves on the M103 head?
If so, what did you upgrade to and how did it affect the engine?
Do you have any other upgrades, like a header or modified intake?
Anyone do a camshaft upgrade? To what specs and how did it affect the engine? (Now this M103 had a camshaft recall, due to lobe wiping, so I am not talking about just replacing the original, I'm talking about an upgrade)

Yes, I know there are a few archived threads on general performance upgrades, but none that I saw that address this particular issue. I can't be the first one to approach this subject or even make these changes,

so I am hoping there will be some good information shared here. Thanks for looking and sharing. I would love to see some pics of the stock head or heads with modifications or anything like that, since I haven't pulled mine yet.

I have a 5 year member badge and I'm listed as a Newbie?!? LOL!!!!