Cams

trophy · Post by **trophy** » Sat Jan 26, 2008 2:07 pm

Just purchased from Jampot Spares inlet and exhaust camshafts for a 1956 500 twin I am sorting out for a friend. As both cams rotate clockwise looking at the pinion end, can any of you technical wiz- kids out there explain why the exhaust cam lobes quietening ramps are on the downstream (closing) portion whist the inlet seems OK on the upstream side (opening).
Another query if I may. Why does the hardened part of a shaft wear proportionally more than it's not so hard bearing bush ie Camshaft or crankshaft bigend journals. The logic has always escaped me.

Biscuit · Post by **Biscuit** » Sat Jan 26, 2008 2:17 pm

Don't know the answer to the cam question.

The harder parts of a shaft and bush wear because any particles become imbedded in the softer material and gently grind away the harder material. For this reason, when lapping, the lap should always be of softer material than that being lapped.

GSAX1 · Post by **GSAX1** » Sat Jan 26, 2008 3:46 pm

Trophy, the cam question, I read somewhere that it has to do with the pivoting points of the cam followers. The inlet ones contact the cam with their "heel" first as the camshaft rotates, and the exhaust ones with their "toes" first. I don't know the exact mathematics behind this though.

Biscuit described why the harder material wears in the previous post, I just want to add that one should never ever use emery paper or grinding paste to "open up" tight bronze bushings for this very reason. That is bad practice but many do it.

Michael

trophy · Post by **trophy** » Sun Jan 27, 2008 7:16 pm

Thanks for the replies Alan and Michael. It's good to know that people are prepared to answer what may seem (to some) pointless questions.
I can see the logic re the hardened journals wearing but can't get my head around the reply to the cam forms. Like you say Michael, the mathematics do not seem to add up. I will do some research on the subject and report back. Thanks again.

bjork · Post by **bjork** » Sun Jan 27, 2008 11:17 pm

ref the illogical cams, I had the same doubts when I did a twin for a mate a couple of years ago. No one could explain it for me either, but Jim at JSL said they are all like that sir, so I had to accept it fitted them and they were spot on timing wise. The bike is still ok too! Hi Fred.