5 speed ribbed tranny rebuild

[georgesgiralt]

Hello !
One of my valves where noisy even with tight clearance. Turned out to be a rocker with a pit on the valve's contact part. The valve start to open then close again due to the hole in the rocker. Then opens up when clear from the hole.
A friendly BMW mechanic polished it and treated it thermally to make it look like new. No more strange noise.
You'll soon know if I'm right when removing the rocker...

[Ken in Oklahoma]

Hello !
One of my valves where noisy even with tight clearance. Turned out to be a rocker with a pit on the valve's contact part. The valve start to open then close again due to the hole in the rocker. Then opens up when clear from the hole.
A friendly BMW mechanic polished it and treated it thermally to make it look like new. No more strange noise.
You'll soon know if I'm right when removing the rocker...

Hmmmm, I'm trying to figure out what "treating it thermally" might entail. I can only imagine that it would mean heating the rocker up to critical temperature (cherry red in a darkish room) then quenching the rocker in oil or water to lock the carbon into the steel instead of slowly cooling which will allow the carbon to precipitate into carbon nodules) and then a re-heat of the rocker to a certain lower temperature to relieve the stresses to the desired degree ("tempering" the steel). The problem is that red hot steel moves around and uneven quenching will lock in any distortion induced by uneven quenching. If done right the steel has achieved the right amount of hardness and ductility for a given application.

Typically, hardened and tempered steel items with critical dimension requirements are then ground to the final dimension. I would expect the fit of the rocker to the rocker arm shaft to be such a critical requirement. That and making the tip of the rocker hard where it "taps" the end of the valve stem.

However my understanding of how things are done is mostly "self-taught" stemming from an interest in things mechanical and shop applications. (For example I have a nice Starrett divider whose points were incapable of scribing a line onto mild steel. My remedy was to heat the tips of the dividers red hot (one at a time) then quench them in water. After quenching I tested the tip with the unused portion of a file. The file wouldn't touch the hardened points. I then made no attempt to temper the hardened tips. I simply ground them to the sharp point I needed. The result scribed mild steel easily. (I am of course as careful as I can remember to be, not to drop the dividers onto a hard surface, for fear of snapping the brittle point off.)

My question is not pointed to georgesgiralt. Maybe somebody else can suggest how an airhead rocker arm can be re-hardened without screwing up the dimensions.



Ken

[georgesgiralt]

Hello !
It is only the tip of the harm which has been re-hardened and polished.
The guy used some sort of induction heat to achieve correct temperature on the valve end of the rocker arm.
He does this "every time" .... and return to normal life rockers that could belong to the scrap bin.... So I'm happy !