I read this on page 109 of Roger Williams new book.
===========================================
Premature wear in the guides is usually the result of valve gear being
at the wrong height for the valve lift that will be generated by your
cam shaft. Triumph got the height of the valve gear correct in the first
place for the cam shafts it was designing into its engines, however if
you fit a cam shaft that increases the anticipated valve lift, then it
follows that the valve gear height will be incorrect. You need to get
the rocker height halfway within the stroke of the valve if you seek
maximum mechanical effect and minimum sideways loads on the valves and
valve guides. If the cam you get increases the valve's opening from
standard by 0.125" (3mm), it's likely the rocker shaft pedestals will
need to be reduced by 0.623" (1.5mm).
===========================================
This makes sense but I have never heard on these lists any talk of
reducing the pedestal height due to increasing the valve lift. I have
heard of adding shims under the pedestals to compensate for shaved
heads, but that would seem the wrong approach considering the above.
Shortening the push rods would seem the better approach.
Any thoughts on this?
Seems to me that the cam lift varied on the TR250/TR6 over the years.
The early TR6 and TR250 valve lift was .310, so plus .125 = .435. My
overall valve lift will be .425 or under. Is Roger talking about an
extreme case? Seems to me that lots of you guys are going over .435
valve lift.
I only see one part number for pedestals in the Moss catalog. makes me
wonder whether this is a real issue and at what valve lift I should
start getting concerned about it.
Don Malling
|