Gary R. Burrell writes:
> On Wed, 2 Aug 1995, Dave Morris wrote:
> > A friend of mine is building an engine for his '74 MGB.
> > cam and lifter (cam follower) kit with 10.5" pushrods.
> > the pushrods are not long enough to allow him to adjust
> > the valves. It appears as though the pushrods are about
> > 0.375" too short.
> The MGB used 2 sizes of lifters and pushrods over the years:
> 1962-70/71 (G-GK engines) 8.75" pushrods and long lifters
> 1971-80 (V engines) 10 21/32 pushrods and short lifters
Ah- Engine building. First, we'll remind everyone that an engine is a
*system*- any changes require looking at the effects on the rest of the
system. Cam changes usually require matching changes in carburation, as
well as other changes in the valve train, and sometimes intake/exhaust
manifolding, and compression to work properly. Right now, lets check the
valve train geometry.
First, he can measure his pushrods again, and make sure they are the 10
21/23" (10.65625") rods. Sounds like they are.
I don't think there were more than 2 combos, so he's probably got the
right bits. It's possible the lifters are shorter than they should be, but
probably not. I think I do know what may be wrong tho-
He has an "aftermarket" cam- this usually means a stock cam that has been
re-ground, probably with more "lift". The way this is acheived is not by
adding metal to the "peak", or "lobe", but by removing it from the rest
of the way around. This reduces the "Base Circle", or the diameter of the
cam not including the lobe section. OK, say they remove 0.200-0.300 or
more from the base circle- you have now dropped the lifter by that much,
and need to compensate somewhere else in the valve train geometry. Some
ways are:
-Mill the head- when you shave the head for higher compression, it makes
up for part of the cam loss. (But probably not all)
-Longer lifters (While this could be a good match for a re-ground cam, it
would increase mass, and might interfere at the top of lift. I don't
think it gets done.)
-Shorten the rocker arm pedestals- This should be approached carefully,
as there may valve spring and other interference. You only need to
shorten by about 1/2 the needed distance, as they are near the middle of
the fulcurm.
-Longer pushrods. This is often the solution, but is a custom operation,
to be done by competent machine shops. You may be able to by custom
lengths from a race parts supplier.
________
/ _ \ Roger Garnett (Roger-Garnett@cornell.edu)
/ /|| \ \ Agricultural Economics | The Wayward Sports Car Centre
| |\/ || _ | 3 Warren Hall | http://www.wayward.team.net/
| | || | | Cornell University |
\ ||__/ / Ithaca, N.Y. 14853-7801 | (607) 533-7735
\________/ (607) 255-2522 | Safety Fast!
|