Dave Lapham" sez:
> As usual, if it wasn't for the last minute nothing would get done.
Saw a good one on a chalkboard last week- "Nothing expidites like the last
minute"
I got the Bugeye running last night at 11:00, while also working on making my
"new" trailer race ready, so we can leave for Mosport tomorrow. I spent the day
standing in front of a surface grinder and milling machine, after learning a
bunch more of details about valve train geometry than before...
If you have, or are thinking about offset rocker arms, listen up!
My head (948cc BMC A-series block) was build with offset rocker arm bushes.
The purpose of these, is to give a little more rocker arm ratio, and therefore
some more lift and better airflow. All this without spending a mint on high
ratio rockers. The offset increases the stock 1.245:1 to around 1.28-1.30 ish.
(And, while the 1275 can benefit from 1.5 rockers, that's too much for the
small blocks.) Fine.
Now, if you do this, it's also *suggested* that you get offset pedestals, or
overdrill your current ones so you can pull the shaft & rockers back away from
the valves, thus recentering the rocker pads over the valve tops. I now know
that this is *required*, because it wasn't done on my engine, with the result
that the rocker pads hit the valve tops off centre, at an extreme angle, and
thus *scraped* metal from the rocker pads towards the outside! I sent my
rocker arms (Stamped style, welded) off to Rocker Arm Specialists in CA, where
they regorund & profiled the pads, with about a 2.5 week turnaround. But, this
process worsened another problem in my valve train geometry, which I discovered
as I assembled everything on Friday night of the holiday weekend...
Before, I was close to the limits of the rocker arm adjusters- this is
probably due to the Kent 649 cam, which is a stock cam, regorund with a new
profile, which reduces the base circle, which in effect "shortens your
pushrods". You can compensate with longer pushrods ($$$) or longer adjusters
($$), but your rocker will be at a steeper angle, the adjusters are more prone
to bending, the extra length increases the effective arm length on that side,
and you loose some of the extra ratio that those offset bushings (above) gave
you.
Well, loosing 0.050 off the rocker pads took the ajusters past the acceptable
limits. With the adjuster nuts close to the top of the threads, I had about
.070-.100 "valve lash". Two days to race, and it's time to visit the machine
shop.
My first step was to shorten the bottom of the pedestals by .0625 on a surface
grinder. This compensates for both the cam and rocker pad losses, and put me
back in the middle of the adjuster range. Next, rather than simply overdrill
the pedestals holes, which will make things pretty slopy, and make for a fairly
thin wall around the smaller stud hole, I used the milling machine to extend
the holes 0.030, only in proper direction, resulting in ovals. A lot more work
than drilling, but about the "most correct" short of buying a set of pre-made
offset pedestals. (The proper thing to do, if you have the time tho. And
they're not real expensive, especially compared to the price of a rocker arm
set!)
So, it's done, and running again. And, the valve train is a lot less chattery
than it was too.
See you at the track...
________________________________________________________________________
Roger Garnett (Roger-Garnett@cornell.edu) http://www.wayward.team.net/
"Wayward Sports Car Racing"
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