67 427/435 Tri-power BB

[Donald W.]

Have a question concerning solid lifter valve lash adjustment. The engine has 3,200 miles since being completly rebuilt, including new cam, lifters, valves, springs, rods, pistons and connecting rods, crank and all new bearings. All parts are OEM type. I averaged driving around 1,000 miles a year since rebuild in 2001. My main question is when is the norm for going through and readjusting the valvetrain. The engine builder's yellow markings on the rocker studs and nuts are all still in line with each other, if this is relevent information or not, thought i would mention it. With all you professional's out there, lets see a long thread response.

Thanks for your info,

Don

[Duke W.]

Re: 67 427/435 Tri-power BB

Chevrolet service publications from the sixties recommend checking and adjusting mechanical lifters ever 12,000 miles, and there is no time interval as there is with fluids (X miles or Y months, whichever comes first). I also recommend a check somewhere in the 500 to 1500 mile range for a freshly rebuilt engine.

The clearance ramps on the BB SHP mechanical lifter cams are .012" above the base circle (both inlet and exhaust as the lobe are identical), and the "1.7:1 rocker ratio" would mean the maximum operating clerance should not exceed 1.7 x .012" = .0204", however, the actual rocker ratio is NOT 1.7, and I am hoping that someone will run the measurements to characterize ratio behavior throughout the lift cycle with particular attention to the actual ratio at the lash point.

SB rockers which are "specified" as 1.5:1 are actually about 1.37:1 at the lash point and 1.44:1 at maximum lift with an approximate 0.3" high cam lobe. If the BB rockers exhibit similar behavior, which I expect they do, I estimate that they start at 1.55 at the lash point and achieve 1.63 at maximum lift.

If the ratios are as above the clearance should be tighter than .020"

In 1965 Chevrolet specified SHP BB valve clearance at .020"/.024", but increased the spec to .024"/.028" in 1966. My current recommendation is to set lash at the early spec until we see some actual rocker ratio behavior data. Even though the inlet and exhaust cam lobes are IDENTICAL, they specified greater lash on the exhaust - probably to account for valve stem expansion during hard running, however, they did not specify greater exhaust lash on the 30-30 SB cam, which also has identical inlet and exhaust lobes including clearance ramps.

Despite reports to the contrary for big blocks, I have never measured a difference in clearance on a SB between cold and "hot" (fully warmed up idling engine). It is likely that clearance will tighten up, particularly on the exhaust side during hard running, but that's why mechancial lifter cams have fairly high ramps. They must accomodate proper operation over a wide range of operation. If the clearance is too loose the valve is lifted off and slammed back down on the seat at higher than ramp velocity which can damage the valvetrain from shock loading and contribute to valve seat recession. If clearance goes to zero or negative the valves will not seat and rapidly burn.

Setting clearances on Chevrolet mechanical lifter cams is trickly because of the long ramps. It cannot by done at TDC except the Duntov cam, but I've worked out an indexing scheme that allows two valves to be set at TDC for each of the eight cylinders, however, neither valve that should be set at each TDC is from the cylinder that is at TDC.

If you e-mail me I will send you the written procedure. It can be applied to ANY mechanical lifter cam, but until someone measures rocker ratio behavior I recommend clearances of .020"/.024". These are also the setting clearances listed on the reference sheet provided by Federal Mogul for their CS-165R cam, which is a clone of the original big block mechanical lifter cam.

Duke

[Donald W.]

Thanks Duke for the info *NM*