What a mystery this was to me when I first heard about it. I thought the point of hydraulic lifters was no setting up, no adjustments required, obviously not!. After a rebuild, factory tolerances, after market parts, skimmed surfaces, and a mix of 30+ years of developed factory parts all fitting the same engine, may lead to your pre-load being out side the factory recommended setting of 20-60 thou.
The pre-load is the amount of compression exerted on the spring in the lifter itself, by the pushrod. Too little pre-load and the valve train will rattle, too much and you valves will not seat properly.
Its very fiddly at first to work out exactly where to measure the pre-load. When the engine and valve train are torqued town, move the first tappet to the heal of the cam lobe. The gap to measure is between the base of the circlip and the flat top of the lifter piston. I used welding wire, paper clips, and just about any wire that could be bent in to a hockey stick shape. Its a bit tight working around the pushrod and good light is required to see the gap and cam lobe position.
I tried to find a set of wire feeler gauges, but to no avail, so various pieces of wire measured with a micrometer were used instead..
The first measurements gave overall too much pre-load, so a shim kit was purchased from RPI to raise the rocker pedestals. If your pre-load is too small you will need to have the rocker pedestals machined or use adjustable push rods.
RPI shim kit come in 16,32 an 48 thou sizes.
After fitting 32 thou shims and about 4 hours of swapping pushrods and rocker shafts from side to side I ended up with the following settings.
|
Post adjustment |
|
|
Left bank |
Right bank |
|
50 |
50 |
|
70 |
50 |
|
55 |
65 |
|
70 |
50 |
|
50 |
60 |
|
60 |
35 |
|
65 |
40 |
|
50 |
50 |
On advice from RPI I have allowed the setting to go as high as 70 thou instead of 60 thou. I believe this is because the valve train very quickly settles down once the engine is running and the gap reduces to within the factory range.