PROJECT 51: Rocker Arm Assembly Installation. A while ago I posted about rebuilding the rocker arm assembly with new bushings and a new pivot shaft. It has been sitting on the shelf since then, waiting for the engine to be reassembled.
Yesterday I put the new tappets (or lifters) into the block, sitting against the new cam, and then slid the new pushrods through their holes and into the tappets. Lastly, I set the rocker assembly onto the block, and torqued down the bolts. That "lastly" was wishful thinking!
When I backed off the adjusting screws in the rockers to set the .012" valve clearance, I found that they wouldn't retract far enough to even get to "zero" clearance. That meant that all of the valves were being held partially open by the combined height of all of the new and reconditioned parts.
I removed one of the tappets, and measured it against one of the old ones. I found that the new one was .017" taller that the original. That's kind of a wide tolerance for a precision-machined part, but the adjusting screw in the rocker should easily have taken care of that.
I realized that the real problem was that the block and the head had both been machined down (by two different shops) to make them perfectly flat. Neither gave me a report on how much they needed to take off. Then, it occurred to me that Brutus had most likely had the same thing done when he rebuilt the engine. The combination of four machinings dropped the head (and therefore the rockers) down so much that when added to the extra-tall tappets, there was no more adjustment in the screws.
I added shims under the rocker posts, and determined that I needed a minimum of .030" added height to be able t adjust the valve clearance to .012". One option would be to grind the bottom of the tappets to make them shorter, but I am always reluctant to make a change like that, which I can't undo if I don't like the result. Also, it would likely have gone right through the case-hardening on the bottoms of the tappets, leaving them "soft" and subject to wearing out very quickly.
So, I decided to make a set of spacer shims to raise the rocker posts up. I was able to use a piece of 18-gauge steel that I had on hand, which gave me a boost of .048", allowing for a little more adjustability.
The rocker assembly is now torqued in place and the clearance set on all of the valves.
|Measuring height of old tappet on the mill.|
|Shims under rocker post to determine the "boost" required.|
|Final spacer shims on head.|
|Final assembly (or is that finally assembled?)|