Hydraulic Tappet Adjustment
I had just installed my new engine and ran it up. It ran rough and was very noisy as if it had a highlift cam which isn't a bad thing but wasn't right. The reading on the vacuum gauge was fluctating back and forth wildly. This indicates an incorrectly adjusted tappet (rocker arm) or a sticking valve.
When I had adjusted the tappets, I wasn't happy with the procedure, it just didn't feel right. This webpage is going to discuss 3 different methods of adjusting hydraulic tappets for a small bock chevy (SBC) which could be applied to almost any model of engine.
Rotate pushrod method
In this method, each pushrod is rotated lightly by hand while its rodnut is tightened (in a specific order). When you cannot easily rotate the pushrod with your fingers, then supposedly you are at 0 freeplay. At this point, tighten the rod nut, 3/4 turn.
I use lots of engine lube when I build an engine. The top of the lifters have lube and the rocker arm had lots of lube. The engine is also pre-oiled by running the oilpump with an electric drill. So oil is everywhere. I could tighten the rod nut until the cows came home and still easily turn an oil drenched pushrod. I know that I overtightened most of the tappets. It doesn't take much as hydraulic lifters only allow about 0.050" of adjustment from what I've heard.
This is the method that most engine building manuals suggest but I know from experience that it is just not conclusive enough.
Adjust while running
This method has you adjust the tappets while the engine is running. I have a set of tappet oil plugs which clip on the rocker arms and stop the oil from being sprayed all over the place. The procedure is to remove one valve cover, plug up the oilholes on the eight rocker arms and start the engine.
With the engine at idle, each rocker stud nut is loosened until it starts to click. Tighten the nut slowly until the click just disappears, then turn the nut 3/4 of a turn more. This will cause the engine to stumble since the valve is being kept off the seat until the lifter can compensate.
Rocker arm clip on oil plug
Oil plug in place on one rocker arm (old junk engine used for example)
Cut up valve cover
This didn't work as well as I liked as the oil plugs would fall off and then the oil would squirt out about 2 feet in the air. It was like trying to adjust the valves with someone constantly squirting you with an oilcan. A better solution would be to drill or punch out holes that just fitted the socket rather than cut a long slit like I did. You wouldn't have to use the oil plugs either.
There's some caveats with this method:
- The tappets must be reasonably adjusted to start with.
- The engine must be reasonably quiet to hear the clicking
I found that the engine exhaust was loud, the noise from 8 rocker arms clicking and the air being sucked into the carb drowned out any hope of hearing a clicking noise. All in all, it was pretty much a waste of time.
Measure freeplay
The last method was simple and worked the best. It requires you to measure 0 play by using a 0.0015" feeler gauge (that's pretty close to 0 play). Each tappet is adjusted in a specific order. The stud nut is adjusted so that the feeler gauge is a tight fit but can be removed. There is about 1/64 of a turn of the stud nut between a tight fit and a loose fit on a 0.0015" feeler gauge. then the feeler gauge is removed and the nut is turned an additional 3/4 turn. This works beautifully.
Just a quick note: There are two TDC on the crank. One is at Cylinder #1 and the other is 360 deg later at cylinder #6. This is due to the cam rotating at 1/2 the speed of the crank (4 stroke engine and all that stuff)
The order of adjusting the tappets for a SBC is:
- Starting with the crankshaft at Cylinder #1 TDC
Cylinder #1 valves do not move when the crankshaft it rocked around TDC. Adjust the black valves as indicated on the following image.
Valve adjustment order - black are adjusted at Cylinder #1 TDC, Red are adjusted when Cylinder #6 is at TDC.
- Starting with the crankshaft at Cylinder #6 TDC
Cylinder #6 valves do not move when the crankshaft it rocked around TDC. Adjust the red valves as indicated on the following image.
Rocking around TDC
Don't use the harmonic balancer bolt to rotate the crankshaft. You'll end up stripping the threads. Take out the sparkplugs so that you aren't fighting the compression.
Now, most power steering pumps have a hex head socket in the pulley. You can use a large hex head wrench with a pipe extender and easily rotate the crank. The ratio between the crank pulley and power steering pump pulley gears down the effort needed. If the power steering belt slips, tightened it up or just press down on it while turning. It works quite slick.