Dry tappet clearance preference?

rjcthree · #1 05-03-2015, 06:03 AM

I'm setting dry tappet clearances now, and have plenty of room to be anywhere in the specification range I choose. I'm trending toward setting them all to the lower end of the tolerance band (all between 0.028"-0.044"). Is there any technical preference to high/middle/low in the band?

Interestingly, all the exhausts were good with no spacers, all the intakes took a spacer or two. All the pushrods are the same pn, new lyc cylinders, first cut on the case. H2AD (I know, don't start).

Rick

curtis · #2 05-03-2015, 06:14 AM

I know with my H2AD I just had to work to get the clearance to the range. You have the shims and you add or remove to get the required dimension. You will not have the luxury to dial in which end of the tolerance, just shoot to be in the tolerance. I have over 500 hours on mine using this method and have had no issues.

Mike H · #3 05-03-2015, 07:42 AM

You want to be close to the middle of the tolerance. When the valves and seats wear the clearance dimension will decrease. Obviously the hydraulic lifter unit will adjust for this wear, but the lifter will not be able to adjust if it it set up too tight. If you are not indicating really close to the mini,um or maximum dimensions then I would leave it alone.

lr172 · #4 05-03-2015, 08:50 AM

I would aim for middle. Yes, valve wear will decrease the gap. But, if this is a new enginge or rebuild, you will have some initial wear on the tappet faces and that will increase your gap. If you look at a new tappet body, you will see that it is ground concave and it wears mostly flat in the first couple of hours. They do this to set up a contact pattern that forces the tappet body to rotate. The most common tappet/cam interface failure is when the tappet fails to rotate.

Larry

curtis · #5 05-03-2015, 01:27 PM

It is very difficult to hit the middle of the tolerance with the H2AD style of lifter system. The rocker arms are almost identical to the Ford 351C engine, with the only difference is Lycoming has shims that you place under the fulcrum of the rocker arm. This is the only way to change the valve lash. You need to keep in mind that the H2AD series of engine is completely different from all other Lycoming lifter and rocker arm setups. The lifter is a barrel type lifter vs the typical mushroom lifter, which is why you can remove the entire lifter assembly from the engine without splitting the case halves. I found that I would be out of tolerance and would either add or remove one shim and the valve lash would come into range but you don't have control whether it is in the lower, middle, or top end of the tolerance.

rjcthree · #6 05-03-2015, 02:24 PM

I'm running a new cam, new lifters, new cylinders, OH pushrods. The comment about either being in or out is proving correct. If I add one shim to a lower end of the spec-but in- it goes out on the far side. So, it will be what it will be. Spec is 0.028-0.080, I'm mostly 0.032-0.040. If only there were half thickness shims....Rick.