Ant CB

Ant CB

Member
I had my valve seats re-ground when the valve guides were replaced. I'm now on the minimum limit .028 on my dry tappet clearances. Should I be concerned?
 
If you are in the range that Lycoming has said is acceptable for your engine I would not be concerned.
 
You didn't say if it was one cylinder or all of them. As stated above if it's within you have no worries. However if it's one or two locations that you have a maximum clearance and you have other tapets (more than one cylinder) that are close to a minimum clearance you can do some pushrod swapping to get them all more in the middle.
Ryan
 
Nah, it's on all of them because the cylinders were all removed for re-honing and had their exhaust guides replaced valve seats re-ground. It reduced the dry tappet clearance and some are right on the lower limit. I can just get the .028 feeler under the rocker on a couple of valves so its right on the lower limit.

I'll see about getting shorter rods.
 
Just to be clear, you did bleed all of the oil out of the lifters right? I am sure you did but its worth asking because its the only way to check them properly. Even if you have a little residual oil in the lifter body it will tend to pump up the lifter when rotating the crankshaft to check clearances giving a clearance that is to tight or non existant.
Ryan
 
Ant CB said:
Nah, it's on all of them because the cylinders were all removed for re-honing and had their exhaust guides replaced valve seats re-ground. It reduced the dry tappet clearance and some are right on the lower limit. I can just get the .028 feeler under the rocker on a couple of valves so its right on the lower limit.

I'll see about getting shorter rods.
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Anthony,
Check out Lycoming SI 1060K. This outlines the lengths and part numbers of the available push rod lengths for all engines. I would definitely recommend that you try to get as close to the middle of the range as possible. While you now have the "minimum" clearance, future wear may reduce that to below the minimum. This is to be avoided at ALL costs. Insufficient dry lifter clearance can cause a rough running idle condition. You would waste a LOT of time and money trying to diagnose the source of the problem. The push rods for your engine come in lengths which vary in .027" increments. Subtract .027" from your current .028" clearance and you now have .055" clearance. That is much closer to the middle than you are now.
As a previous lister mentioned, you might want to measure the actual lengths of all your push rods. You might be able to be a better fit by swapping some of the push rods around.
Lycoming SI 1060K can be downloaded at:

http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1060K.pdf

I hope this helps

Charlie
PS Re-surfaced camshaft/lifters/rocker arms/ valves and valve seats [or a complete cylinder] are the items which will alter the lifter dry clearance.
 
Rocker arms are not 1:1 ratio.

chaskuss said:
The push rods for your engine come in lengths which vary in .027" increments. Subtract .027" from your current .028" clearance and you now have .055" clearance. That is much closer to the middle than you are now.
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Pushrod length and valve clearance is not a 1:1 ratio.
A pushrod delta of .027" will not yield a valve clearance delta of .027.
 
Seeing as though .028 is the minimum at overhaul, and from overhaul the engine can run out to 2000HRS then I assume Lycoming is happy that there is enough margin to account for valve seat wear in that time.
 
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I have 60 roller rocker arms I had made along with custom tapered pushrods. The rocker arms have adjustable cups and I can make the dry tappet clearance whatever I want, without having to change pushrods. We're in long term testing now. Over time the valve seats do not wear but the valves stretch a couple of thousandths particularly the exhaust valves.
 
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