lostpilot28

lostpilot28

Well Known Member
I posted a message about pushrod length about 7 years ago before my airplane ever ran. Back then, I had two cylinders that had intake valve lash clearance that was at the very upper end of the spec (they both measured at .080"). Here's my post.

Anyway, I'm doing my annual condition inspection and these same two cylinders are now a bit out of spec. One has a .09" gap, and the other is at .1". The engine appears to run just fine, but obviously the gap needs to be closed up a bit. A 3rd cylinder is at .08 exactly. All of these are on the Intake side. The one remaining intake and the 4 exhaust gaps are all within spec around .07".

My question is, would going up to the next length of pushrod be enough? I've heard that the AEL73435 is .03" longer than my current AEL73434. I'm not sure if the rocker gives an exact 1:1 ratio, so I just want to make sure that up-sizing by .03" would be enough.

Last question is on the .08" gap...should I swap that one, too? Or leave it alone because it's technically "in spec"?

Thanks in advance...any help is greatly appreciated!
 
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1.28:1

Stock rockers are 1.28:1 ratio. If it were me I'd put any that were close to the published 0.080" maximum dry tappet lash and get longer pushrods to move them to the middle of the range. The risk is that you run out of extension in the tappet and start hammering the valve drivetrain. Pushrods are inexpensive enough.

Beware though, that Lycoming and Superior pushrods with similar part numbers are NOT the same. I went through this with Duane from Superior quite a bit, and he could not explain it, but the Superior parts are shorter that the similar numbered Lycoming parts by a good 0.030 to 0.040". Beware, and be ready to swap parts in the mail a few times until you get the right fit.
 
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Thanks, Kevin. I've read that it's better to be close to the high end. Other than the one that is .08", I won't touch the others. I'm still on the fence about touching that one. The two that are out of spec definitely will get replaced.

So, can you clarify the 1.28:1 ratio? Is the valve side the 1.28 or the pushrod side? If it's the valve side, then my .1" gapped intake valve will move down to about .06". If it's the other side, then it will be .08". Big difference!
 
Valve to Pushrod Travel

The valve travels 1.28 times the pushrod travel. Look at the rockers, they're longer on the VALVE side, from the center pivot. So if you're looking to close the 0.080" (measured on the valve side) to the small side spec (0.028" if my memory is right), you'd need (0.080" - 0.028")/1.28 = 0.040" longer pushrod.

Generally from new you'll see the dry tappet clearance get slightly smaller. This is because the valve wears into the valve seat and the valve spring pulls the valve closed a bit more, hence the general acceptance of being on the high side of the spec, but staying away form the low side.
 
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Thanks again, Kevin. I would probably aim more for the middle of the spec and go with .030" longer pushrods.

But, I'm still not sure whether I should replace the one that's at .080". Can anyone who knows this stuff tell me which way they would go if it were their engine? :confused:
 
Sonny,

One of the purposes of having adjustable pushrod lengths is to be able to adjust the hydraulic plunger some were close to the middle of it operating range, instead of at its limits of operation. As you probably know cylinders get longer when they get hotter, shorter when they get colder. The hydraulic plunger is trying to maintain zero valve lash at all temperatures and is also compensating for wear in the valve train. In your case with your clearance at the maximum when tested on the ground, it means that at high cylinder temperatures your valve clearance is most likely out side of the plunger?s ability to compensate. This tends to hammer the valve train like a collapsed lifter does.
There are only a couple of things that can noticeably increase valve clearances in a Lycoming type of engine. Worn rocker arm bushings, or the cam or lifters starting to spall and wear. Did you have more than normal metal in your last oil filter exanimation?
If not your cam and lifters are probably OK. But you should most defiantly adjust the pushrod length to get the .028-.080 clearance when the lifters are completely collapsed at cold engine temperature.
 
Thanks, Bobby. That's exactly what I'm attempting to resolve with the 2 gaps that are over .080".

The question that I still have is whether to replace the one that is at .080". I'm assuming (probably incorrectly), that the spec exists "as is" even when considering heat induced expansion. I believe that if I didn't measure my clearances, but rather used the Go/No-Go gauge, it would be a "Go", and I wouldn't be asking this question.

Regarding your question about cam wear...I haven't seen anything in the oil or screen. I don't believe there is abnormal wear in the engine...most likely the changes from my initial measurements 7 years ago are caused by the engine actually getting some use. The gaps I had back then had zero wear...not a single revolution. So, I would expect that I cannot use that initial measurement for any type of trending. If I measure my gaps again next year and they change, then I'll start looking for a cause.
 
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You're looking for Lycoming SI 1060K (I think that's the latest revision). Should be able to find it on the internet - if not, send me a PM with an email address where I can send an attachment.

Dan
 
Dan, that was very helpful...this is what I found regarding part numbers and their lengths:

P/N inches
73434 12.483
73435 12.510
73436 12.537
73437 12.564

Interesting that they're all .027" difference...why didn't they just round it up to .030"? :rolleyes: The math would've been easier!
 
Dry Tappet clearnce?

May I ask were these dry tappet clearances? By the book the hydraulic units should be fished out and cleaned & dried. My book has the 34,35,36 listed for the 0-360 parallel valve.Middle of the road to the high side of spec would be preferable.
RHill
 
rhill said:
May I ask were these dry tappet clearances? By the book the hydraulic units should be fished out and cleaned & dried. My book has the 34,35,36 listed for the 0-360 parallel valve.Middle of the road to the high side of spec would be preferable.
RHill
Click to expand...

Yep, they were dry. Removed each one and cleaned out the oil, then put them back before measuring.
 
lostpilot28 said:
Thanks, Bobby. That's exactly what I'm attempting to resolve with the 2 gaps that are over .080".

The question that I still have is whether to replace the one that is at .080". I'm assuming (probably incorrectly), that the spec exists "as is" even when considering heat induced expansion. I believe that if I didn't measure my clearances, but rather used the Go/No-Go gauge, it would be a "Go", and I wouldn't be asking this question.

Regarding your question about cam wear...I haven't seen anything in the oil or screen. I don't believe there is abnormal wear in the engine...most likely the changes from my initial measurements 7 years ago are caused by the engine actually getting some use. The gaps I had back then had zero wear...not a single revolution. So, I would expect that I cannot use that initial measurement for any type of trending. If I measure my gaps again next year and they change, then I'll start looking for a cause.
Click to expand...

Sonny,
If it were my engine and I was changing other pushrods in it already. I would change the one that?s at the max limit too. To avoid going outside the limit if there is wear in the engine in the future
 
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