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Pushrod Contacts Shroud Tube

DanH

DanH

Legacy Member
Mentor
Installing new Superior cylinders on a Superior XP-320 and noticed something interesting. The pushrods are contacting the inside of the shroud tube at the end where they enter the case spigots. See below.

This is a roller engine, first generation lifters with snap ring bodies. Seems like an obvious geometry issue, but why? First guess would be the hydraulic units were operating flat all the time, but that seems highly unlikely, and anyway, the design geometry should accommodate it. Valve spring pressure pushes a few lifters flat every time the engine is shut down.

I have a call in to Bill Ross, but we may as well speculate over the weekend ;)

The drawing below is from the Lycoming manual. Don't be confused; I'm merely illustrating where the contact is taking place...6 o'clock in the shroud tube.
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It would be interesting to see the pushrod travel pattern without the tube installed. With the lifter
collapsed and full.
 
Last edited:
JonJay said:
Wrong lifter?
Click to expand...

Certainly seems that way. Did some checking. The pushrods removed with the old cylinders were all -31's, interesting becasue they are nominally 1/2" longer than pushrods intended for a 320. A dry tappet check with the new cylinders says 33's and 34's are required. It appears the tappet assemblies are too short. More research required.

First generation roller tappet bodies can't be removed without splitting the case. This could get interesting.

More later.
 
It can't realistically be due to collapsing of the lifter. That rod is about a foot long and the lifter can only collapse about 75 thou when installed correctly. Not willing to do the trig at this point, but the vertical change of the pushrod in the collpased state vs the extend state has to be inconsequential at that movement level. My $ is on a case machining error, as the design should allow for plenty of clearance and the pic shows more than just incidental contact. Further, it looks like it has worn a nice channel into that tube. You need motion to get that kind or wear and once the pushrods are moving, the lifter is no longer collapsed, assuming there is no defect in the lifter.

Larry
 
Last edited:
DanH said:
Certainly seems that way. Did some checking. The pushrods removed with the old cylinders were all -31's, interesting becasue they are nominally 1/2" longer than pushrods intended for a 320. A dry tappet check with the new cylinders says 33's and 34's are required. It appears the tappet assemblies are too short. More research required.

First generation roller tappet bodies can't be removed without splitting the case. This could get interesting.

More later.
Click to expand...

Wait till you find out how much new ones cost.
 
Also Dan are you aware that the 0.028-0.080 dry tappet clearance is not the same for roller lifter engines?
 
rocketbob said:
Also Dan are you aware that the 0.028-0.080 dry tappet clearance is not the same for roller lifter engines?
Click to expand...

That's a good point. No, I didn't, but I just looked it up, L08-01B, 0.028-0.210. You're suggesting the pushrod might clear if the lash is set at the 0.200 end of the scale, so the hydraulic unit extends to its max? Or just offering a heads up?
 
DanH said:
You're suggesting the pushrod might clear if the lash is set at the 0.200 end of the scale, so the hydraulic unit extends to its max?
Click to expand...

Yes exactly. But you may still get rubbing. With Lycomings there's not much shroud tube clearance at the ends especially on the cylinder side. I had some custom roller rockers made and experimented with custom pushrods also due to the wider arc of travel with a higher ratio rockers. Solved this problem with pushrods from Manton with a tapered tip.

I expect the shroud tube ends creep up over time. Maybe new seals center the shroud tubes in the short term.
 
rocketbob said:
Yes exactly. But you may still get rubbing. With Lycomings there's not much shroud tube clearance at the ends especially on the cylinder side.
Click to expand...

A mystery remains. If a large dry tappet clearance is necessary for the pushrods to clear the tubes, why do they publish 0.028 as the small end of the range?

I expect the shroud tube ends creep up over time. Maybe new seals center the shroud tubes in the short term.
Click to expand...

That too is a good insight.

I'll report after I talk to Bill. The -31's were factory installed.
 
The rest of the story...

Apparently the pushrod-to-shroud tube contact (photo first post) is not unusual with the first generation Superior rollers. An engine shop buddy had three kit engines do the same thing. To get happy they machined the cases to accept 2nd generation roller lifters. Here the owner did not want that expense.

Measurements said -28 pushrods (shortest available without custom machining) would push dry tappet clearance out past 0.165", with some just under 0.190". The max for the 1st gen lifters, as Rocket Bob noted earlier, is 0.210". The idea is to get the hydraulic unit extended as far a practical to improve the geometry. Bill Ross ran it past the engineering department. They agreed it might work, so I gave it a try.

An absolute answer will require run time. However, visual inspection seems to show enough clearance to work...maybe We'll see.

Cyl 3.jpg
 
DanH said:
Apparently the pushrod-to-shroud tube contact (photo first post) is not unusual with the first generation Superior rollers. An engine shop buddy had three kit engines do the same thing.
Click to expand...
Seems this wear on the shroud might show up in oil analysis, or perhaps it just looks like normal initial engine wear. I have roller tappets in my Mattituck IO-360 from about 2010 - I wonder if I have this shroud wear. Is this only an issue with the 320?
 
rv8ch said:
Seems this wear on the shroud might show up in oil analysis, or perhaps it just looks like normal initial engine wear. I have roller tappets in my Mattituck IO-360 from about 2010 - I wonder if I have this shroud wear. Is this only an issue with the 320?
Click to expand...

I don't know how common it might be. No one at my airport had seen it before.

As a practical matter, I don't think it's cause for significant concern. First indication on a running engine would be an obvious oil leak as the shroud tube wore through, but that leak would not be subject to pump pressure. The rubbed spot in the photo (above first post), was about 1200 hours.

If you think you have first gen rollers and want to inspect, it should be possible to remove the nut on the spring retainer under the valve cover, then slide the shroud outboard enough to peek into the crankcase end.

1st gen rollers:


2nd gen rollers:

 
My concern for Rocket Bob is? Is his stack up fire proof? Exceedingly inside humor that only he will appreciate. Dan he does have good suggestions. Occasionally:)

cheers
Howard
 
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