ASP120FS sheared tappet screw

[Mike L]

I was on 3rd flight this morning with ASP120FS in Wot4xl, on approach, engine cut at about 1m, no issues, but unusual though as engine normally runs really well. Starting it up again, no compression, engine just turned freely. Took off rocker cover and lo and behold this is what popped out. Sheared tappet screw, but in an odd location to my mind. Got back to workshop and it now has compression. I can turn the screw stub and there is enough thread to get the nut on. Spares seem to be in short supply, will give JE a call later. However the whole thing baffles me, why would it shear in first place and why is there now compression?  I would be grateful for any observations! 

[Denis Watkins]

I doubt the shear was in one go Mike, as a shiny sliver looks like that was the last bit.

The sudden return of compression can be the piston ring popping back in open position.

I oil the top end, under the rocker usually each month in the flying season

Despite oily mist vapour and oil pressure claims up to the tappets, I have seen rust up there in the past, so I oil.

Edited June 11, 2021 by Denis Watkins

[J D 8]

  It could be that compression was lost when one of the loose bits found its way to a position were it jammed a valve in an open position. When you remover rocker cover the bit or bits came out freeing the valve.

  

[Bob Cotsford]

I had one shear a few years ago, I put it down to overtightening the locknut.  I'd go with JD8 on the nut stopping the valve closing.

[Engine Doctor]

The tappet screws are a bit rubbish on the SC/ASP . If you are really stuck then an OS screw should fit. I have seen them made from small cap screws suitably ground down on the head to form a cup for the pushrod to sit in. 

[Mike L]

Thanks for the feedback, much appreciated.
 

ED - goods heads up on OS as it looks like JE are offering the OS replacement tappet screws for the ASP so that could well be a result.

 

It’s a good excuse to drop in the Saito 100 that’s been waiting in a box for a home. ASP will be earmarked for another project.

 

thanks again 

[Richard Acland]

Had exactly the same thing happen on an SC 70 FS not once but twice. The first time it also cracked the rocker cover. This was a while ago when spares were easy to get but when I ordered parts the second time Just Engines were out of stock of tappet adjusters for my engine which made me wonder if this was a common problem. I later bought four just in case. I still have some as it has never happened again despite the engine having a lot of use.

[Martin Harris - Moderator]

I wonder if there was a faulty batch too.

 

I had this problem with a friend's 120 fairly recently - sudden stop and total lack of compression - took it home to have a look at for him, fearing a major problem - 5 minutes after removing the rocker cover, with an obvious internal witness mark where the nut/stud had been wedged between the rocker arm and the cover, I'd found a spare adjuster and had it fit for further service!

[Don Fry]

I like ASP motors, cheap as chips. Production process and materials reflect this, with softer metals to ease tool wear and speed the cutting, and less attention to metal consistency. Good enough for a dangerous career fronting up a hooligan machine controlled by the likes of me.

[Engine Doctor]

When you think of the probably millions of these engines that have been made over the years the failure rate is really quite low.