Broken motor mount bolts on lower mounts

[Bob Wood]

i fired up my XL and taxied and ran up to test the balance master disk I installed last year . Seemed a lot smoother and all checked out well .  I shut down to continue my inspection before picking up my flight testing from last year.  I pulled on the engine and found both lower bolts broken.  I installed the high grade metric bolts I read about in the forum . Tightened them so the rubber mounts bulged a little under pressure and all 4 felt the same.

I pulled the engine today and found both lower bolts sheared off flush with the case.  I got the stubs out with a screw remover tap and everything cleaned up fine and threads are in good shape..

Starting over here, now do I go to a softer bolt that will bend instead of shear..
I bought the bolts at Fastenal. They were called HCS.  I asked for high grade or grade 5 bolts and this is what I got.

Did I get the incorrect bolts?  where do I get the correct ones?

thanks for the help
Bob

[Steve]

I bought my bolts at NAPA...

Among the Joe Engleman shop drawings on the Yahoo group files there is a made stainless stud alternative for the lower mount bolts...

Extraction from the Engleman shop drawings attached:

[scottiniowa]

  I pulled on the engine and found both lower bolts broken.  I installed the high grade metric bolts I read about in the forum . Tightened them so the rubber mounts bulged a little under pressure and all 4 felt the same.

I pulled the engine today and found both lower bolts sheared off flush with the case.  I got the stubs out with a screw remover tap and everything cleaned up fine and threads are in good shape..

Starting over here, now do I go to a softer bolt that will bend instead of shear..
I bought the bolts at Fastenal. They were called HCS.  I asked for high grade or grade 5 bolts and this is what I got.

Did I get the incorrect bolts?  where do I get the correct ones?

thanks for the help
Bob

I have tried to touch this subject before, but perhaps I can add to the question a few more things.
    In reality, the engine is bolted on just like Champ would have with 65 HP,  i.e. same bushings, same bolts?  And they seldom break.
but often we hear about broken bolts in the XL like just listed.

    With the smaller engine, did we go through the process differently that what is used in the Champ?  It would seem so without the rubber bushings being mated up with the same angles as what is called out for in the champ.  I have searched in vain for the latter exact call out…so presently have set on the back burner.  

    But if someone asked me, I would say, “ do your bushings, the metal face angle and bolts exactly the same as the CHAMP” and  you should be good to go.  By the way, that would be a great post, to tell EXACTLY what that method is for angles of faces. Presently I think there could be a binding of the vibration isolation set up that is causing the bolts to break.  I wouldn’t fault the bolts just yet. But again, there is a prescribed bolt for the Champ. With more HP by a little, done in similar fashion, we should be good.

[Bob Wood]

Steve,
Did NAPA call your Bolts HCS?  I am going to  go back and see if there are other grades, and be sure what I got. I cut my aluminum bushings to mach the rubber angle on the mount rubbers....All pieces look ok when I took them out, no shavings or rub marks after 12 hours of running.....

 I will for sure never fly this without safety cables on the motor.....

Maybe now with the balance master disk this is solved......

I was impressed on how well it worked.

Bob

[Steve]

Bob,
My only discussion with the NAPA staff was that the bolts would be comparable to the bolt used on the Vw car... Leonard used Vw bolts on the prototype...

I added the Engleman stainless fastner drawing to my last post...
Steve

[Bob S.]

I had one bolt break on the bottom one time... Had to drill out and put heli-coils in the bottom mounts....

[914pete]

Bob, just out of curiosity, what did you decide on for new bolts?  I've been packaging fasteners for over 20 years and know a little bit about them. Were your original bolts tap bolts?  Grade 5 bolts will have head markings with 3 slashes and are typically zinc plated.  I'm not an aircraft engineer by any stretch but I personally wouldn't use them to mount an airplane engine.  Grade 8 bolts have a tensil strength far greater, probably 50,000+ more psi.  and are marked with 6 slash marks and are usually zinc yellow or cadmium plated which is a better weather resistant plating.  A head marking of HCS could just be the manufacturer and really could have been made in china.  I would think VW engine mount bolts would have been metric socket head cap screws either grade 8.8 or 12.9.  The latter being far stronger.

[Bob Wood]

I ended up with standard Hardware grade metric bolts. Grade 3 maybe...

I am not sure what a tap bolt is??

I want into Fasenal and asked for a high grade bolt grade 5 I think and that is what broke.

Every builder I spoke to said to use standard grade so they can flex and not break...

I check them before every flight..
Bob

[scottiniowa]

I would entertain thoughts back to my reply #3- I asked some fairly important questions, but presently don't think any of them were covered.

That is fairly typical of forums, so not to much concern on my end.

But to toss out an idea.  There are places to push the low-budget ways, and places not to.  In this case, wouldn't it be prudent to use 4 AN grade aircraft bolts/nuts and methods.. I mean in one swift blow, you got all your questions covered, answers already researched and life goes on.  I would be hard pressed to answer someones questions and seemingly weeks of research to save $15.00 on 4 bolts.  But then again, it does open the door pretty big for the old blame game.

"someone said that someone said these "here bolts" would work" - " I don't know what they are, but by golly they are suppose to do everything I need them to do"

[Bob Wood]

Scott,
I am with you all the way on this and used AN Bolts on the top. But where do you find AN Metric Bolts....?

Bob Wood

[scottiniowa]

Hi Bob,
   Without diving into to this far on my part as of yet, what  decimal sizes are we talking? between what "we need or motor needs in metric vs what is available"?

[Dan_]

Hi Bob,
   Without diving into to this far on my part as of yet, what  decimal sizes are we talking? between what "we need or motor needs in metric vs what is available"?

Scott,
I believe this is the correct stock item...  http://www.mofoco.com/category/Hardware/c165

I take the spec to be 10 mm diameter by 1.5mm thread pitch and 85mm long...

Metric thread data...

[Poorman2]

I don't know for sure but, the hcs probably stands for high carbon steel, If I had to guess. And for what it is worth, I once built a dune buggy and put a bus transaxle in it. I had to make  adapters to make the swap and I used grade 5 bolts. They sheered off soon after installation. I replaced them with grade 8 and had no more trouble. My .02!

[Bob Wood]

All these studs would need is a cotter pin hole on the end with the nut inside the plane. the length seems about right. Thread lock the end in the case and tighten them up inside on the firewall.

So I just have to find out what grade they are. The diameter and pitch are correct..

Thanks for the info

Bob

[hawkaleuge]

I used the stock VW studs that came with the case and they both broke at about 50-60 hr.  The top bolts were 3/8" AN hardware from Aircraft Spruce and they held the engine on.  I noticed that climb out was faster but cruise was slower and there was a thudding feel at the rudder pedals.  It's a alarming thing to find broken bolts that were supposed to be holding an airplane engine on.
I replaced the studs with McMaster Carr 94358A350  150,000 psi tensile strength studs.  I tapped the case to 3/8 nc and mounted the stud with thread locker.  Then drilled the stud so I could tie wire it to the frame.  This stud is a bit long so extra washers are needed.  I used an AN castle nut and tie wired through the stud and castle nut and through a hole in the plate supporting the rubber mount.  I'm past 150 hours on this setup and it is still holding.  It's easy to inspect that tie wire is still there, so there is some piece of mind.

Ed L.