Matt Ward

+20 year old donuts (1966 M20E)

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I'm just into owning my first Mooney and I'm hearing that I need to replace my donuts.  The donuts appear to indicate a manufacturing date of April 1998 by the "4 98" on them.  That would make sense as I have a logbook entry on 8/21/98 (2563 tach hours ago) that says "landing gear shock discs (all)".  Before I spend a lot of money on doing this, are there conditions I should be aware of?  I did a bit of searching on the donut replacement and it sound like old donuts are often accompanied by problems steering.  I've not noticed that at all.  There doesn't seem to be a lot of visible evidence (to me) of major cracking/wear.  What condition should I be looking for as I consider replacing them?  Or, is it simply a time-of-life issue?  Thanks!    

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Matt,

What were you listening to when you heard the donuts need replacement?

There are a few threads around here that cover donuts and several quirky side stories...

The short story...

There is actually a procedure in the maintenance manual to follow... a simple measurement of compression...

Over time the donuts compress, and just don’t come back...all the way.

Short body donuts last longer than long body donuts... same donut, just a few more pounds of compression...

20 years is a long time...

Realistically, the date of manufacture can be misleading...

It is time under compression that wears them out...

average temperature while under load will help them compress faster...

Read through your airframe log to find out when the were swapped in...that will give the real feel for how old they are...

Read through the MM to see how to measure the compression...

Search for donut pics around here... a few owners have put new stacks next to old stacks...

Some stacks are really ugly...

Get ready to see what else can go wrong... some times the tube the DuPont mount on can get damaged... by rust...

To start... your donuts look nice... they aren’t aged, cracked, or painted over...   :)

New donuts are really cool... comfy when taxiing over cracked pavement...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Thanks for that. FWIW, I have manual gear and not electric. It seems like the gear retraction issue would be relevant to electrics, wouldn’t it? The leaky fuel tanks seems notable. That’s not a problem for me now but good to know! The article didn’t explicitly mention a way to measure these, any insight on that?

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Thanks @carusoam  I saw those threads too. Those donuts appeared much worse! I’ll search out the MM as you suggested. The reco was from an A&P estimating my first annual. 

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10 minutes ago, Matt Ward said:

Thanks for that. FWIW, I have manual gear and not electric. It seems like the gear retraction issue would be relevant to electrics, wouldn’t it? The leaky fuel tanks seems notable. That’s not a problem for me now but good to know! The article didn’t explicitly mention a way to measure these, any insight on that?

the procedure for measuring is in the Maintinace Manual, it's available in the download area, if not I can email it to you

Edited by RLCarter

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Worn metal parts are causes of steering issues...

worn donuts are not noticeable by anything other their ability to absorb jolts...

The more compressed they get, the less they absorb jolts...

Use the search function for changing out donuts... tools needed or not needed...  lots of pics...

Find a parts manual...

find a maintenance manual...

They should all be around here somewhere...

When that doesn’t work, your local MSC should be able to get you an electronic copy...

Best regards,

-a-

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I just replaced mine last year. They finally failed the gap test. They had a date code of 7-77. They were the originals.

If you jack the plane and the doughnuts don't fill the gap, they need to be replaced. 

Contrary to a lot of other peoples results, I didn't really notice a difference in the ride after I replaced them. It might be because it is hot here, they behave better when it is hot.

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Thanks. I found the below in the MM. It says I need the plane on jacks. Were you guys thinking of another way?2FFBDCF3-389B-469F-AC4F-CB5674E857F4.thumb.jpeg.51db26bc3f0fbf917a243c39a3a4a4e2.jpeg

Edited by Matt Ward

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Following the MM is the best way to not be a test mechanic...

As imperfect as the manuals, procedures and drawings are...

Jacking the plane up allows things to hang with gravity opening up all the available space...

The donuts will expand to fill some of the space...

The measuring is done under that set of conditions...

Done exactly like the book says is a go / NoGo type gauge...

realistically, the donuts compress slowly over decades... so it’s not like they just broke and need to be fixed immediately...

annuals and pre purchase inspections usually take this measurement into consideration... 1 amu for donuts keeps us from wanting to do this often...

Got anything else in mind?

Somebody was selling a set of jacks around here recently...

Best regards,

-a-

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If you do put it up on jacks take a look at the "free play" left and right of the nose gear. More than 3 to 5 degrees of twist is time to fix the wear in the nose gear steering linkage up in the top of the nose wheel well. If you look up there while twisting the nose gear you'll see how worn the parts can get. Makes a BIG difference on landing     Look up "The 8 Second Ride)  by Don Maxwell  What the heck I looked it up for you

http://donmaxwell.com/the-eight-second-ride-sb-m20-202/

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Thanks everybody.  FWIW, in 2013 the plane did have the LASAR nose gear truss oversize bushing service kit installed.  I'll check the track this morning and look for any free play.  

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Rubber products simply aren't designed, or able, to retain their original properties for more than 10 years. And 22 years in service? Just replace them.

This goes for everything. Hoses, door gaskets, engine isolators, etc.

Show someone a 22 year old shock disc that costs $5, and he'll replace it without question. Heck, it's a 22 year old rubber part! Toss it!

Show someone a 22 year old shock disc that costs $110, and he'll try to justify keeping it in service awhile longer...to the detriment of the landing gear system and everything riding on it.

Edited by philiplane
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Mine were replaced all around at annual a few weeks ago. The date code was 1998 so I’m sure they were due. My landings have been smoother but I’ve noticed a little bouncier. You can rent the tools from Lasar & they are not too difficult to replace. 

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If it weren’t for Al Mooney choosing International Harvester rubber donut engine mounts to put on the M18 gear legs, they might not have migrated into the M20’s.....B)

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M18 Mite owners would love to have a part number for the IH mounts... 

there seems to be a shortage of replacements for them...

Best regards,

-a-

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Hmmmm..... I believe it was dear Mr. Terrible (Bill Wheat) himself  that told me the original donuts on the M18 were from IH tractor engine mounts, along with some other tractor parts incorporated into the Mites.  I’m sure the Mite folks know a lot more about the donuts, and lack thereof, than myself.

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The Mite section of MS has been on the look out...

I thought they were from a Mack Truck for some reason...

Best regards,

-a-

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I could be wrong...... I was wrong once before, I think, but I really don’t remember!!:lol:

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I had the same issue with my E. Very, very old discs. A&P said, "Time to think about replacing." Did so and the handling and landing improvements were noticeable. Old discs were hard as rocks. I still have them.

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13 hours ago, carusoam said:

M18 Mite owners would love to have a part number for the IH mounts... 

there seems to be a shortage of replacements for them...

Best regards,

-a-

They’re as rare as unicorns and when available sold for $1000 for 6 of them.

Clarence

  • Haha 1

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16 hours ago, carusoam said:

I thought they were from a Mack Truck for some reason...

I seem to remember reading the ones for the M20's were from Mack trucks. I had to replace mine which were original, meaning "Mo Money", Dan at LASAR hooked me up with new Disks, Shock Links, Collars, Bushings and Hardware

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Welcome aboard Skyd.

Lord sells a bunch of rubber parts to the industry...

Trying to figure out which ones are the right size for the Mooney isn’t very difficult...

Getting them for a lower price than $105 each, is the challenge...

We need 11 of them for the whole plane...

Are the Vans people using the same dimensions as the Mooney?

PP thoughts and wonderment... trying to find a better solution...

Best regards,

-a-

  • Thanks 1

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Very interesting! The part number looks a little different. Are they compatible? And any less expensive?

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