When to replace landing gear shock disks

[PT20J]

Unless they are obviously cracked or otherwise degraded, it isn't necessary to know what the date code is or when they were last replaced. They should be replaced when the static compression is excessive. There is a procedure and a specification in the Service and Maintenance Manual. Be sure to use the correct manual for your model/serial number because the specification for the main gear has changed from 0.125" for early models to 0.85" on later models. For reference, my M20J disks are about 9 years old. The spec is 0.6"max and mine measure 0.4".

[N201MKTurbo]

I don’t think the manual mentions it, but the disks are very temperature dependent. Almost any disk will fail the test in the dead of winter in a cold hangar and almost any disk will pass on a hot day here in Arizona.

[Fritz1]

Interesting topic, I replaced the discs when I bought the Bravo, they were 17 years old and they did not fail the test at 70F, but the shop that had done the prebuy recommended replacing them, ball park they said replace every 7 years, ride became significantly smoother after replacing the discs, I put the plane on jacks if I do not fly it for more than 2 weeks, maybe that helps a little, or just makes me feel good, so if the test in the service manual is not really reliable since it does not specify a temp, when do you replace the discs?? 

[PT20J]

I haven’t noticed that much temperature effect on mine, but I’ve just been eyeballing them and this is the first time I actually measured them. It was 40 deg F when I took the measurement. I’ll take measurements at different temperatures this year to get some data. 

I’m pretty sure the ride will improve after replacement if the disks have a few years on them. But, given the cost of the things, it would seem prudent to get as much life out of them as possible. 

They will certainly last longer on the earlier airplanes with lower weight. Just look at how the limit has changed by nearly 3/4 inch with no change in the design.

[kortopates]

It’s important to note there are really two independent test for the shock disks and only one is in the service manual - the amount of compression under load. I don’t think compression is much affected by temperature. Temperature affects how quickly the disk expands after removing the load. Many mechanics feel the better test is to jack the plane up and see if the disks expand back to taking up all the slack such that you can’t twist the disk and feel any free play. This test is temperature dependent but remedied by waiting awhile. There is no precise measurement for this test but it replicates the old issue going back to squat switches where the weight on wheels switch didn’t close right after takeoff preventing the gear from coming up. So the test has been in use as long as we’ve had Mooney’s.


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[PT20J]

Paul is correct, and I should have pointed out the second test. Attached is a section from the M20J SMM with the entire procedure outlined in red. No temperature or time limit is specified in the extension test, but since the text states that the gear must be fully extended for retraction, it seems self evident that the delay should be minimal. 

I highlighted in yellow a note that I find humorous as it describes a critical approximate measurement. 

[Mufflerbearing]

I purchased the nose gear tool which I will be glad to share if someone is in need.

[MB65E]

I have a nose gear tool available to rent in SoCal if anyone needs one. 
Just installed a shim on a J that didn’t have one. Learned some things! 
-Matt

[Flyler]

Hopping on this thread... If anyone has a *new* or unused puck, could you measure the thickness with a vernier?  

[201guy]

Disks are 1 inch thick.

[Flyler]

Thank you, but I'm an ex-machinist and 1 inch can mean a rather broad range to me... Not trying to be snarky, just hoping for more significant figures.

[EricJ]

9 minutes ago, Flyler said:

Thank you, but I'm an ex-machinist and 1 inch can mean a rather broad range to me... Not trying to be snarky, just hoping for more significant figures.

I have a bunch in my hangar from two different batches.   If I remember I'll try to get some measurements tomorrow, but I doubt there's much precision to them.

 

[Hank]

I'm with @Flyler on thickness. For a useful comparison reference, a tape measure in this instance has no meaning. Even a cheap Harbor Freight caliper would get us in the ballpark.

On the other hand, someone who has never used calipers won't provide much useful information. And I don't know about the precision of rubber molding (although I've spent decades very precisely molding various plastic parts).

The shock discs also have flat bronze plates on the upper and lower surfaces.

[Flyler]

For reference I have some used pucks that were manufactured in 2007, and removed from service in 2025. They measure about 1.03" thick on average.  I'm most curious about new ones.

[Hank]

55 minutes ago, Flyler said:

For reference I have some used pucks that were manufactured in 2007, and removed from service in 2025. They measure about 1.03" thick on average.  I'm most curious about new ones.

The sooner after removal that you measure them, the better. Rubber flows and deforms under pressure, and will relax some of the movement away when the pressure is removed.

[EricJ]

On 3/1/2026 at 5:48 PM, Flyler said:

Thank you, but I'm an ex-machinist and 1 inch can mean a rather broad range to me... Not trying to be snarky, just hoping for more significant figures.

I put a dial caliper on a couple new ones from different batches today, measuring total thickness including the plates around various spots to find the max and min thickness.    Both were about 1.110" - 1.120".   

[N201MKTurbo]

1 hour ago, EricJ said:

I put a dial caliper on a couple new ones from different batches today, measuring total thickness including the plates around various spots to find the max and min thickness.    Both were about 1.110" - 1.120".   

It sounds like 1” rubber with two 1/16” plates.

[Flyler]

1 hour ago, EricJ said:

I put a dial caliper on a couple new ones from different batches today, measuring total thickness including the plates around various spots to find the max and min thickness.    Both were about 1.110" - 1.120".   

Thank you! So if there are four pucks in the mains, that means my stack shrunk by about .32"

They also measure a harder durometer than the new ones in the plane.

[Mooney in Oz]

On 2/23/2026 at 4:48 AM, kortopates said:

Many mechanics feel the better test is to jack the plane up and see if the disks expand back to taking up all the slack such that you can’t twist the disk and feel any free play. This test is temperature dependent but remedied by waiting awhile.

This was the test my mechanic recently used to determine the condition of the main gear disks after 17 years of use.

Those disks were replaced, however the nose gear disks had passed the test and remain.

[EricJ]

9 hours ago, Mooney in Oz said:

Those disks were replaced, however the nose gear disks had passed the test and remain.

I think the nosegear pucks will always pass that test unless they crumble and fall apart.    They're always under a significant amount of compression, which is why it's much harder to install new ones in the nose gear.