PCM Engine Mounts

Hi,

I was about to align my engine, but it turns out that one of the rear engine mounts (on the transmission) are totally seized.

I removed the mount, and have stripped it completely. I am replacing all the rubber isolators, washers, etc. In the process of stripping the heavily rusted mount, the lower stud retaining nut has been damaged (item #9 in diagram).

The PCM part description says:
RS1082 Nut, Two way Locking, Stud Retaining {Special} 5/8" - 18.

Is this a normal 5/8 UNF nut? Why does it say "special"?

I sourced a 5/8 UNF locally, but it seems to get quite tight as I turn it onto the stud. Just want to be sure I don't damage the threads (which are already not in great condition).

Thanks,
Greg

I had a better look, and it seems like the threads on the stud are higher than those in the nut - i.e. the nut threads are shallower.

So by turning the nut onto the threaded stud, the nut is squashing the top of the threads on the stud. It will probably work, but its not ideal. You can see on the photo below that the nut has squashed the first 3 threads flat.

You think it is okay?

Greg,
That is tight!!!

The bottom nut doesn't carry any weight of the engine. It's just handling any torsion loading. Can you source a 5/8-18 adjustable die over there? Is the "UNF" (18 pitch) out of the book? IE: are you sure it's a 18 pitch?

I just double checked, and the thread pitch is definitely 18 for both parts... I find it strange the the thread depth can be so different :( I suppose the word "special" does mean something...

Another supplier says: NUT, LOCK PREV. TORQUE 5/8-18 Not sure it "torque" refers to anything?

Pete, thanks for all the advice, once again

I put some metal polish on the thread of the stud, and worked the nut a little back and forth, and it ended up hardly wearing the stud thread much at all (much less than the first 3 threads) - so it fits great.

I'm replacing my engine and transmission mount isolators. When I removed my old rear (transmission) lower isolator (part 29 in the diagram above), it was installed upside down from the diagram. Has anyone else seen that? I'm guessing the diagram is correct and mine was installed backwards in its previous life.

Thanks! -Rob

Thanks Benj!

It's also interesting that the ring at the top of the bracket is closed on yours, like the diagram shows. Mine are open, much like a flarenut wrench. Same functionality, just not an enclosed circle.

1975 or 1976 (I can't quite determine for sure) with PCM 351. However, a previous owner had, at least, rebuilt the engine, which doesn't have an engine tag anywhere. So for all I know, it could have been swapped from an original Waukesha 🤷🏼‍♂️

I just replaced my rubber isolators. How much torque do I need to use to tighten down the nut ( #9 in the diagram) to properly compress the rubber bushing and get it to the correct snug-ness?

And how about the equivalent nut on the front (engine) mount?

robdubbleu wrote: 1975 or 1976 (I can't quite determine for sure) with PCM 351. However, a previous owner had, at least, rebuilt the engine, which doesn't have an engine tag anywhere. So for all I know, it could have been swapped from an original Waukesha 🤷🏼‍♂️

So far your boat has taken turns being a 74 then a 76 and now a 75 or 76

Why don't you post the HIN and it should be relatively straightforward to figure it out.

robdubbleu wrote: KENO wrote: robdubbleu wrote: 1975 or 1976 (I can't quite determine for sure) with PCM 351. However, a previous owner had, at least, rebuilt the engine, which doesn't have an engine tag anywhere. So for all I know, it could have been swapped from an original Waukesha 🤷🏼‍♂️ So far your boat has taken turns being a 74 then a 76 and now a 75 or 76 Why don't you post the HIN and it should be relatively straightforward to figure it out. Ok, you caught me! Forgive me, as I initially was told it was a 75 by the previous owner. Then the official title shows 74. And the hull number indicates (I BELIEVE) that it was built in December of 75 so I don't know if that was sold as a 75 or 76 model. HIN: CTC05834M75E SNK-76. And to muddy the waters further, there's conflicting and ambiguous info on the webz regarding the decoding of the HIN    As for the mounts, thanks so much for that picture!!! That should be all I need. I took pics of almost everything but not these. Now is it too much to ask for a similar picture of the front mounts?

As far as the front mounts go, the nut on the bottom is put on the same way and has same amount of threads showing.

2 lock nuts are used, one above and one below the trunnion stud

I'd take a picture but the fronts are in a boat and you can't see the bottom nut when it's mounted to the stringer    

That HIN says to me that it's a 75 model year built in December of 74. but I could be all screwed up.

Maybe it never left the factory that year and they magically turned it into a 76 at the factory since in some of your previous pictures it looks like a 76, especially the windshield frame

My 76 ended with SNK and no numbers after it.

Let's see a picture of that HIN

KENO wrote: Let's see a picture of that HIN

As requested!

KENO wrote: link I just don't know what it takes to get something added to the reference section

I've also found that parts manual exceptionally useful, and have also requested Keith to add it to the Reference section, but nothing so far.

Does not look like Marine Parts handles PCM parts anymore probably why the list is gone.