The GMC CCKW

What size bolts fit the rear motor mounts? I bought new motor mounts for my truck and went to buy new bolts but nothing will thread into the mounts. The original bolts from the truck are unusable. Any advice appreciated.

Thanks,
Nick

G'day Nick! The bolts should be 9/16"UNF threads, unless the after market mounts are different. Cheers!

Interesting. I tried the 9/16" UNF thread and it only threaded in about 1/4". Is it supposed to be hard to thread in? Possible to ensure the bolts don't come loose?

Thanks,
Nick

The mount I tried accepted the 9/16" bolt with ease. No difficulty encountered. Maybe there is a problem with the thread?

I'm thinking that I need to buy a 9/16" thread chaser and run it into the motor mount to clear up any possible debris such as some kind of preservative like hardened cosmoline.

It is hard to admit, but I've already screwed up one pair of rear motor mounts when I had this same problem . I just figured they were supposed to fit tightly. Two bolts went in all the way (using an impact wrench) and come out fine and the other two will not thread all the way and are stripped. I made sure not to cross thread them but I can't figure it out why they won't thread. In the process, I ruined two of the original bolts and I am going to replace all of the bolts with new ones.

While the mounts aren't the most expensive thing in the world, I don't want to screw up another set. I also bought the two sets from seperate distributers (the newest set is from Memphis Equipment) and I am having the same problem.

Will the thread chaser idea work fine or will I end up screwing up another set of mounts?

Thanks,
Nick

Well, I went ahead and ran a thread chaser through them. I can't believe the gunk that came out which I believe caused me to cross thread the bolts on the first set of replacements. I also bought new bolts and had to buy them much shorter than the originals. It seems that at some point they started to put a metal stop in the mounts which is why they wouldn't screw in enough to tighten up all the way. After I cleaned the threads out, the new mounts went into the truck flawlessly.

Moral of the story is always run a thread chaser into the mounts before you install them and you won't ruin a good set of mounts. I learned the hard way.

Thanks for your help Ross.

--Nick

There are NOS (sort of) motor mounts out there that have no known thread size! I bought 4 of them and nothing US or metric will fit. I took them to a machine shop and they could not match anything to them.

Maybe wirtworth....


I finally gave yup and bought new oned from Memphis Equipment

Thanks for that info Steve, it is good to know.

--Nick