Jhiltonn

I was checking my starter today and noticed the center bolt that goes into the block on my RH motor mount is missing?!? I've got a 72 base engine 350 4speed.
So I looked at the drivers side and it seems the center bolt is there, but it's coming out. i.e., it's not tightened down.

Does anyone know the size of this missing bolt? also what this bolt should be torqued down too?

I plan on replacing it asap, and tightening down the one of the drivers side too. I checked the other bolts on the mounts and they seem fine, but the ones in the center have issues.

thanks! JH

Alan 71

Hi jh,
I believe the engine mount to engine block bolts are 3/8"x16x7/8".
Torque 22-30 lbs.ft. according to the AIM.
Regards,
Alan

1974CorvetteJimCr

I have a similar issue where the passenger side motor mount center bolt fell out and the the other two were loosing up. Mine is a 1974 and the AIM for my year states it a #331246 bolt, 3/8" -- 16 thread and a length of 15/16". The torque is 30 to 40 foot-pounds. You may check your 1972 AIM (or get one if you don't) since there seems a different values from 1971 to 1974, where 1972 may fall in with 1971 or 1974.

Alan 71

Hi,
I don't know why the difference in the AIMs. Bolt part # changed.
CJC, what's the motor mount part # in your 74 AIM.
In 71 it's 3980701. Did it change?
Regards,
Alan

Peterbuilt

Hi Alan,
The 74 AIm shows the motor mount # as 330973 so it must be 1/16" thicker.

20mercury

My 72 AIM shows on UPC 6 Sheet B3

screw 3748038, 22 to 30 ft lbs.

motor mount 3980701 and I do not see any rolling changes for the motor mount either.

How do you guys know the bolt size from the AIM? I suppose if I had a part number list that has it?

I usually put permatex B on bolts like these that see vibration.

Hope this helps. BTW, getting a torque wrench on these was a trick but enough different wobble extensions will get it done, LOL's!

Alan 71

Hi 20,
I use a couple of old GM parts books.
There are 'some' exploded drawings that show many of the fastener sizes, but not the hardness spec.
Also, 'some' of the fasteners are shown by part number in the parts books.
I also use J Tripoli's Chassis Book which lists a lot of fastener sizes.
As a last ditch method I take the bolt out and size it.
Regards,
Alan

Jhiltonn

Thanks everyone for your help!

I suppose the easiest way to get the bolt size might be for me to remove the drivers side bolt, which is loose anyways, match it up, and then screw it back in? Do you think that's an ok way to go, or will I have problems lining up the LH bolt again if I remove it?

20mercury

Much thanks Alan!

Probably the last method for me, most of the time, might be the quickest, ha!

Alan 71

Hi jh,
I think you're safe going out and buying a 3/8"x16x7/8" bolt as was suggested.
Just be careful that you get it started straight and don't force it.
Since it's a blind hole I'd take a moment to clean it out a little first.
Regards,
Alan

Easy Mike

Part numbers are shown in the AIMs; sizes are not. The Corvette Parts and Illustration Catalogue lists sizes.


These can get pricey. Originals are paper, but CDs are now available. Handy to have around.

20mercury

Much Thanks Easy Mike! Will keep my eyes open and see if I can come across one.

Alan 71

Hi 20,
There's also earlier catalogues called Corvette Chassis and Body Parts Catalogues.
I bought a used one in nice condition dated 1977 at Carlisle last summer for $10. (The cover says 53-77)
It's been marked up a little in a way that makes me think it spent at least part of it's life at a dealership.
It has the specs for many fasteners so is certainly worth the $.
Good Luck!
Regards,
Alan

Peterbuilt

Jhiltonn I'm glad you posted about that bolt. It was a nice warm (60*) day so I decided to change the oil and filter and your post made me look at that bolt and my drivers side was loose.
Thanks again for posting or I would have lost that same bolt.
Regards,PG.

gcusmano74

I would try to find a Grade 8 bolt for that.

Solid LT1

I would go a minimum of 30ft/lbs on that bolt maybe even more.....22ft/lbs is too low for a 3/8ths fastener to properly stretch it. 22ft/lbs.....no wonder they are falling out!

thegavel

Have a look here.
http://www.oldirononline.com/

Jhiltonn

Thanks again everyone for all the great help!!

I wasn't able to find a 7/8" long bolt, which seems a little short to me(but what do I know?!?!), so I chose a 1" long, 3/8" 16, grade 8 bolt, instead of a 3/4" length version. Hopefully the 7/8" vs. 1" won't be a problem. I'll try the permatex B on the threads too!

BTW: Where would I find an original bolt if I wanted to go that route?

Lastly, I found a copy of the Chevy Corvette Parts & Illustration guide catalog for $25 on eBay... Sounds like a great must have book, especially if it lists all the fastener , screw and bolt sizes! Thanks for that tip too!

Joe

Solid LT1

I would seriously consider adding an 1/8" high stack of washers to that bolt, the bolt screws into a " blind hole ' tapped into the engine block if it bottoms out and your still trying to tighten it, you could possibly crack your block!

20mercury

Best of luck to you!