What is Transmission mount bolt size? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

'65 chevy lover · 10-07-2014, 08:27 PM

I'm having a difficult time finding the correct bolt size for the rear mount of my TH400. Not sure what year model it is. Bought it at Bishops International. It didn't have bolts in it. I've tried 3/8 and 7/16...neither worked. I'm wondering if they might be metric. From what I can tell, they appear to be fine thread. I don't want to screw up the threads trying to put a boult in there that's the wrong size/thread count. Anyone know what size and thread count?

Thanks

**Captainfab** · 10-07-2014, 11:44 PM

It is possible they could be metric. If they are, they should be either M10 or M12.

'65 chevy lover · 10-08-2014, 02:53 PM

Got some M10 bolts. One side , the bolt wouldn't thread in, the other side, the bolt slipped in there about half way without threading it in. Obviously, the threads are screwed up. Decided to just go with 3/8" bolts with lock nuts. Wanted to go with 7/16 but, a 7/16 nut wouldn't fit into the area where the hole is. Had to slightly reem the hole with 3/8 bit.

Kenfuzed · 10-08-2014, 04:01 PM

I believe it may be the same as the TH350 mounting holes.

For a TH350
1976 and earlier is 7/16-14
1977 and later is metric M10-1.5

A 3/8 will thread into the metric threads but it will be loose. I've never tried it but if at least some of the threads are still intact you could try some Loctite Form-A-Thread to give the 3/8 bolts a better grip. They claim that stuff will withstand 128 ft/lbs of torque. I'm sure others may suggest a stronger and more permanent thread repair but this would at least add a little security over your current fix. You don't want the tail of the trans to rip loose from the mount.

'65 chevy lover · 10-08-2014, 04:43 PM

Thanks for the reply. It's already a done deal. I think it will be ok. Wether it bolts into the trans or, the bolt goes all the way thru with double nuts on top, it will still be a 3/8" bolt holding the mount, at least in my mind should be close to, if not equal to it threading into the trans. If someone has prof that this will not work, I'd love to hear from them because, I don't want it to be subject to break! I really wanted to use a 7/16 bolt but, the nut would not fit between the strengthening web ( don't really know what else to call it ) without grinding away some of that material( wouldn't have to be much) which, seems to me that might not be a good idea but, it might be ok...what do ya'll think? Thanks

10-07-2014, 08:27 PM	#1
'65 chevy lover Registered User Join Date: Apr 2011 Location: Ft. Worth, Tx Posts: 536	What is Transmission mount bolt size? I'm having a difficult time finding the correct bolt size for the rear mount of my TH400. Not sure what year model it is. Bought it at Bishops International. It didn't have bolts in it. I've tried 3/8 and 7/16...neither worked. I'm wondering if they might be metric. From what I can tell, they appear to be fine thread. I don't want to screw up the threads trying to put a boult in there that's the wrong size/thread count. Anyone know what size and thread count? Thanks Last edited by '65 chevy lover; 10-07-2014 at 10:23 PM.

10-07-2014, 11:44 PM	#2
Captainfab 60-66 Nut Join Date: Mar 2007 Location: Coeur d'Alene, Idaho Posts: 23,252	Re: What is Transmission mount bolt size? It is possible they could be metric. If they are, they should be either M10 or M12. __________________ Power Steering Box Adapter Plates For Sale HERE Power Brake Booster Adapter Brackets For Sale '63-'66 HERE and '67-'72 HERE and '60-'62 HERE and "60-'62 with clutch HERE Rear Disc Brake Brackets For Sale. Impala SS calipers HERE Camaro Calipers HERE D52 Calipers HERE 6 Lug HERE Hydroboost Mounting Plates HERE

10-08-2014, 02:53 PM	#3
'65 chevy lover Registered User Join Date: Apr 2011 Location: Ft. Worth, Tx Posts: 536	Re: What is Transmission mount bolt size? Got some M10 bolts. One side , the bolt wouldn't thread in, the other side, the bolt slipped in there about half way without threading it in. Obviously, the threads are screwed up. Decided to just go with 3/8" bolts with lock nuts. Wanted to go with 7/16 but, a 7/16 nut wouldn't fit into the area where the hole is. Had to slightly reem the hole with 3/8 bit.

10-08-2014, 04:01 PM	#4
Kenfuzed Registered User Join Date: Jun 2014 Location: Glendora CA -> Tampa FL Posts: 587	Re: What is Transmission mount bolt size? I believe it may be the same as the TH350 mounting holes. For a TH350 1976 and earlier is 7/16-14 1977 and later is metric M10-1.5 A 3/8 will thread into the metric threads but it will be loose. I've never tried it but if at least some of the threads are still intact you could try some Loctite Form-A-Thread to give the 3/8 bolts a better grip. They claim that stuff will withstand 128 ft/lbs of torque. I'm sure others may suggest a stronger and more permanent thread repair but this would at least add a little security over your current fix. You don't want the tail of the trans to rip loose from the mount. __________________ 1966 C10 Short Fleetside 1988 Sportster, 2008 Street Glide Ken's 1966 C10 build "Red Ale" Follow on Instgram: @cv_performance https://instagram.com/cv_performance Harley Performance Parts: CV Performance (Harley related) Enjoying the ride!

10-08-2014, 04:43 PM	#5
'65 chevy lover Registered User Join Date: Apr 2011 Location: Ft. Worth, Tx Posts: 536	Re: What is Transmission mount bolt size? Thanks for the reply. It's already a done deal. I think it will be ok. Wether it bolts into the trans or, the bolt goes all the way thru with double nuts on top, it will still be a 3/8" bolt holding the mount, at least in my mind should be close to, if not equal to it threading into the trans. If someone has prof that this will not work, I'd love to hear from them because, I don't want it to be subject to break! I really wanted to use a 7/16 bolt but, the nut would not fit between the strengthening web ( don't really know what else to call it ) without grinding away some of that material( wouldn't have to be much) which, seems to me that might not be a good idea but, it might be ok...what do ya'll think? Thanks



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