258 Block thread size at motor mount

[DaveMueller]

I'm picking up a motor this weekend and am making a brace that will attach to the holes for the motor mount. It just occurred to me that the new engine (early 80's - it has 7/16 head bolts) may not have the same thread size as mine (3/8 course). The early 80's were a transition time for many auto manufacturers from SAE/English hardware to Metric.

Anyone know for sure what the thread size is in the block where the motor mounts go?

Thanks
Dave

 

[caveman]

I can't imagine that it has 1/4" bolts to hold the motor mounts. Sometimes the 3/8 aren't enough. I had to drill and tap mine out to 5/16" because I was breaking the 3/8 bolts.

 

[Posi]

3/8-16, use grade 5 or 8

 

[Lumpy Grits]

I can't imagine that it has 1/4" bolts to hold the motor mounts. Sometimes the 3/8 aren't enough. I had to drill and tap mine out to 5/16" because I was breaking the 3/8 bolts.

:laugh:

 

[Posi]

I can't imagine that it has 1/4" bolts to hold the motor mounts. Sometimes the 3/8 aren't enough. I had to drill and tap mine out to 5/16" because I was breaking the 3/8 bolts.

I think you mean drilled out to 7/16"

 

[DaveMueller]

3/8-16, use grade 5 or 8

Thanks for that. I don't want any surprises on top of 15 hours in the car!

 

[Lumpy Grits]

I'll add, use split-ring lock washers also.
LG

 

[Posi]

Thanks for that. I don't want any surprises on top of 15 hours in the car!

I would also suggest asking the seller to run a 3/8” 16 bolt in just to be positive before heading off. It’s already been mentioned here that folks enlarge the holes.

 

[DaveMueller]

I plan on taking a handful of SAE bolts/nuts, just wanted to make sure the chances of it being Metric were slim to none.

 

[caveman]

I think you mean drilled out to 7/16"

I think you're right.

 

[DaveMueller]

Thanks for the help. My contraption worked well, and bolted right up to the block. It was an epic 22 hour trip to Connecticut and back in a 20 year old GMC 1500 pickup with a quarter of a million miles on it.

The engine has about 10K miles on it after a rebuild and has lots of blue paint.

My co-pilot (and owner of the getaway vehicle) said he'd do it again, but not this year ;-)

Dave