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Engine mount block bolt stripped out.

74 454

Active member
Joined
Apr 25, 2007
Messages
31
Location
Michigan
Corvette
1974 Brown Coupe 454 4-speed
I have a 74 454, just purchased. Owner says one of the driver side engine mount bolts is stripped out and missing. This is one of the 3 bolts to the block. Don't know which one yet.

Any ideas of how to fix this without pulling the engine and heelicoiling it! I eventually will pull the engine in a few years to rebuild, but not now.

Some have said;
if enough room, tap the next size metric bolt.
check with locktite, they have some thread fix.

Anyone know exactly what size bolt goes in the block? Length? Fine or coarse threads? Should be harden?
 
Joined
Jul 5, 2002
Messages
264
Location
Dayton, Ohio USA
Corvette
'74 383 4 spd coupe
Hey 454, I can't see where one of the three bolts holding the mount to the block would cause any problems. Are you planning to race this car? If not make sure the others bolts are ok and test drive it. Go somewhere you can sidestep the clutch and see how she reacts. A broken mount will allow the engine to raise up on sudden acceleration and may let the throttle stick.
peace Bud
 

74 454

Active member
Joined
Apr 25, 2007
Messages
31
Location
Michigan
Corvette
1974 Brown Coupe 454 4-speed
No, I don't plan on racing the car. But I do plan on pushing it to the max. every once in a while, say 5 to 10% of the time I drive it. It is on the drivers side which takes all the pulling torque. I don't know if that's how the bolt stripped out to begin with or if the previous owner cross the treads or over torque it when replacing the mount? He believes he had a broken mount at one time and replaced it. Lastely, why would the design have three bolt attachments if two does job? Yea, your probably guessing I'm an engineer now. I am.
 

bossvette

Gone but not forgotten
Joined
May 19, 2003
Messages
3,233
Location
West Unity Ohio
Corvette
1968 1997
Locktite does have a thread restoration kit that should work for a temp fix. Next best is to jack up the motor, remove mount and helicoil the hole, you should be able to get a 1/2 long helicoil in there. If the thread is completely stripped out you should be able to get a helicoil tap to run in as the tap drill size for helicoils is just over body dia. Cast iron is soft and taps easily.

BTW welcome to CAC
 

vette427

Active member
Joined
Apr 30, 2007
Messages
30
Location
NW New Jersey
Corvette
1969 silver big block (390) coupe
Locktite does have a thread restoration kit that should work for a temp fix. Next best is to jack up the motor, remove mount and helicoil the hole, you should be able to get a 1/2 long helicoil in there. If the thread is completely stripped out you should be able to get a helicoil tap to run in as the tap drill size for helicoils is just over body dia. Cast iron is soft and taps easily.

BTW welcome to CAC
I had the same problem: Lower bolt, right side (passenger) of engine. My buddies at NCRS say this is a common problem on the c3 big blocks. I researched thread repairs and thread inserts. The best and simplest solution is to tap the hole with for a 7/16x14 thread bolt that is 15/16 or one inch long. The hole in my block was so stripped, I didn't even have to drill it. You really need two taps (same size) The first one is a tapered tap to start the hole and tap it as far as it will go. Since this is a blind hole, you need to finish the threads to the bottom with a square end tap. These are used specifically for these blind holes. I didn't want to be messing with a drill in my block for those other fixes so I might have actually been lucky that the hole was in such a stripped condition.

Two things:
FIrst GO SLOWLY!! BE SURE TO USE "TOO MUCH" OIL..DON'T EVEN THINK ABOUT BREAKING A TAP IN THAT HOLE. They are hardend and do not drill out easily. If you've never run a tap in before, get someone who knows what to do to at least coach you. Hekicoil or thread inserts require the same process, by the way...

Second: When you think about all those "quick fixes" think about the stresses that stripped those threads in the first place. Heli_coil and other thread inserts and replacements are good products, but again...what stripped those threads in the first place?..Good luck...Joe
 

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