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sbc 350 in a 1973 nova with stripped crankshaft bolt

4K views 12 replies 5 participants last post by  BogiesAnnex1 
#1 ·
I have a 1973 chevy nova with a 350. The crankshaft bolt is stripped and just keeps turning is there any other way to put it on time. Or what are my options
 
#6 ·
Get it out of there, throw the bolt away and buy a new bolt and thick washer from your favorite vendor. Its a 7/16 X 20 bolt approx 3.0" long. Get a 7/16 X 20 bottoming tap of this same thread size and chase the threads. Reassemble. Hopefully it is not so buggered up in the crank end hole.


If this works, then use the bolt to turn the engine over gently with the spark plugs removed.

It can be tapped out to a 1/2 X 20 if nec.
 
#11 ·
huge hint... invest in 2 different 1/2-20 taps.. if you are not experienced invest in a starting tap so it will go in straight. then finish the threads with either an intermediate or finishing/bottoming tap.




when pulling dampers in.. you always use a stud threaded most of the way in and turn a grade 8 tall nut.. or select several bolts starting 1" and then 1.5" longer than the stock bolt so you can grab more threads. before you start pulling the converter in so it does not strip the threads..

i hope you have it all done by now.. and have a tap handle.. to do this with.
 
#12 ·
I agree with wayne 100%.

I had to do this on a 307 when I was 16. I was just going to run a 7/16 tap through the threads and be done. Well, lying on my back in the yard and in a hurry; I goofed it up pretty bad. Thankfully my grandad had a steady hand and a 1/2" tap
 
#13 ·
You can clean this out with a 29/64ths bit then retap to 1/2x20 and use the bolt for the BBC engine.

The reason for the thread failure is that the bolt was not up to torque allowing the damper to work about the shaft. This fails the weaker threads inside the crankshaft as it pulls and twists on the bolt. You should carefully dress the snout of any gouges and replace the damper with a new unit. It must be a press fit on the crank and installed with the proper tool which is not a hammer. This must be a press fit if you cannot get that the shaft is junk. You need to check the condition of the Woodruff key and its slot in the crankshaft. If the key is damaged replace it but also the slot needs to be good and hold the key tight it should take light hammer force from a copper or brass headed hammer to seat the key it should never just slip in. If the slot is pounded out you're sol; the crank will have to be removed to either make a shop repair to the snout (fill with weld and machine a new slot) or replace the crank if repair is not possible. Otherwise this will just happen again.

These tight fits are necessary to transfer the harmonic vibrations on the crankshaft into the damper if this does not happen they will reflect back into the crank. This usually ends up by showing itself as a wipe out of the number 1 main and rod bearings. The bolt is just there as insurance that the forces involved do not drive the damper off the shaft. For many years the SBC simply used the press on force to retain the damper, it was not successful especially as additional loads were imposed on the snout by power steering, air conditioning, and higher output generator and alternator power take offs hence the insurance bolt.

Bogie
 
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