350 small block with m12 crank bolt?!

Bruce Wingate

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Put in a Vintage Air serpentine belt system this weekend and it went mostly OK but the supplied crank bolt does not fit. It will go in a turn or so by hand and then binds up. I can thread the old bolt in by hand so the threads aren't boogered up. (And it's too short to reuse) it looks like the old bolt is M12 but that doesn't make sense.

Am I missing something? Should I just go get a longer M12 bolt?
 

BillK

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Every small block Chevy engine I have ever seen (and I have seen plenty :) ) uses a 7/16 x 20 bolt. Unless someone messed it up and rethreaded it or simply used the wrong bolt. See if you can find a longer 7/16 fine thread bolt and try it. ARP makes a stud that you could use also.

Where are you at on LI ? Go see John at Ace Crankshaft . . . he should have a bolt or maybe even the ARP stud.

https://www.facebook.com/Ace-Competition-EnginesMachine-World-146234515426095/
 

Bruce Wingate

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Every small block Chevy engine I have ever seen (and I have seen plenty :) ) uses a 7/16 x 20 bolt. Unless someone messed it up and rethreaded it or simply used the wrong bolt. See if you can find a longer 7/16 fine thread bolt and try it. ARP makes a stud that you could use also.

Where are you at on LI ? Go see John at Ace Crankshaft . . . he should have a bolt or maybe even the ARP stud.

https://www.facebook.com/Ace-Competition-EnginesMachine-World-146234515426095/

I know 7/16 is the standard that's why I'm confused but the new bolt that I know is 7/16 fine pitch goes in by hand and then jams like it is cross threaded. The other bolt I can get in pretty far by hand. I can measure size but not pitch at home, and the old bolt is definitely bigger.

I'm not far from Deer Park - I'll have to swing by and see what he says.
 

BillK

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Do you have a 7/6 x 20 tap you can try running in there ? Are you sure the one you took out is Metric ? If it is somebody probably screwed up and put it in there and that might have messed the threads up.

If you do go see John tell him "double wide" sent you. He will get a kick out of that :) :)
 

Bruce Wingate

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I thought about chasing the threads, but the old bolt goes in nice and smooth. I would hate to rethread it and really mess things up. Unless the prior owner's solution was to go up in size to fix his mistakes (totally possible with this truck).
 

BillK

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What doe the head on the bolt look like ? Have you measured to to be sure it is metric ? What size wrench or socket fits it.

When you say it is M12 is that on the bolt head or did you actually measure the threads and it is 12mm ? 12mm is a fair amount larger than the 7/16 so maybe it has been re-threaded.
 

Bruce Wingate

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I measured the shaft of the bolt with a cheap plastic gauge. The old bolt rubs the sides of the 7/16 hole and slips easily into the M12 hole (dirty minds . . . .). Both take a 5/8" socket. I'm going to take the bolts to Home Depot to see what their bolt checkers say.

Chevy's drive me crazy - half metric, half SAE. I'm used to German stuff - all metric all the time. I think the Bug only has 3 major bolt sizes, maybe 4.
 

BillK

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If it takes a 5/8 socket then it is a 7/16 bolt. Chevy engine internals are all SAE. The only metric in those years is the bolt on stuff. I dont know if the HD bolt checker will have 7/16. Its kind of an oddball size except for automotive. The threads on the stock crank bolt I have here measure .431 od It is about 2.300 long under the head. 12mm is quite a bit bigger and will not fit into a 3/8 nut

I bet its 7/16 and just half buggered up.
 

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Maybe someone stripped the crank threads trying to suck up the balancer with the bolt, then tapped it for 12 mm.
 

Bruce Wingate

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I went out and measured them - both are 0.43" across the tops of the threads. Now I'm totally confused - how can one go in but the other bind up? It is hard to explain, but it feels like when you put a coarse threaded bolt into a fine thread.
 

BillK

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Have you tried the new bolt in a 7/16 fine thread nut ? Not something most guys have sitting around though. If you put the two bolts against each other do the threads look the same ? I think you might just have a worn out old bolt and a maybe a little bigger new bolt that needs to be finessed into the hole. If I was working on it I wouldnt think twice about getting a tap out and running it into the hole. Just use lots of tapping oil and work it back and forth slowly. I bet it will go in.
 

Bruce Wingate

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Looks like I'll be buying a tap . . . 7/16-20 right?

This is the newest old vehicle I've worked on and also in the best shape. I try not to do "destructive" things when working because half the time I find out that there was a better way to do something without breaking something to do it. Accidentally cutting threads the wrong size, or cross-cutting threads would snow ball a simple project into a much larger ordeal.
 

Bextreme04

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Looks like I'll be buying a tap . . . 7/16-20 right?

This is the newest old vehicle I've worked on and also in the best shape. I try not to do "destructive" things when working because half the time I find out that there was a better way to do something without breaking something to do it. Accidentally cutting threads the wrong size, or cross-cutting threads would snow ball a simple project into a much larger ordeal.

Post a pic of what the head of the bolt looks like. The one that threads in easily. Also post a pic of the threads in question. You should be able to lay the two bolt threads against each other, and if they are the same thread they will fit perfectly together.
 

BillK

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Bruce,
If you are close enough to John at Ace take the two bolts to him and tell him what you have going on and ask his opinion. I would do that first. You might have something really oddball going on but I dont think so.
 

Bruce Wingate

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Ace is maybe 20 minutes away - I will have to wait until Saturday morning

Thanks @BillK, @WFO and @Bextreme04 for all your help - I'll post any new developments.
 

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