MGB Stud Threads ?

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VWNate1
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MGB Stud Threads ?

Post by VWNate1 »

I am replacing the early soft head studs in my engine block and want to run a tap in the block's deck holes before I do , what's the correct thread size and pitch please ? .

TIA ,
-Nate
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Re: MGB Stud Threads ?

Post by Steve Simmons »

UNF 3/8-24
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Re: MGB Stud Threads ?

Post by Larry Kluss »

Steve, Are you sure about that? I thought they were coarse 16 pitch threads in the block. I know they are fine 24 pitch on top.
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Re: MGB Stud Threads ?

Post by Steve Simmons »

Whoops, I think you're right. I know the nuts are fine thread and that's what I was thinking of. I'll see if I can find a stud around here somewhere. In any case they are standard, not metric, so it should be very easy to figure out which they are by looking at the thread count.
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Re: MGB Stud Threads

Post by VWNate1 »

Thank you .

I am asking about the 5/15" 16 NC studs that hold the head to the block .

The thinner 3/8" rocker tower studs appear to be O.K. ~ the big ones are stretched and one is stripped so I want to change 'em all and do a good job of it .

I have an American made tap but it really was stiff going into the block's hold in spite of me cleaning the hole well and lubricating the flutes of the tap so I was concerned it maybe had some odd ball thread pitch ? .

No ?

So I should just gently work it into the block's holes then ? .

The stud came out easily so I wonder if maybe I don't need to chase these threads , I still need to find a fill set of 11 dimpled studs , I know where a '77 MGB is sitting way out in the Desert and can go get those but I'd rather grab some off a bare block not take a possibly good (stuck) core 1800 engine apart as once I taker the head off they'll bin it .

What's the current opinion of the Fel-Pro (Payen) black composite head gasket Vs. the copper one I got from Moss Motors ? .
-Nate
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Re: MGB Stud Threads ?

Post by Steve Simmons »

If the studs go in and out easily then I wouldn't worry about it. My feeling is that every time you run a tap through, you're wearing out the threads a little more. If the threads are dirty then I like to run a cleaning brush in there with some solvent and then vacuum it out. But sometimes a chasing tap is inevitable...

I've got a block here but the threads don't look very fresh. I wouldn't use them myself, considering how important they are.
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Re: MGB Stud Threads ?

Post by VWNate1 »

Thanks Steve .

I usually just wire brush then oil the upper threads and run the nut up and down to check for stretching , most of these studs do not pass the test .

Do the improved studs have dimples or will they be stamped " 22 " ? .
-Nate
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Re: MGB Stud Threads ?

Post by VWNate1 »

Progress ! .

I got a taper tap and a bottle brush , some Q-Tips and brake cleaner , taped over the cylinder holes and then cleaned and cleaned and cleaned , there was an amazing amount of crud in the bottom of the hole .

I scraped the accumulated carbon and crud off the deck too .

I'm going to use ARP studs , they come with very narrow hardened flat washers , I think I'll use the wider original hardened flat washers and nuts .

Bending over the block kinda hurts my back so it'll be a while before I get back to it but I'm keen to get this done so I can be ready for Summer Driving :drive .

This is a fairly low compression engine so I was thinking of using 45 Lb. Ft. torque ? .

TIA ,
-Nate
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Re: MGB Stud Threads ?

Post by Larry Kluss »

Nate, I recommend following ARP's installation instructions and using their special lube. Also use their washers and nuts. If you don't, you risk not have the correct tension induced in the studs. This is because ARP threads are precision class 3 and their lube reduces the friction or K factor. The lower the friction factor, the more tension or stretch in the stud at a given torque value. They have calculated the torque based on stud stretch tests, but those values are no good if you change anything involving the joint (i.e. stud/nut/washer, materials, coatings, or the lube). Do not over-torque it either. And be sure to lightly coat both surfaces of the washers. This affects the K-factor too.

Also, use Permatex thread sealer (not silicone or loctite) on the threads into the block and hand tighten (literally). Do not torque or wrench tighten the studs into the block. The stud will not “torque” into the block when the nuts are tightened. The stud will simply pull on the threads and stretch. The reason for the sealer is in case there is porosity in the blind holes in the block. That’s the coolant leak issue I had in one stud hole in my B, before using the sealer. It’s cheap and easy insurance.

For anyone interested, the torque calculation is T=KDP, where T is torque (in-lbs), K-factor (friction coefficient), D nominal bolt/stud diameter (in), and P load/tension (lbs) induced in the bolt or stud. A dry black oxide bolt into a bare steel threaded block has a K-factor of around .23. I believe the K-factor for the ARP lube is probably half of that. All else being equal, that can make a huge difference in the torque need to induce the proper stretch or load into the stud.

Examples: This is only for illustration and not a recommendation for you application.
Using a 3/8” stud and a 5000 pound load into. T=KDP

Using .23 K-factor, .23 * .375 * 5000 = 431 in-lbs (or 36 ft-lbs) torque
Using a lower .12 K-factor, .12 * .375 * 5000 = 225 in-lbs (or 19 ft-lbs) torque

You can see by the above examples that the friction can have a huge impact on the stud load or stretch.

ARP’s has developed their instructions specifically for their product, and I recommend following them.
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Re: MGB Stud Threads ?

Post by spitfire »

And I was just going to say "don't over torque it wouldn't make any real difference in compression and possibly damage the bolts prematuey". But you said it so much better.
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Re: MGB Stud Threads ?

Post by VWNate1 »

THANK YOU very much ! .

I did use their torque spec. of 45 # .

I also used their special washers and lubricant .

I got the head on and the water pump in to - day , much thread chasing was done first .

I hope to get back on it this week , I want to drive it to Golden Cove when I get back .
-Nate
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