Improved head bolts

ludwig said:

Different pitch = different torque.
What value would you suggest for 20 TPI ?

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I could provide 12 point ARP head bolts for Molnar cylinders cut with 20 TPI threads similar to these:



Or I could get these high tensil BSA A10 bolts with rolled threads and reduce the shanks but they are nowhere near as pretty (recommended torque is 30 lbs in BSA A10 iron cylinders).

I’ll have a set of those ARP bolts for Molnar barrels Jim.

I would check with somebody on that front center bolt for Molnar 750 barrels. I'm not sure (although it probably is) that particular bolt is 20 tpi. I would check, but I have to remove more stuff than I want to remove to check today. I don't have a spare set of Molnar barrels to look at... yet.

baz said:

Ok I'm quite prepared to be knocked down and sit in the corner
But is there a reason you can't machine a set of standard head bolts to do this?

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I did this set of standard head bolts 8 or nine years ago. Worked fine in a landspeed race motor. Later started using waisted ARP bolts by Jim Comstock. I also like the ones done by Jim Schmidt, and might use them for the next engine build I do.



Ken

I do a lot of research on metal work being a fledgling machinist. I watch a lot of cool youtube stuff where guys heat up hardened parts to cherry red to soften them, then let the part/s cool, machine to spec, then reheat and quench to reharden the newly modified part/s... Is this the basic process you followed when you made your own Ken? Is there some secret to a stretch bolt beyond just waisting it? Is there hardening applied to a waisted bolt or some other property that allows it to stretch? (sorry for all the questions, OCD)

Looking at Ken's bolts they have not been annealed, machined and then rehardened as the original colour is on the unmachined sectioned and the machined sections are fresh cut. Tungsten carbide allows you to cut hardened steel as long as you use the right speeds and depth of cut plus a machine that can cope with the loads. Grinding is another option.

Schwany said:

I would check with somebody on that front center bolt for Molnar 750 barrels. I'm not sure (although it probably is) that particular bolt is 20 tpi. I would check, but I have to remove more stuff than I want to remove to check today. I don't have a spare set of Molnar barrels to look at... yet.

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On Molnar barrels:

All five head bolt threads are 3/8 BSF.

The two 5/16 front studs are 26tpi Cycle thread.

Fast Eddie said:

I’ll have a set of those ARP bolts for Molnar barrels Jim.

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I second that, although you need to be careful about the thread diameters. Andy's bolts have a too loose fit, obviously not made to spec (I found out by comparing with rolled thread BSF bolts), so I am reluctant to using them.

- Knut

This company makes custom bolts, offering rolled threads, BSF, and high strength steels. Worth an inquiry.

- Knut

Schwany said:

I used 30 ft lb on the Molnar head bolts, but I definitely had to retorque them with a composite gasket. Oops, just noticed you are asking Jim. Forgive the intrusion.

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30 ft/lbs is for std 26 TPI bolts. If you use the same value on 20 TPI bolts tension and clamping force will be considerably less.

Is there any reason not to take a set of original (used) head bolts and machine a "waist" on them?

This is intended to be a different question than Baz's, where he asked: "But is there a reason you can't machine a set of standard head bolts to do this?" I am assuming he was asking about "new" reproduction standard bolts.

Stephen Hill said:

Is there any reason not to take a set of original (used) head bolts and machine a "waist" on them?

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Used bolts could have been over torqued at some point and sent them into the plastic zone and permanently distorted them. Even if over torqued just for a few secs before being backed off then they are deformed, not worth the risk.

ludwig said:

30 ft/lbs is for std 26 TPI bolts. If you use the same value on 20 TPI bolts tension and clamping force will be considerably less.

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Right, but I had no other information, so 30 ft/lbs is what I used. 60's vintage Norton heads are made of cheese and I didn't want to distort the surface any more than usual.

Probably have to ask Andy Molnar what he recommends. I didn't ask Andy when I was putting my little 750 engine together using his barrels and bolts.

I wonder what the odds are of a stock head bolt experiencing any significant inelastic deformation, given that the bolt is pulling a soft aluminum head on to a copper or composite gasket.
And after a number of heat cycles most engines are probably under torqued.
Using new vs old head bolts in a rebuild is a bit like an oil thread, but what the heck.

After 48+ years (1974 850 Commando motor) I am still using my original head bolts with copper head gaskets, I only retorque at 500 miles and leave it alone after that and have never had a blown head gasket and only once I had to retorque the head after getting a slight seep and it stopped, my last head removal I forgot to anneal the copper gasket and had the head torqued down before I remembered so let it be, but I had sprayed copper gasket sealant on it that was well over 15 years now (only retorqued after first 500 miles) and the head hasn't seeped or been retorqued since with over 35k miles on it, I have never had a copper head gasket blow completely out but had blown a fiber head gasket completely out that came with my Norton from the factory.
In fact all my motor studs and nuts are still original except the front centre barrel stud and nut and all my timing case and outer gear box case had the screws replaced with allen head bolts back in 1978 and are still using the same bolts today.
Maybe if using alloy barrels then maybe I would use the waste head bolts, but my head has been shaved and had major port work done but still running the original valves and pushrods with a 2S cam grind and 40thu over size flat top Hepilite pistons and balanced crank, been built this way since 1982.

Ashley

The advantage of the "waisted bolts is that they can stretch and avoid pulling the 3/8" studs out of the head. They also need less re-torquing because they can stretch when going through the heat cycles. A serious performance motor may need them because the common grade stainless can break as shown below (which resulted in the loss of an expensive heavy duty case. Do it yourselfers see Ken's post #24 above. Otherwise the stock bolts are fine in a stock motor (except for pulling out the 3/8" head studs which I have experienced).

I am not a fan of SS bolts on motors, they might look pretty and as I have said after 48+ years the stock bolts, studs and nuts have done their jobs without any problems even with the work that has been done to my motor, have never had a stud pull out and the only one I had was the front centre stud on the crankcase but the 1/2 stud hole had broken away on the inside of the crank case and was rolling around the top of the crank when idling but was flowing when the revs were built up in between the bottom cylinders walls, was like that for 2 years before pulling the motor down and the broken piece was nice and polished by then, I still have that broken piece somewhere, that was back in 1979.

Ashley

o0norton0o said:

I do a lot of research on metal work being a fledgling machinist. I watch a lot of cool youtube stuff where guys heat up hardened parts to cherry red to soften them, then let the part/s cool, machine to spec, then reheat and quench to reharden the newly modified part/s... Is this the basic process you followed when you made your own Ken? Is there some secret to a stretch bolt beyond just waisting it? Is there hardening applied to a waisted bolt or some other property that allows it to stretch? (sorry for all the questions, OCD)

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As kommndo concluded, I turned them on the lathe with fresh carbide inserts (round inserts, to get the smooth finish and nice radii at the ends). The finish was very good, but I also polished them afterwards, just because it seemed like a good idea.

I only ever did one batch, just to see how well it worked out. I suppose I could have done the same thing on my other builds, only starting with ARP fasteners instead of stock, but with both Jim Schmidt and Jim Comstock offering them ready to go, I didn't see any reason to try doing my own.

Ken

Schwany said:

All my comments on this site are typed while sitting in the corner wearing a dunce cap. Saves me time not having to move back and forth and keeps my head warm.

If you did this stuff in a small shop using standard head bolts, my guess is they would stretch funny or break. When I torqued down the long skinny JSM barrel through bolts I thought they would snap, but they are processed to take the torque applied with the reduced shank.

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The JSM bolts are triangular because that cross section shape is stronger than just reducing a cylindrical bolt to a smaller diameter. The triangular shape still has the original diameter at the tips of the triangles and so it resists twisting more than a bolt which is simply a smaller diameter. The triangle shape also keeps everything centered in the round bolt holes and that allows for reduced shank studs.



Reduced shank studs (triangular)

jseng1 said:

The JSM bolts are triangular because that cross section shape is stronger than just reducing a cylindrical bolt to a smaller diameter. The triangular shape still has the original diameter at the tips of the triangles and so it resists twisting more than a bolt which is simply a smaller diameter. The triangle shape also keeps everything centered in the round bolt holes and that allows for reduced shank studs.

Reduced shank studs (triangular)

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Oops What I said came out wrong. I actually wasn't complaining at all. I was saying your reduced shank bolts worked in a bass akwards kind of way. If I were to build another Norton engine for whatever reason, I'd use your bolts everywhere I could.