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Through bolt head stud

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I’ve been toying with the idea of using a through bolt or stud setup for a Porsche 968 engine. It’s an all alloy inline 4 cylinder 104mm bore and 88mm crank 3.0lt. They have a fairly stout crank girdle from the factory but the big hp builds are suffering from oil pressure drop off under boost. Ultimately the block is cracking at the base of the main stud. My theory is the long stud will remove all tension loads from the alloy block preventing the block from letting go.

I’m looking for anyone with experience with a through stud setup. The issues I’m going to have to deal with once drilled through is that one side travels through the main oil gallery. I’ve read some people just use thread sealant but have also seen o-ring retainers machined in to the stud itself. I imagine with everything secured in place there isn’t anywhere for the oil to escape anyway but how far does one need to go to ensure the oil is only going where it’s meant to.

I’m also limited to the 6 central studs, the 2 at each end of the block either do not line up or pass through the water and oil pump. It’s a 10 stud setup. Are there any issues here with different torque values. As there is additional strain put on the through studs during the combustion stroke, which is ideal if all 10 are the same. Do I apply equal torque to all fasteners or slightly more to the outer studs to account for dynamic factors.


My thoughts - not gospel.

Extending the thread engagement is usually a good idea, but I'm not familiar with that engine, other than the general details.

Do you know if this is a common remedy, and would it be feasable to only partially extend the engagement so it's short of the oil gallery? If you run it right through, thread sealant may work perfectly well if it's applied to the stud(s) back from the gallery area. Overall, it may not be significant either way as any leakage past the block threads, the nut, and maybe the mating face is going to be very minor compared to the rest of the oil's "internal leakage" through the bearings, etc. and I assume within the crank-case?

I'd be at least as concerned with the loss of oil pressure, which suggests an oil-pickup problem and/or the oil being held up elsewhere, such as in the head, and so lowering the oil level in the sump - it is wet sump? Carefully relieving the drain holes in the head (usual problem) and maybe opening them up in diameter, may help, as may adding a crankcase breather as a common problem is the ring blowby pressurising the crankcase (doesn't need much) and 'blowing' up the drain holes, impeding the oil return. This can also compromise the drain from the turbo' bearing(s).

The crankshaft is a fairly big piece of metal, what sort of damper are you using - I'm thinking of the crank harmonics putting additional stress on the caps and their studs where they're mounted in the block?

I assume you will have studs made to suit the longer engagement in the block - assuming they are all made of the same steel, and heat treatments, etc. they should all use the same torque value - the stud' supplier should be able to give you that value, it's most unlikely to be the same as the OEM.

This engine was the foundation the "THOR" engine was developed, have you checked that out, there may be useful mention in their videos that could be of use?

You’ve mentioned many things that have crossed my mind already but unfortunately this engine doesn’t have a mass following so development has been limited. The engines I’ve seen crack have had a substantial amount of thread engagement. It makes me wonder if the studs may have come loose from the harmonics, as this is a real issue with this engine. Im running an ATI damper instead of the stock dual mass flywheel and also retaining the balance shafts. I know one of the engines that suffered the cracks had the balance shafts removed.

I have a supplier who is willing to make the studs to my spec. I was just hoping someone here had built a Cosworth through bolt engine as I know this type of stud has been developed for them.

I’m well aware of the Thor engine and have had some discussion with the owner of rp968 about the development of his 968 engine prior to the Thor engine. He mentioned the through stud idea had been discussed but they didn’t have time to develop it. A stronger billet girdle was also mentioned but I feel the development of this would be cost prohibitive for just myself. I would really like to replicate the factory 944-gtr sump which incorporates the girdle but again cost prohibitive.

At the end of the day even though these studs aren’t going to be cheap they look to be a cost effective solution that should solve my problem and also help the head seal as well.

You have, indeed, been getting into it indepth!

Forgot to ask before, was there any evidence of fretting - shiny spots - between the cap/girdle and where it was clamped to the block?

Trying to find more info' I did come across this - http://performancedevelopments.com/porsche-944-968-engine-block-lower-member-upgrade/ - Don't know if you know about them, but may be worth a call or e-mail as they also seem to be battling similar issues.

These folks seem to be concentrating on the top end, but if you don't know them, might also be worth a call? https://speedforceracing.net/product/3-0l-closed-deck-bored-and-stroked-engine/

Worst comes to the worst, depending on just how easily Elmer's billet block accepts OEM heads, cranks, oil pumps, etc, it may actually be a viable alternative for a bunch of you to get together and finance the design and machining of billet block assemblies, with "solid deck" - as in the tops of the sleeves, stepped or straight, are held by the top of the bore to give additional support - sleeves fitted. I said "viable" as any OEM block failure could come to a pretty penny on it's own, and avoiding one, or more, failures would be cheaper in the long run.

Starting with a reasonably clean sheet of paper would allow you, or rather the billet designer, to add material, fasteners, move oilways, water jackets, etc, as thought wise for durability - Elmer may already have done the work, but not sure just how many special parts are required to use their block.

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