1971 Porsche 911 flywheel information needed

[Jon B]

Somewhere down the line the bolt grading on factory bolts changed from 140 marking to branding letters. The aftermarket bolts grading went to 12.9. We use to torque the 140 graded bolts to 150 lbs on race engines and never had any issues. The 12.9 grade bolts heads would pop off at 120 lbs.

Original INBUS XZN 140 flywheel bolts were pre-ISO. The 140 rating indicated a tensile strength of 140 kgf/mm2 (1400 N/mm2). Bolts graded ISO 12.9 indicate a minimum 1200 N/mm2 tensile strength.

Bolts for pull-clutch flywheels were slightly shorter than push-clutch flywheel bolts. The shorter bolts were identified with a groove around the bolt head. The 6-hole disc under the bolts was 1.0mm thinner for pull-clutch flywheels as well.

Jon B.
Vista, CA

[304065]

I worry that the ARP bolts at 90lbs just aren't tight enough? Gordon

Gordon,

ARP's assembly procedure calls for a strip of Loctite 242 on the threads and their proprietary thread lubricant under the head of the bolt and a torque to 95 foot-pounds. Contrast this with the dry state of the Factory procedure.

Torque (you know this but it bears repeating for fun) is an estimate of the force required to achieve a specified pre-load or stretch in the bolt. It is influenced by thread design, surface finish and whether the fastener is lubricated or not. I think, but I do not know, that when developing these bolts ARP figured out how much they wanted them to stretch in the as-installed condition and backsolved the torque number to achieve that stretch. Expressed another way, the preload is probably equivalent to the factory's bolt. (I sure hope so, it's holding my flywheel on!)

[304065]

Original INBUS XZN 140 flywheel bolts were pre-ISO. The 140 rating indicated a tensile strength of 140 kgf/mm2 (1400 N/mm2). Bolts graded ISO 12.9 indicate a minimum 1200 N/mm2 tensile strength.

Bolts for pull-clutch flywheels were slightly shorter than push-clutch flywheel bolts. The shorter bolts were identified with a groove around the bolt head. The 6-hole disc under the bolts was 1.0mm thinner for pull-clutch flywheels as well.

Jon B.
Vista, CA

Jon,

Hmm now let me remember a 901 is a push, 911 is a pull, 915 is a pull.

It looks like ARP has made the bolts shorter than the Factory. Steve Weiner told me to expect to have to shorten these bolts by a tiny bit, but as you can see, they are much shorter than the (stretched beyond yield point, pink loctite chips and burn marks from MAPP torch used to melt the loctite for removeal) factory Kamax.

[Jon B]

Hmm now let me remember a 901 is a push, 911 is a pull, 915 is a pull...
It looks like ARP has made the bolts shorter than the Factory. Steve Weiner told me to expect to have to shorten these bolts by a tiny bit, but as you can see, they are much shorter than the (stretched beyond yield point, pink loctite chips and burn marks from MAPP torch used to melt the loctite for removeal) factory Kamax.

In terms of transmission clutch release, yes, 901 and 914 are push, 911 and 915 are pull.

According to the 911 Workshop Manual, Vol I, page SE54 (early editions might not have this) under Removing and Installing Flywheel - 2.2 Liter Engine: "Make sure that machine washer has thickness of 3mm. The retaining bolts must have an overall length of 35mm and head thickness of 8.0mm". Doing the math, this meant an under-head length of 27mm for pull-clutch bolts. All flywheels I have here measure roughly 8.5mm where the bolts pass, so with 3.0mm disc/washer the bolt engagement was 15.5mm. This is somewhat less than general rule for machine screw engagement, 1.5 x diameter, but Porsche felt it sufficient. If using early 4mm disc under bolts, you lose 1mm from this.

Probably best to check with ARP, find out what their recommended thread engagement is. Realize that less thread engagement = less torque resistance, and 90 lbs-ft might have different stretch/tensile results at different engagement dimensions.

I'm curious to know how your KAMAX bolt is marked, if pre-ISO. All original flywheel bolts I have here are INBUS 140, similar to Gordon's experience. INBUS was product name of Bauer & Schaurte (INnensechskantschraube Bauer Und Schaurte) as I think you know.

May we call you John here, or would you rather not use your name on this forum?

Jon B.
Vista, CA

[304065]

Jon,

I don't have any pre-ISO bolts, I imagine they were long ago discarded. The ones I have sourced lately are KAMAX 12.9 property-class and they measure up in uninstalled condition like this:

[Jon B]

It looks like ARP has made the bolts shorter than the Factory.

Do you have an ARP flywheel bolt nearby to measure its under-head length? Do you know the ARP bolt's tensile strength rating? The rough equivalent to ISO 12.9 would be 180,000 psi.

Jon B.

[Jon B]

Original INBUS XZN 140 flywheel bolts were pre-ISO. The 140 rating indicated a tensile strength of 140 kgf/mm2 (1400 N/mm2). Bolts graded ISO 12.9 indicate a minimum 1200 N/mm2 tensile strength.

Two original INBUS XZN 140 flywheel bolts, one of each version. Bolt pictured on left has groove around head, indicating it is shorter version for pull-clutch flywheels.



Jon B,
Vista, CA

[304065]

Do you have an ARP flywheel bolt nearby to measure its under-head length? Do you know the ARP bolt's tensile strength rating? The rough equivalent to ISO 12.9 would be 180,000 psi.

Jon B.

Jon, I looked in the catalog, 200,000 psi, 1" (or 25.4mm) underhead length. 25.4- 8.5 -4 = 12.9 or basically 1.075 times the diameter. I don't have the tensile strength of the crankshaft, however, and I think the equations require that also?

[304065]

Two original INBUS XZN 140 flywheel bolts, one of each version. Bolt pictured on left has groove around head, indicating it is shorter version for pull-clutch flywheels.



Jon B,
Vista, CA

Jon, where do you GET this stuff! Between that and your collection of original tools I am in awe. That screw on the right even has the yellow witness mark! Very cool.

[Jon B]

Many 2-liter engines end up with the later, somewhat shorter bolts, although the difference isn't much. Most people are unaware of the disc difference.

So at 200,000 psi the ARP bolt is obviously an excellent option. I'd question what their suggested engagement is for a 12mm bolt/screw, maybe they're fine with 12mm (1:1) engagement. I'd be more concerned about this with a large displacement, high performance engine than with your stock 2-liter. Gordon says he used to torque original bolts to 150 lbs-ft without problems, so probably wouldn't worry about the crank yielding first. Unless he's lying.

Jon B.