914 - bus flywheel to crank bolts

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Piledriver
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Re: 914 - bus flywheel to crank bolts

Post by Piledriver » Wed Nov 11, 2020 1:54 am

Apparently this was beaten to death already by the usual suspects on TOS.
https://www.thesamba.com/vw/forum/viewt ... p?t=717876

Short: Per Ray Greenwood, flanged bolt, metric class 10000.
(above 10.9, just below 12.9)

Really needs to be a flanged bolt on std cast iron flywheel.
The OG ones have a dimpled underside to dig into the steel 5 hole underlay.
(acts as a bolt locking mechanism and protector for the flywheel)

The 914 versions are a little longer (3mm is mentioned)
I would shorten those, real Porsche bolts can be had for ~$6.50ish.(each)

I personally recall seeing 12.9 on the OG bolt heads on VW motors at least, more than once.

The 911 bolts are supposedly single use, at least the factory ones.

You can get similar OE flanged bolts in 12.9 for newer cars, sometimes a lot cheaper.
Addendum to Newtons first law:
The longer a vehicle rests on jackstands,the more likely it is to remain that way...

Ol'fogasaurus
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Re: 914 - bus flywheel to crank bolts

Post by Ol'fogasaurus » Wed Nov 11, 2020 12:00 pm

Pile is right on a lot of the single use bolts. A lot of the high torque bolts are limited in use as a "just-in-case" as you are stretching the bolt when you torque it. Add to that the hard jarring/shock loading of the flywheel line when "dumping the clutch" can weaken things and the fastener does not show the damage. Seen quite a few of bolts that when you expose them to being able to see them, the bolt head is missing and laying somewhere else.

Lee

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Piledriver
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Re: 914 - bus flywheel to crank bolts

Post by Piledriver » Wed Nov 11, 2020 1:36 pm

Also note that when you tighten a bolt, you are always stretching it to some extent, in the materials elastic range.
How much it stretches and how much clamping force it applies are down to grade, surface finish of the threads (both sides) and what lube you use.

ARP use a specific moly EP grease as a std lube, and sets their own torque specs accordingly, very likely based on stretch measurements..

Tighten to yield bolts simply limit the clamping force and probably provide better consistency for OEM by stretching the bolt into the materials plastic range, where it doesn't snap back to the original length when removed.

The flanged T4 crank to flywheel bolts are not tighten to yield bolts.
Addendum to Newtons first law:
The longer a vehicle rests on jackstands,the more likely it is to remain that way...

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