Jason Moyle Racing

I managed to shear 6 out of 9 with a missed gear change on my old ITB'd 16v- scary biscuits

I have known other valvers to do the same- though only those with 2L bottom ends & revving to 7800 instead of the KR 7200 limit

Maybe an issue of the slightly increased torque combined with the weak bolts & poor stock locating dowel.

I used to own the OP's 16v & that flywheel has been lightened & modded with 2 steel dowels & the bolts were correctly torqued/loctited on assembly- however at a recent Curborough sprint day we attended I sensed all was not well

It also has a 2L block so we shall have to do some investigations!

An access hole in the bellhousing is what my 2 16v trackday friends have gone for

I recently had the opportunity to tour the ARP manufacturing facilities in California and following some discussion with Jason I am in the process of drawing the standard OEM fasteners and sending them to ARP for them to evaluate some uprated items 

Excellent info and progress guys 

^ This 

I've popped flywheel bolts even on a mostly standard 1.8 KR, with a lightened flywheel only, on track - so I'm not surprised they go on something a bit revvier. Only found out when I heard a rattle from the gbox coming off track. Thought the box was eating itself, but no - flywheel bolts rattling around. I do drive like a prick though, in a car not prepared for it. Give me a dose of that Loctite, eh Dave?

I've never had a problem with them, but the latest engine upgrade might change that…

Just a quick update......Jason and I met with ARPs Director of Speciality Products at Autosport earlier in the week and handed over some sample OE bolts for them to analyse and come up with a proposal for uprated fasteners.

Will keep you updated as soon as we hear anything 

I hate to put a damper on things folks, but higher grade bolts may only improve matters a little in as much as might only delay the onset of fracture. I say may, because higher strength materials can sometimes be more prone to fatigue failures than softer materials. The failure mode of this joint is without doubt due to fatigue.

The mechanism which causes fracture of these bolts is very complex, but, is in the main, due to flexibility in the standard system. The fatigue is due to flexture at the joint face which allows small rotations of the flywheel on the cover plate. Because the bolt is not a tight fit in the flywheel, the head of the bolt will be pushed backwards and forwards which will generate bending fatigue where the bolt threads into the cover plate. If the bolt was of a design where it was a perfect fit in the flywheel, such as a precision shoulder bolt in a reamered hole, the the loads would be transferred as shear and not bending.

Firstly, the torque permissible in the joint is governed entirely by the material of the cover plate.  Compared with the standard bolts it is of a lower grade so limits the torque which can be applied.  At the same time the material of the standard cover plate is only 5mm thick, and even with the slight upsetting of the plate where the thread is, only gives approx 6.5mm of thread length. Usually one looks for about 1.5 x thread diameter to prevent strip out at normal torque. In this instance, the thred length is less than diameter so is at the limit for the torque specified in VAG documents. If one attempts to exceed this torque to a figure applicable to the high grade bolts used then strip out will occur.  The competition cover plate is 5.5mm thick and is also better made at the threaded area giving 8mm of thread length.

Another factor in the complexity of the system is the stiffness of the cover plate arrangement.  The standard unit is 5mm thick and fastened to the crankshaft with a thin plate under the heads of the bolts.  The competition cover plate is 5.5mm thick and fixed to the crankshaft using a much thicker plate under the heads of the bolts. Now, as the cover plate hangs free off the end of the crankshaft it will bend down slightly due to gravity and when rotating will tend to whirl.  The competition arrangement is some 34% stiffer, due to stiffness being proportional to the cube of the thickness. Also due to the stiffer bolting arrangement at the crank has, in effect, a shorter lever-arm carrying the loads and has a much higher whirling speed. Whirling will cause the system to see unwanted loads in a radial direction in the bolted joint.

To sum up.
1: POWER - The standard set-up comprises a badly design joint with a massive void in the flywheel casting where the bolt passes through.  It is limited in the presuure applied at the joint by the material of, and thread length in, the cover plate. These factors lead to the joints which flex and are unable to carry high rotational loads from a very powerful motor.
2: REVS - The standard set-up is less stiff than the competition arrangement and is more prone to whirling. This leads to the systems imposing higher radial loads from a higher revving motor.