Triumphrat

IrlMike

First off, I've recently returned to the saddle after a few years off & just bought a 98 Tbird....

....and I want to say BIG thankyou to everyone who's been posting away on all the topics esp the tech ones....what can I say...a massive help !

And before I launch into my ruminations, I have to say what fantastic bikes these are - just fekin' love it !...as they say hearabouts.

Yes, not only have I just joined the Tbird club I'm another proud affiliate of the sheared-Alternator-shaft-bolt-club, lesser-post-VIN-566blah-not-clutch-side-section....

So....after reading all your posts....I offer de following 'umble torts (I'm from Ireland btw)....

Sheared bolt stuck in Alternator shaft has so far resisted removal by 'easy out' stud/screw remover tool....

so....I plan to take said Alternator to a machine shop & have the shaft drilled & tapped for a size up - M8, as suggested...

....tho' I have in mind to use an 8.8 strength bolt not the 10.9 of the original M6. My thinking here is that I suspect the bolt shears mainly due to the relatively large sideways (90deg to axis) movement allowed by the combination of big space between the end of the shaft threads & a bellvedere washer which also allows it a bit of movement.
This all happening in a shock load kind of way whilst the more 'brittle' 10.9 is torqued pretty close to it's max spec.

8.8 has about 20% less tensile strengh than 10.9 but the M8 has about 75% more area, using the following table of torque specs, in FOOT POUNDS....

Bolt size Grade 8.8 Grade 10.9 Grade 12.9
6mm 7 10 12
8mm 17 24 29

.....I'm planning to torque up the M8 to about 13 ft lbs & allow a bit of sideways 'give' where the bolt is not fully 'sprung' (stretched) lengthways

It follows from what I've said that the even more brittle 12.9 in M6 size would not be much improvement or even worse than than the original, as a possible upgrade....

Any advice on this idea gratefully recieved ! (& apologies if ye are all bored to death by this issue..!)

Best,

Mike

IrlMike

After reading my post again, I've realised I could clarify it a bit.

The key point in my post is that I think that the alternator (impeller drive) shaft bolt sheared when it was still tight & not 'came loose & then broke' after hitting the nut holding the other half of the impeller drive.

The main evidence for this is that the remaining piece of bolt has proved impossible for me (& others) to screw back out with an 'easy-out' stud/bolt remover. How so if it was loose before the head hit something ?

This suggests to me that the problem is maybe not keeping this bolt tight, but that the bolt has insufficient strength & maybe too much brittleness in the sideways direction. It also appears to be a recurring problem for those affected.

I would really appreciate the thoughts of the tech gurus here on the solution I've suggested, esp. to do with the correct torquing & use of belleville washers etc.

jimmyj900

I think you may be confusing 'toughness' with 'hardness' and coming up with 'brittleness.'

Usually when moving to a higher grade bolt you're moving to a higher alloy level that makes the bolt much tougher. For example, a SAE grade 3 bolt is a mild (low carbon) steel, a grade 5 might have a bit more carbon and a different temper while a grade 8 or 9 might be an alloy with molybdenum, manganese and tungsten and a very precise heat treating to prevent it from being brittle.

The usual specs for bolts are give in terms of 'tensile strength' -- the amount of tension which can be placed on the bolt before it either breaks or deforms (stretches) beyond limits.

I'd also suggest that unless you have access to a metal lathe you're going to have an impossible task in drilling and tapping the alternator shaft exactly on center. If you're off a fraction of a millimeter you're going to be placing some severe angular stresses on the alternator bearings and drive gearing.

I'd say your best bet would be to take the alternator to a good machine shop where they can remove the broken bolt with an EDM machine and precisely cut threads for the larger bolt you want to use.

They might also be able to tell you if the larger bolt is a good idea or not.

Jim

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IrlMike

Thanks for the info there regarding the drill & tap out to M8. I have found a good local engineer whose basically said the same as you have re the need for an accurate, careful job, so have left it with him. He was reccommended to me & it turns out he's into bikes ! Looking forward to chat there when I go to collect the alternator. He agreed the upgrade to M8 was a good idea, given the recurring nature of the problem & thought 8.8 strength at that size would be ok.

Yes, I am a bit unsure about bolt strengths. Especially in this case where I think the problem may be 'sideways' strength. Most bolts I figure are pretty much held from moving sideways by what they are bolted into, but this bolt has a good bit of fresh air around it near the head with nothing much to hold it. Not sure how different tensile strengths work here.

My source of info re bolts was boltdepot.com & they have a section 'About Fasteners' where they do use term 'brittle' to describe higher tensile bolts ?

Anyhow, I think I'll have a repair I can count on & can get back to riding - we had sunny weather last weekend & all I could do was tinker about with odd bits of maintenance / cleaning

Can't wait to try out the Thunderbike pipes coming in a couple of weeks too

Thanks again, Jim, & hope your winter isn't keeping you out of the saddle too much.

Mike