Fly wheel bolts conventional thinking...why

[OldTee]

Conventional thinking is to always replace fly wheel bolts and not reuse them. Why? These particular bolts were replace about 20K miles ago with a new clutch and I see no reason not to reuse them. They came out clean and the threads are undamaged.

Any thoughts, other than conventional wisdom.

[hcoles]

there are some threads on this....if you can get the seach function to find them.. many mechanics seem to reuse them... someone may have reviewed the stress analysis to see if they are "yeilded" in this application and torque. Then there is the use Loctite conversation. I went by the book and used new bolts and Red Loctite. Would be interesting to find the origination of the "always use new" recommendation. Must have started somewhere.

[YTNUKLR]

Like rod bolts, they are a stretch type fastener. The first time you torque them, they stretch, and give you a mechanical preload, like a spring. The next time you torque them, that mechanical preload is less *at the same torque*. When subjected to high stress, there is potential for the bolt to go beyond its elastic limit and snap.

That said, I have seen brand-new ones fail...

[OldTee]

Ummmm. Considering the crap steel being shipped to this county now, maybe these have already proved themselves. New may be the bigger gamble.

You pays your money and makes your choices.

[88911coupe]

I suppose what would be good is to hear from someone who resused the bolts and then they failed...anyone who can claim this has happened?
I have new ones on order for a rebuild but how do I know the new ones will be better than the old ones which aren't all that "old".

[sww914]

I've reused many, many flywheel bolts but I'm sorry, I can't report a failure.

[Steve@Rennsport]

Originally Posted by sww914

I've reused many, many flywheel bolts but I'm sorry, I can't report a failure.

It all depends on how hard you spin the engine,...

[304065]

When the correct preload of the bolt is achieved it has been stretched beyond the yield point.

Further torquing will elongate the bolt and neck it down. At this point its capacity to hold the flywheel to the crank is diminished below that of a new bolt.

Try it with a new bolt ($$ experiment). Measure the bolt length with a micrometer before installation. Then install it and torque it. Then pull it out and measure it, it will have permanently stretched. Now throw it in the garbage.

OK, like everyone claims on Pelican they can reuse things like rod bolts and flywheel bolts and "get away with it." My perspective is, if the flywheel comes loose its going to do a number on the end of the crankshaft and bugger the transmission housing and that's just the opening act, to say nothing of the propensity of the engine to go from 7000 RPM to zero, which would tend to upset your line through a corner. Twelve bucks a bolt? Cheap insurance if you ask me.

[Jeff Alton]

The ones for the 9 bolt cranks are a whopping 2.00ea..............

John, who here claims they re use rod bolts?

[Brant]

I know of a race car
(not stories, but actual friend)
whom lost both the bell housings as well as engine cases from a flywheel mishap

I figure its worth 80 bucks for the peace of mind over my custom tranny and aluminum case motor.

ssw914, are you reusing them on a -4 or a -6
I've reused -4 bolts myself and this is a common practice on the -4 (they don't fail)
I'm not certain why, but sometimes the -6 can and do

perhaps rpm
perhaps the number of pulses per revolution combined with harmonics or ?

brant

[jaybird840]

All very nice, but how do I get the damn things out??? The engine yoke is too close to the flywheel to allow me to get the tool in there... Do I dismount it from the yoke and do it on the floor??? (EGAD!) Here's the issue....

[OldTee]

I would. This is the purpose of having football player sized friends.

[KFJ_SC]

Originally Posted by jaybird840

All very nice, but how do I get the damn things out??? The engine yoke is too close to the flywheel to allow me to get the tool in there... Do I dismount it from the yoke and do it on the floor??? (EGAD!) Here's the issue....

I used a dremel to cut down the length of the triple square bit so it would fit with a ratchet. Worked like a charm.

[911 tweaks]

on floor w/ air wrench,,, gonna throw out the bolts anyway, right guys!!!!!! I hope you all don't re use these... do as someone already said and measure a new bolt vs. one torqued up to full spec... BIG difference in length... I am no gambler with a 6-7k RPM spinning 15# weight in the most vulnerable part of our drive trains...right where the engine and trasn join together... what are you guys smokin'??? toss the used bolts, or better jet, use them with your slinhshots to hit the neighbors cat that always jumps up on your car and leaves sandy paw prints all over your nice now scratched clear coats... I dont get why some try to pinch where they do...
just my $0.02 here guys... not my car or money...
Bob

[KTL]

Same can be said for CV bolts and people continually reuse those. CV screws very easy to acquire too, considering they're not all that special- a basic 12.9 class socket head screw.

Jaybird,

As far as removal, have you tried getting at the flywheel screws through the end of the yoke? My yoke is hollow so that's how I get to the flywheel screws on the stand.

[sww914]

Originally Posted by Brant

I know of a race car
(not stories, but actual friend)
whom lost both the bell housings as well as engine cases from a flywheel mishap

I figure its worth 80 bucks for the peace of mind over my custom tranny and aluminum case motor.

ssw914, are you reusing them on a -4 or a -6
I've reused -4 bolts myself and this is a common practice on the -4 (they don't fail)
I'm not certain why, but sometimes the -6 can and do

perhaps rpm
perhaps the number of pulses per revolution combined with harmonics or ?

brant

Both, but only on street cars, easy rev's. I've replaced many flywheel seals on 6's during a clutch replacement and re-used the flywheel bolts. People say that they're stretch bolts like rod bolts but they're not necked down in the middle like rod bolts, they're normal bolts as far as I can see. I have taken to replacing CV bolts now and the hard to find Schnorr washers, I've seen too many come back with 1 or 2 finger tight. It's not funny when an axle comes apart either, sometimes you get a quick trip off the track at an extreme angle and at high speed.
If there is any question, I would defer to Steve Weiner's advice over mine. I spent 20 years in bodyshops while Porsches were a hobby, he spent those years working on Porsches. I might know more about banging out dents but he appears to know virtually everything about Porsche repairs. I'm just sharing my experience.

[jaybird840]

Porsche mechanic shop rate= $125/hr...... Factor of two for the dealership to make any money...... Wise advice from fellow pelicanites=priceless!!! Got the first three done in under five minutes. (Don't fret over the angle of the wrench.... I had to let it hang to do the photo )

Originally Posted by KTL

Jaybird,

As far as removal, have you tried getting at the flywheel screws through the end of the yoke? My yoke is hollow so that's how I get to the flywheel screws on the stand.

[KTL]

There you go.

[Porschekid962]

I would replace them with some ARP hardware with external wrenching heads, clean the holes, clean off the new bolts and install with red loctite for bolts of 1/4 to 3/8 diameter, I think the number is 242 or 243, I use so much of the stuff the numbers get confusing after a while but it is the best. Never ever reuse a flywheel bolt as people have mentioned above they are designed to stretch in order to hold the proper torque, if you reuse them and over torque they will have exceeded their elastic modulus and fail. If you reuse them and torque to the same value as with new hardware they will back themselves out or the heads will shear from the grip length and threaded portion of the fastener. Its not cheap insurance, its the proper way, reusing old stretch bolts is simply asking for problems I really dont care how many people have said they reuse their old stretch bolts or dont replace nylocs. This applies to the whole vehicle, as my father told me when I first started driving, I can replace the car but I cannot replace you.

[dtw]

Just as a semi-related aside - I measured two OEM 2.7 rod bolts the other night. One new, one torqued to spec and removed. The one that had been torqued was 0.09mm longer.