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michaeljf

Attaching a flywheel to crankshaft on TR6

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Hello, I am in the process of rebuilding/upgrading? the motor in my TR6. Upon inspecting the attaching threads in crank (before sending it off to be treated )found that the first 1/2" of the thread in two of the four "locators"is damaged.So longer bolts,8 bolt attachment or ? Suggestions.Regards,Michael.

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Damaged as in the thread is a bit tight? Or damaged as not enough left to hold a bolt?

Longer bolts is one option provided there is enough threaded depth left to give at least 1.5 x D and you can source suitable bolts.

More holes is better, especially if you are intending to use it hard. For full competition use, get another crank.

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Hi Michael,

How many grinds has the crank had ? As per Nick, you may be better getting a better crank.

For the record, X4 ARP bolts with locktite applied, is more than sufficient on a competition engine.

My engine has done 8 seasons and big revs without any issue.

 

Cheers.

 

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Hello all, Thank you all for replies. Starting with Nick. A good engine builder cleaned the thread with a tap and passed them for use....but my son (a motorcycle mechanic/own shop of 27 yrs. + builder of quite a few rally cars and engines) said not good enough. The threads are 1.0" deep,the flywheel is 0.4" thick (in this area) and the bolts used were 1" (under head to end) so were only using half the thread. Have an enquiry with ARP at the moment regarding longer or an 8 bolt (with 5/8 hex head to fit socket with 8 bolts).          Trtom2498efi. One. The car is 1968 built TR6 US spec. Had Kastner S4 cam,chrome moly pushrods/alloy caps,extractors,(6into1) 2 1/2" exhaust,balanced, head to P. I.+ specs. numbered roll bar,alloy wheels,etc.when I purchased it in San Francisco in 1994.  I have had the lot rebuilt.2.7 L (modified Mazda pistons), modified P.I. head (original was narrow port) the list is long but I'm sure you get the idea.                                                                                                                                                                                                                       John D,   Will try and show some pictures.                                                                                                                                                                                                                                                 2.5 Piman, Don't know that helicoil would be strong enough on a 2.5 crank. Regards,Michael.        

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Helicoils not strong enough, but these solid ones with the anti rotation tangs are, thats one being fitted to a lower wish bone mount.

 

QRCjoFcI.jpeg

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Hello RedRooster, I don't think there is enough metal to fit helicoils.But as I said to John I will post a picture of A crank and the old bolts.Regards,Michael.

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That solid type you drill over size quite a bit so it doesn't matter if your existing thread s u/s, normal helicoils won't work in that application as they don't torque up enough.

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I think the worry is the thickness of material left between the edge of the holes and OD of the crank after drilling out for inserts?

If the thread is basically there but a bit chewed then longer bolts will probably suffice, though if sustained high rpm/ competition use is planned then 8 bolts (or 7 +dowel) is better. You’ll likely need internal hex heads for space.

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I had a lightened balanced flywheel come loose on my race 1500 Spitfire during a rolling road session in around 1990. I was supposed to be getting the twin 40's re-jetted. The engine was well over revved immediatly followed by a loud knocking. The threads where undamaged, so fitted new bolts.  The engine builder recomended fitting a couple of additional dowels, which he did, and I remembering him saying what a job he had because the crank was so hard, it was previously tuftrided. 

Edited by Mark

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Hello all,Pictures tomorrow I hope. As you will see there is not much room for oversize anything,but we will check out all the options. Yes Nick you are right about the thickness. This crank had had some treatment (balance etc.) before I got the car and I had it crack tested,balanced again new bearings 10 thou.and was in the process of sending it off to be  tuftrided? polished and checked for straightness when we noticed the threads.I also have another spare crank but it is an unknown quantity but will do some checks on it as another option. I also had a suggestion of using 12.9 tensile allen bolts of 3" (for the shoulder) cut to size of hole with good loctite (flywheel and hole are 1.4") Regards,Michael.

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Hello, Attached are a couple of photo's.The one of the bolts isn't very clear but you can see the worst one and the one next to it has a similar length of the thread "flattened" on the leading edge.Regards,Michael. (the crank has corresponding damage).

Photo0514.jpg

Photo0516.jpg

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To me, that has to be due to a fly wheel loose on the crank.     Only a bolt that has extracted itself half out of its bore could be damaged that way, and the others must have loosened as well so that the flywheel moved on the crank, and the worst bolt chattered in the threads.   That bolt is scrap, the others?   They must have been highly stressed, so junk them too.

How to retrieve the crank?    More, undamaged threads are needed, as the existing ones will have also been stressed.      Redrill and thread for wider bolts?    Drill deeper and thread for same size?    Finally, thread locker on reassembly.

The question of why the flywheel and bolts came loose is more speculative.    Have you inspected the damper pulley on the front of the crank?    One that has failed will subject the whole crank to much more torsional vibration, just the component that will shake bolts loose.   Mere visual inspection is not diagnostic, although if the TDC mark does not correspond with true TDC, that shows movement of the outer ring for certain.    Contact me if you would like damper tested!

John

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Hello John,  The bolts are 1" long from the "base" of the head,the flywheel is 0.4' thick in that area so that is as far as they went into the crank. They were ALL extremely  tight ( to specs I think) AND the worst one was the hardest to get out. Looking at it and the corresponding thread in crank it looks like they were "fused" together? Have an email with ARP to make some new bolts to go ALL the way into the threads which are 1" deep.Have a new std.damper but may have to use an ATI one.$$$+$. Have been in touch with Jon Wood but lost the link. Have been in touch with you too John about dampers ? REGARDS,Michael. 

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From your description it’s possible that the damage actually occurred when the bolts were removed due to galling (basically friction welding). Galling is more normally Associated with stainless steels, but possibly it could happen having been assembled dry, maybe slightly over-torqued and then subjected to many heat cycles.

Doesn't change my views on the recovery options.

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Ok, Michael, they were tight, and NIck's explanation is most likely.    If they are the standard 7/16" bolts, and the damage occurred throught the engaged length, then they barely get the minimum one-and-a-half times the width of engagement, and longer bolts most desirable.

I don't recall you asking me anything about dampers, but please do!

John

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Hello,The damper appeared ok . I have this other crank (the one in the photo) seems to be std. sizes, "cross" drilled? but is short backed ,the numbers on it are, 311313, 17740K1JGSB, W2152   HNP . The original one is long backed. I used the flywheel of the original one on the P.I. motor (with some machining to fit the P.I. crank (short back) and have now had a spacer to fit it back on the original crank, long back). Your heads spinning yet? Two motors,Three cranks.Regards,Michael. 

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5 hours ago, michaeljf said:

John did you do a survey about dampers a while back? Regards,Michael.

Yes, and I reported on it last year: 

I've also reported on the research that the survey led to, in articles in the Courier (October/November 2019) and in print at TRaction.

There, I describe the often overlooked purpose and function of the crank damper, and the service I'm offering to test dampers, that is available nowhere else.

John

 

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Hello John,  Glad to hear about the outcome of your survey. I didn't ask you about dampers. I live in Australia and had a serious accident on one of my motor cycles in 2018. I made a comment about an article in Triumph World about Jon Wood and his crank damper. Regards,Michael. (will have to see if I can find a copy of your report) 

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Hello, Back to the task at hand. Looks I will be going with 7 bolts + the dowel. 5/8" hex head x 1.375" long. Will keep you all informed. Have a nice cool Christmas,we are having a RED HOT one.Approx.2 million hectares burnt so far. Regards,Michael. 

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Sounds like the best engineering solution has been chosen.

Have a good one - hope the bushfires stay away!

Just been looking at the alert maps and ...... wow! :ohmy:

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