ausmini :: View topic - cracked crank damper

I think i'll be using a lot of loctite and making sure it is all torqued correctly.

Michael,
Screw the bolt in without the damper on, the head should then touch the end of the crank. Unusual for it not to, the bolt is lots shorter than the thread depth.

checked OK

The only time I ever cracked one (back in the 80s) I was using the lockwasher, and the bolt came loose.
These days I leave the crappy soft washer out, put Loctite 262 on the threads, key, and the crank nose. Torque up to 75-80 lb/ft. did that when i assembled it
I've not had one come loose since.
GR does the same... :wink:

Did it actually come loose? it could rotate on the crank with the bolt still done up. But that could be because of the crack after the fact.
If not- it might have had that crack already starting before you fitted it.
Another possible cause of it coming loose is the oil slinger, they are soft and can collapse with a big rev, like the lockwashers do.
Some race engine builders ( :wink:

) leave them out on race engines for this reason. left it out for this reason

How much endfloat did you give the stroker crank? .003" is NOT enough, I have the overheated centre main shells and thrust washers to prove it (yes and I saved the crank too...)
3 bearing long stroke Mini cranks flex lots at high rpm. They grab the thrusts if not enough clearance.
.006" is a realistic minimum for a 1275 crank if you are gonna rev it.
My 1360 and 1412 both have .007". GR gives race motors .008".

David R has a terrifying knowledge

Author:	mickmini [ Mon Jan 18, 2010 8:12 am ]
Post subject:	cracked crank damper
Not happy Jan! After having it rebuilt just under 3 years ago to finding it loose on the crank as I am stripping the engine for another problem. Not sure what I got wrong, but I am pretty certain it would have been the way I fitted it rather than a problem with the damper itself. It has cracked from the corner of the keyway, so I suspect there may have been a bit of play there. Any advice for fitting the new one when i get it, like what kind of clearance between the key and the key way in both the crank and the damper? cheers michael

Author:	66S [ Mon Jan 18, 2010 8:40 am ]
Post subject:
Hi Michael, There are two possible causes that I can think of. The most likely is that the pulley hasn't been properly tightened and it was loose on the key. The seond possible cause would be that the woodruff key was too high and split the pulley when it was installed. Regards Al

Author:	zeus11 [ Mon Jan 18, 2010 9:38 am ]
Post subject:
I just pulled my engine apart after having the flywheel nut come loose. I also have a cracked damper like yours, the key way in the damper was also flogged out. The crank pulley nut was hand tightened at best so i'm putting it down to who ever put the engine together didnt fully torque the flywheel or crank damper nut leaving me with a frigged crank, flywheel and damper $$$$$. Cooper s crank too! I feel your pain mate. I think i'll be using a lot of loctite and making sure it is all torqued correctly.

Author:	mickmini [ Mon Jan 18, 2010 5:25 pm ]
Post subject:
zeus11 wrote: I think i'll be using a lot of loctite and making sure it is all torqued correctly. I did that I measured .001" clearance between the key and the crank keyway. At the uncracked end of the keyway on the damper it is more like .006". Just wondering if this could cause a problem, but probably not if it was all tightened properly. Also wondering if the bolt might have just bottomed in the thread before it fully tightened up the damper. How do i tell if this is the case? cheers michael

Author:	drmini in aust [ Mon Jan 18, 2010 5:51 pm ]
Post subject:
Michael, Screw the bolt in without the damper on, the head should then touch the end of the crank. Unusual for it not to, the bolt is lots shorter than the thread depth. The only time I ever cracked one (back in the 80s) I was using the lockwasher, and the bolt came loose. These days I leave the crappy soft washer out, put Loctite 262 on the threads, key, and the crank nose. Torque up to 75-80 lb/ft. I've not had one come loose since. GR does the same...

ausmini https://www.ausmini.com/forums/

cracked crank damper https://www.ausmini.com/forums/viewtopic.php?f=2&t=60392	Page 1 of 2

Author:	mickmini [ Tue Jan 19, 2010 8:07 am ]
Post subject:
drmini in aust wrote: Michael, Screw the bolt in without the damper on, the head should then touch the end of the crank. Unusual for it not to, the bolt is lots shorter than the thread depth. checked OK The only time I ever cracked one (back in the 80s) I was using the lockwasher, and the bolt came loose. These days I leave the crappy soft washer out, put Loctite 262 on the threads, key, and the crank nose. Torque up to 75-80 lb/ft. did that when i assembled it I've not had one come loose since. GR does the same... hmmm

Author:	drmini in aust [ Tue Jan 19, 2010 6:33 pm ]
Post subject:
Did it actually come loose? If not- it might have had that crack already starting before you fitted it. Another possible cause of it coming loose is the oil slinger, they are soft and can collapse with a big rev, like the lockwashers do. Some race engine builders ( ) leave them out on race engines for this reason.

Author:	mickmini [ Wed Jan 20, 2010 6:08 am ]
Post subject:
drmini in aust wrote: Did it actually come loose? it could rotate on the crank with the bolt still done up. But that could be because of the crack after the fact. If not- it might have had that crack already starting before you fitted it. Another possible cause of it coming loose is the oil slinger, they are soft and can collapse with a big rev, like the lockwashers do. Some race engine builders ( ) leave them out on race engines for this reason. left it out for this reason I'm going to go with the already cracked or a result of the other problem. The other problem was a disintegrated crank thrust washer, for reasons unknown. Crank might be salvagable, sent it to the machinist to check today..... cheers michael

Author:	drmini in aust [ Wed Jan 20, 2010 3:30 pm ]
Post subject:
How much endfloat did you give the stroker crank? .003" is NOT enough, I have the overheated centre main shells and thrust washers to prove it (yes and I saved the crank too...) 3 bearing long stroke Mini cranks flex lots at high rpm. They grab the thrusts if not enough clearance. .006" is a realistic minimum for a 1275 crank if you are gonna rev it. My 1360 and 1412 both have .007". GR gives race motors .008".

Author:	GT mowog [ Wed Jan 20, 2010 4:28 pm ]
Post subject:
I have never seen one cracked like that, however I would say that the loose bolt had plenty to do with it. Many engines that I have pulled down I have found 80%+ the HB Bolt only hand tight and often (but no so much) the flywheel bolt loose or at least not as tight as it should be. One posibility for the cause of this is the harmonics produced by the engine. If you think about it, it works at these bolts like a rattle gun, but in reverse and at a much higher frequency. Yes, IMO the locktabs too will cause more trouble than they are supposed to cure. Locktite would be the way to go. Mini Spares also do a slightly longer bolt for more engaugement.

Author:	drmini in aust [ Wed Jan 20, 2010 6:03 pm ]
Post subject:
It could be that the bottom threads in the crank are full of rubbish (old Loctite, wasp nests, etc). The thread is 5/8 x 16 tpi UNS, yes it's an oddball pitch (not UNC or UNF). I have an old 850 flywheel bolt I modded for this job- I ground 4 `flutes' along it with the bench grinder so it works (sort-of) as a tap. Works fine for getting the crud out, but being soft it won't fix damaged threads.

Author:	mickmini [ Wed Jan 20, 2010 6:08 pm ]
Post subject:
drmini in aust wrote: How much endfloat did you give the stroker crank? .003" is NOT enough, I have the overheated centre main shells and thrust washers to prove it (yes and I saved the crank too...) 3 bearing long stroke Mini cranks flex lots at high rpm. They grab the thrusts if not enough clearance. .006" is a realistic minimum for a 1275 crank if you are gonna rev it. My 1360 and 1412 both have .007". GR gives race motors .008". gave it plenty of end float under the eyes of GR himself. He even surface ground the STD thrusts to ensure enough end float. Can't remember ever actually measuring it - but I will give him a call later to discuss. And GTmowog - like i said before, damper bolt was torqued correctly (using a Warren & Brown deflection beam wrench), loctited, no oil slinger or lock washer to collapse. cheers michael

Author:	Phat Kat [ Wed Jan 20, 2010 6:30 pm ]
Post subject:
I don't think anyone was accusing you of shotty workmanship mickmini I think a few people are just trying to work out why it happens because yours is not an isolated case. It doesn't just happen on A series motors. A close friend of mine works on V8's. He has a client who's car he maintains. He and his brother race it. And they went thru a stage where there was one corner on this particular track where coming out of it the Harmonic balancer would fly off. The engine builder couldn't figure out for the life of him why it was happening, it didn't make any sense at all (kind of like your situation yeah? )... so they tried everything, over tightening, loctite, lock tabs you name it they tried it. The engine builder couldn't work out why it was only happening on this turn on this track so he went for a ride along. It didn't happen... they tried again didn't happen.... so after a couple of hours of head scratching Steve asks "which one of you two does most of the driving?" the other brother put his hand up... So he went for a ride with the other brother... It happend this time. It turned out that the the combination of Rpm/load/and gear choice produced just the right frequency to undo the bolt and put the balancer into orbit!!!! What are the freaking chances? So it turned out, that with that exact motor, in that corner under those conditions, if you lugged the motor it would throw the balancer... physics hey? So I think a few people are probably trying to work out if its a similar situtation... David R has a terrifying knowledge and understanding of bottom end harmonics,,, if your lucky he might tune in?

Author:	Morris 1100 [ Wed Jan 20, 2010 6:39 pm ]
Post subject:
Cracking at the keyway is a fairly common problem on the A series when used hard. I think that it is probably a combination of things that could also include the way the keyway is cut.

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Author:	MG Rocket [ Wed Jan 20, 2010 6:47 pm ]
Post subject:
Phat wrote: David R has a terrifying knowledge He don't scare me (Sorry Phat)