flywheel bolt trouble - how to clean thread?

[mikeb]

61 super rocket / LJ crank: i removed the longer flywheel bolt to remove the sludge trap only to find the bolt's thread is quite damaged. Also I can't thread a (26tpi - CEI) bolt into the crank / female thread, i can't clearly see the female thread in the crank and i can't even get a tapered tap down into the hole - as the tap it too wide for the hole (see pic).

clearly the bolt(s) need replacing. but what to do about the female thread into the crank? bit worried about the state of it and don't want to force anything into it.

also, the a65 flywheel bolts are torqued to 30 ft lbs, srm say 45 ft lbs for theirs, and some folk here say add some loctite. I assume that would be blue loctite? as there's no chance of getting heat down on that bolt to break some red loctite... yes?

thanks
Mike

[Rex]

If you have the correct tap (easily checked using the bolt) then how can it be "too wide" for the threaded hole?
The tap should start easily enough, given that the tapped hole is a precision thread.

[mikeb]

yes, that's the mystery Rex - the tap matches the bolt - double checked with other cei bolts and nuts. and still the tap won't fit the hole going down to the thread. I'm very confused about this and also don't see how its possible. the problem is, even with some slight side interference on the tap, I can't ease it into the start of the female thread so risk wrecking it.

??

[Swarfcut]

 Something very strange going on. Could be the flywheel has to come off to inspect the threads. Always a possible there is a fragment of a previous broken tap or a P.O. bodge down there. Threads don't damage themselves... I'd pull the other bolts and check them too for the correct undamaged thread form.

 Swarfy.

[mikeb]

i'm guessing the immediate damage is probably due to enthusiastic loctiting on a previous rebuild - long story but a mate loosened the bolt this time and i didn't see the work so don't exactly know the details.

hadn't thought about removing the flywheel. is that just remove the three bolts and move it to the side or is it more complex than that?

thanks

[Swarfcut]

 The flywheel is a tight fit on the crank and will require a good dose of heat in an oven or on the barby. Protect the crank journals to avoid nicking them when struggling with the hot bits. But really it's just a case of an obstruction in a hole and there must be a simple cause. The best way to fix it is the real problem.

 Swarfy.

[Colsbeeza]

Excuse me if I my eyes deceive me Bill. The shank on the tap looks too large, and would bind before the tap goes fully into the thread.?? ie, the bolt length is longer than the larger diameter of the tap shank.
 Col

[mikeb]

just went back out to the shed to make sure i haven't completely lost the plot.
see pic 1 - the tap (3/8 26tpi CEI) is larger than the hole. (its not about the shank Col)
pic 2 -the tap and another cei bolt fit to the same nut. the flywheel bolt also fits this nut. you get the idea

so the flywheel bolt hole has a smaller diameter than my tap - an hss tap from tracey tools.
I think i better find some other 3/8x26tpi taps and try those first. and get some replacement flywheel bolts.

[Greybeard]

Logic says that the flywheel should not have any thread;  just the crank should be threaded. I reckon you are seeing some kind of debris in that hole.

[RDfella]

Does the bolt fit into the hole? A tap will be slightly larger in diameter than the bolt it's making a thread for, else the bolt would bind. I'm thinking maybe the hole in the flywheel is an exact (reamed?) size but that your tap would fit the thread with the flywheel removed, as GB suggests.

edit - another thought: if you try to 'force' that tap into the flywheel, it will make some sort of shallow thread. Then, when it meets the crank, it will probably cross-thread the thread already there. I'd try another 3/8 bolt in that hole (doesn't have to be cycle) or measure the diameter. Clearly the old bolt fitted OK before, so it's either a small burr or the tap is slightly over 3/8 as I suggested above. Seems to me flywheel off is the only sensible way to go.

[JulianS]

Just check a 3/8 x 26tpi tap in a flywheel. It will only go in 1 of the 3 holes in this flywheel. Visible in the holes is some grey sludge deposit marked with a thread. I think these holes may be finished with a fairly tight clearance.

[BagONails]

As has been said, not a good idea to attempt tapping the thread through the flywheel as there is a good chance of not correctly picking up the original thread in the crank and doing more harm than good.

Although the bolt is damaged that does not necessarily mean the thread in the crank has been damaged too, the crank is made of very high grade material and is likely OK. I would soak some diesel or kerosene into it overnight and try getting a small rotary wire brush (brass or nylon bristles would be best) , they sell them in Dremel kits. Give it a thorough brush out and another good rinse with kero, then a good blow out with an airline. Then once clean and free of any old loctite or dirt try a good quality bolt of the correct thread (but not a tap) and see if that will now screw in OK. If not then the flywheel must come off and the thread will probably need to be recut but this should be your last resort.

[chaterlea25]

Hi Mike,
I would remove one of the other flywheel bolts, cut a groove lengthways along it's threads
Clean off any burrs ,then thread that into the damaged hole to clean the threads in the crank
The bolts are supposedly one use only so buying a set would be sensible

John

[mikeb]

thanks for the many suggestions. and thanks Julian for confirming that a tap won't go into all your flywheel's holes (i was starting to doubt my perception). interesting the tap goes into one hole - suggests a range of diameters so another option could slightly oversize the offending hole...?

I think what I'll do is flash the hole with a gas torch to burn out an old loctite and other cleaning as per Ian's comments. Then try John's suggestion of cutting a groove in an old flywheel bolt and gently starting the thread with that. failing that, it may be flywheel off. It will take a few weeks for new bolts to arrive from srm so some time to play.

final question for now: on reassembly,
30ft lbs (as per a65) or 45 (as per srm)?
red loctite (and risk future thread damage) or blue (and slightly risk the bolt loosening)?

thanks again
Mike

[Jules]

I'm with John on this, the bolt threads are definitely "decapitated" by something in there, but is it just something hard stuck in a thread or are the threads damaged inside, the only way to be sure is removal of the flywheel, but that sounds quite a big job for the uninitiated like me! So my first thought would be to clean out the threads as you want to do, but if the tap doesn't fit and a bolt does, use a high tensile bolt if possible and cut a slot down the threads and use it to clean out the crank thread, this approach has often been a good fallback to clean up threads in general if a tap isn't available.....have to say I'm surprised though that the access hole is sized soooooo precisely to avoid a tap going through!
Good luck Mike....
PS I'm not familiar enough with this assy. but if the flywheel comes off (albeit with a bit of effort), could the (very precise) clearance hole not quite line up with the threaded hole in the crank, thereby creating the flattened threads ie when you try and screw it in the bolt rubs on the "clearance" hole??