Crankshaft Runout Question

[Northracing]

An update, I managed to true a crank before disassembling it and got it to a good spec - took many hours.

I disassembled the crank and no surprises, but unfortunately some light scoring on the timing side pin bore - the crank had been rebuilt in the past, the pin was clearly aftermarket. Good interference fit with new pin and bores according to my measurements.

Still have some cleaning to do, as well as clean out the sludge trap.

Question, how best to dress the scoring on the timing side pin bore?

Thank you.
Derek

[LaceyDucati]

Derek,
The scoring is sadly not unusual. The cause is often sharp edges of oil holes etc. Generally you can take off any high spots with a fine oil stone. I generally use a round stone around 3/8 diameter. Ideally new or in good condition, done carefully, you will see the high spots as you go. With care you won't remove anything significant. I prefer to do this rather than use a hone as you can just work on the high spots. The scores are mostly insignificant in themselves, only superficial.

Nigel

[Northracing]

Thanks, Nigel.

Couple more questions, I have seen some crank wheels with noticeable chamfers on top of the pin bore and others where a chamfer is barely discernable. Does the chamfer factor in to how easy or difficult it is to true a crank?

I have seen some rod kits that use a straight 30mm pin rather than a stepped 30/32mm pin. Does the straight pin have any advantage in crank truing?

Derek

[LaceyDucati]

Derek

Yes the chamfers are very variable, they tend to be there to avoid the internal radius at the step of the pin (there to minimise a stress raiser). Ideally you should avoid pressing the pin up against the shoulder as this tends to increase the possibility of the pin failing at the step. But as I said, sometimes pushing it up hard (in particular on the drive side) can help return the crank to true as that's how the cranks were generally built from the factory. As I said before, it's not really an issue for a road bike, but less than ideal for racing. Any gap to the thrust face should be kept to a minimum, I usually aim for around 4 thou (0.10mm), to avoid loss of thrust shim running area. My pins were always designed with this in mind. The chamfers at the pin holes will not really affect truing in any way.

I like a parallel pin, but it's not always possible/ideal with a Ducati single, with regards to the bearing size relative to pin bore. As I said before, the use of a parallel pin can sometimes make truing a crank more difficult, but that's a manufacturing issue with that particular crank....not much you can do about that. On the timing side the Late Widecase pins sometimes have a radius on that step, so I guess they were never intended to be shouldered and they usually have a large chamfer on that side, at the pin hole.

Regards Nigel

[Northracing]

Thanks, Nigel,
The chamfer issue is interesting, I have one crank where if I pushed the stepped pin fully in I would lose some of the pin bearing surface area into the chamfer.

This information has been very useful.

Derek

[Northracing]

An update, pressed the crank together with new rod kit. With my home made jig, was about 0.008" off on the bevel side, 90 degrees from the crank pin. Some gentle persuasion with a 4lb copper mallet got the bevel side of the crank near perfect, maybe 0.00025 off. Unfortunately, the drive side of the crank is still out 0.004", still 90 degrees to the pin.

Not sure what to do, trying to get the drive side better just made the bevel side worse and I had to back-track.

Any suggestions appreciated.

Thanks.

Derek

[LaceyDucati]

Derek

Sounds like you may need to wedge or squeeze the wheels 90 degree to the pin to get an opposed error and then swing the wheels until the error zeros. It may keep springing, but it may be a correctable error. If the error was inline with the pin, up one side and down the other, then you can't correct that, only balance out. Patience is a virtue here, but very satisfying when you get there. Regarding hammers, I always use an aluminium block fixed to a heavy steel bench ( sort of anvil solid). I hold the timing side in one hand and the rod in the other, then give the drive side wheel a thump on the pad. I tape the aluminium with duct tape to limit splintering (replace when it starts to). Takes attention and getting used to this method, but I far prefer this method to a hammer. Bronze/copper could be used, but they all splinter without tape. Main thing is avoiding any marking of the wheels. If you wedge the wheels, used alloy pieces (2 or 3mm) with a suitable cold chisel to avoid marking the wheels. Also soft jaws in a vice if you need to nip the wheels. Truing up a bevel twin cranks, needs some very heavy treatment to move them, I personally think a copper hammer wouldn't cut it.....not for the faint hearted, LOL. Good Luck, keep trying.

Regards Nigel

[blethermaskite]

What I made up years ago when I was building the odd crank was a skinny cycle thread nut and bolt that would just slide in between the wheels that I could put a little pressure here and there to find out if the crank was 'bowed' 'in', conversely my trueing set up would let me use a fine screw clamp to squeeze the wheels a little to see if it was 'bowed' 'out'.......just my backyard method.......but.....Nigel is your man for this stuff Cheers George

[Northracing]

Thanks, Nigel, George. Some other obligations will keep me out of my workshop the next few days but I'll get back at it.

It's always seemed a bit unnatural to pinch cranks in a vise but necessary at times, I'll approach it very carefully. I'm not that far off. Thankfully the high spot on the drive end of the crank is 90 degrees to the pin.

Derek