Wondering if there's a known power level and run hours that shortens M8 crankpin life in the flywheel bores?

I read through some M8 threads where some of the crankshafts get out of runout specs from the pressfit wearing. I got no experience with HD motors until this 1st bike. Seems kind of like a piston wrist pin press fit into the piston skirt? Never heard of one of those wearing out?

 

JUST MY OPINION, so don't go crazy on me! From what I have read, and surmised from the runout readings on my crank, and others in my small circle, it really depends on how the bike is operated. I am not saying big power does not play a part, but I have one friend who now has 80K on a 150tq/150hp 131 and his runout showed .004" this spring when he had a lifter failure. I convinced him to check the runout after decided to recam at the same time.

On mine, (sigh) I took it apart last winter, runout is .0055" This would be my fault, I am a much more spirited rider than my friend. 25K @149tq/128hp. So, my bike sat there while I had surgery this spring...I put it back together, and have some BIG decisions to make. I was going to install my CR512 and new heads, but the crank has me worried. Anyhow, that is another tale for another time. I digress.

I dove into this with a local builder/speed shop, he agreed that anybody going 130/130 and rides remotely aggressively should look at doing something with the crank. He is starting to see more and more issues as guys start putting big power into these stock bottom ends. He said my Tractor Build 128 probably would have gone the distance if I had not acted like dumb kid a few times.

I have 4 good friends with big bore M8s, and we are now paying the price for them; cranks, transmissions, etc Not complaining, all part of the deal, but I think if you are sensible with the throttle and the clutch, you can stack on the mileage.

 

@NorthWestern

Thanks, sure makes sense to me.

If memory serves, automotive high power cranks are all one piece, usually forged steel, so there is no two-piece rod journal joint. It those fail, they break / crack. They generally know how much power for how long you can put through them, which is why I asked about this unique MoCo crank.

I just went from 107 ---> 117 with 11.0:1 pistons, dyno shows 131 tq / 115 hp. On a softail 100 lbs lighter than a tourer, it is way fun to get on it. Which, of course, is why I asked! Good thing I'm just a cruiser w/o many hours. Sounds like time for a Dark Horse for you. I'll hold off till more of you bust 'em.

 

Been thinking about this - auto cranks, in addition to being one piece, have fillets where the journal bearing connects to the counterweight. The bigger the fillet, the higher the fatigue life of the crank.

Not only does the Harley crank pin not have a fillet, it's not even attached - just press fit. Little wonder there's a sharp upper limit where it comes loose. Ideally, if they'd have left wider spacing between the counterweights, the pin could be TIG welded inside the counterweight. That would provide the solid connection and a fillet radius. Could take a lot more power in that configuration. Since there isn't room, I guess welding external is the only solution for bigger builds.

And then there's this - why isn't the piston pin as power limited as the crank pin? Because each piston pin is only 1/2 the power to the crank pin?

 

I'm curious if you know.. Did the cranks spread, pinch or twist?

 

I assumed in my case that it is spreading. I do not know about the others.

 

Yeah. You like that roll-on torque, the stress is on spreading.. You like to over rev, the stress is on pinch. You like to dump the clutch, the stress is on twist.

 

So when you say your crank has .0055" runout, what is that up from. Did you measure the runout when you first did your big bore? .0055" of runout is well within Harley's specs. I'm not saying I agree with their specs, but they'll tell you that is OK. I have had TwinCam cranks straightened, welded, and balanced and still ended up with .0025" runout.

 

Apologies, yes I had .004 at time of cylinder change, with right around 1000 miles on the crankcase.

I am aware that it's acceptable (now), but it is certainly trending in the wrong direction. I tore it down to add yet more power, and more right side torque, so I stopped feeling this is a recipe for disaster.

I do really ride sanely, 98 percent of the time. But, inevitably, the boys and I get out in the middle of freaking nowhere Montana, etc. and do something stupid. The last time we hauled one back in a Uhaul with 3rd gear gone

If I could keep 2 and half thou like you, with a Johnson crank build, I would be happy. If I knew my 5 and a half would not get worse, I would be ecstatic.

 

Sounds like the line is getting crossed with the bigger-than-factory builds, 121 and up?

I mean, what's the point in doing a custom big bore if you're not gonna get on it now and then? Perhaps advice should be give to rebuild the bottom end when going that route? At least a welded crankpin and bearings, perhaps a higher alloy crankpin as well.