crank stroking

[RM125Guy]

I was wondering if it were possible to stroke my crank from 72.00 mm to 74.00 mm? If it were possible would it be Ama legal? After all its 2.00 mm difference like the bore rule says 2.00 mm does it apply to stroke as well? If its possible will it be a lot smoother and torquer? How much would it cost to have such a service done? And would any motor mods be needed? case clearancing and etc.... thanks....

 

[EricGorr]

The AMA rule is stated for cylinder bores, with a maximum of 2mm or the equivalent displacement with crankshaft stroking or a combination of each.
Recently I built an RM250 with a 69.5mm bore and a 76mm stroke. I had to bore out the cases so the big end of the rod could clear the cases. However there was enough extra material in the cylinder head to accomodate the longer stroke.
I'm putting the finishing touches on a tech article about crankshaft stroking that I'll post in this forum.
The company that I use for crankshaft stroking and balancing is Crank Works in Tempe Arizona. They also do work for the factory teams and many of the big hop up shops in California.
Their web site is www.crankworks.com I highly recommend their work.

 

[MACE]

I just sent this off to Philtech.... Maybe a good discussion can be had here as well.

A few questions if you have some time.....

Is it true that when balancing cranks you must choose a best balance factor to trade primary imbalance verses secondary imbalance?

Is there a "magic" balance factor that always works?

Do you use different factors for different anticipated RPMs?

I'm a trail rider and am more concerned with vibration in the lower half of engine speed range. Is it likely that the stock balance factor on my YZ 250 can be improved upon for me?

I am considering an Eric Gorr big bore kit for my 98 YZ250. Would I be wise to have the crank balanced?

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Eric, do you recommend the 74mm overbore (296cc) or is it more trouble prone than the 71mm (273cc). (1998 YZ 250 @ 68.8mm stroke)

 

[EricGorr]

Hi Mace, good questions, I hope Phill has the time to respond. Theres few people in the motorcycle industry that can answer questions regarding crankshaft balance factors and I suspect that most of that information is competitive in nature. One interesting thing that I picked up from Phill while working on my article about stroking, that most of the Japanese cranks are over compensated on the balance factor, which I perceive as being able to handle a heavier or larger piston. Recently I sent a KX500 crank for balancing at high rpm, and was surprised to see the balance holes on top lightened without weight added to the bottom.
Regarding the different bore sizes, the 74mm is a bit of a problem because the inner o-ring groove margin gets compromised. The 71mm with a stroked crank seems like it would be the logical alternative.

Thanks, Eric