Jump to content
Sign in to follow this  

Stroking a crankshaft, what is there to know?

Recommended Posts

I am thinking of stroking a crank but of course have never done it myself or had it done by someone else. There isn't much talk about it, but many engine shops offer the service.

Does anyone know how it actually affects the crank itself? For a single cylinder it should be fairly easy, weld, machine it flat and bore a new hole. Does it affect the stiffness, how much? Does it need heat treatment or similar afterwards? How easy it is to get the holes aligned?

I know what it does for the engine because ktm used to offer a factory stroked crank but those are very rare and expensive these days.

Measurements in this case are 72mm stroke, 35mm pin. +6mm so it would be 78mm stroke.

I know I could go to the crank shop and ask stupid questions but I'd like to know something before I do.

Share this post


Link to post
Share on other sites

what exactly are you trying to accomplish ?

theres alot of questions that need answered before you do anything. first of all i dont know what bike you have but is there enough clearance in the crankcase to allow the rod to swing around without hitting ? if not youll need to trench the cases. is there enough material there to make the needed trench ? will the exh port be uncovered at tdc ? may need to raise the cylinder or weld the port up. which may add even more problems with airbox alignment, headstay mounts and exh pipe hanger misalignment. then the head needs dome reconfigured. and the list goes on and on. this is hypothetical but what im saying is you need to do alot of homework so you dont get caught with your pants down. another thing is check your rod angle and how the extra stroke would affect it. some motors have steep angle and some dont. longer rod may be needed if the angle is too steep which add even more complication. your simple idea really isnt simple if you look at the little things that need to be considered

Share this post


Link to post
Share on other sites

Thank you for your concern, but all that is sorted out. All I'm worried about are the crank webs and methods to do it. I'm not saying I'll do it myself, I just want to know.

The bikes I'm mainly interested in are rfs ktm's, but the same should apply for four stroke cranks.

Edited by gravelfreak

Share this post


Link to post
Share on other sites
what exactly are you trying to accomplish ?

theres alot of questions that need answered before you do anything. first of all i dont know what bike you have but is there enough clearance in the crankcase to allow the rod to swing around without hitting ? if not youll need to trench the cases. is there enough material there to make the needed trench ? will the exh port be uncovered at tdc ? may need to raise the cylinder or weld the port up. which may add even more problems with airbox alignment, headstay mounts and exh pipe hanger misalignment. then the head needs dome reconfigured. and the list goes on and on. this is hypothetical but what im saying is you need to do alot of homework so you dont get caught with your pants down. another thing is check your rod angle and how the extra stroke would affect it. some motors have steep angle and some dont. longer rod may be needed if the angle is too steep which add even more complication. your simple idea really isnt simple if you look at the little things that need to be considered

As he is just copying a existing factory (4t) crank all that has been accounted for by the factory.

Share this post


Link to post
Share on other sites

+6mm is a HUGE change in stroke that will probably end up with clearance issues with the transmission. Powroll here in the states does Hondas with success and has an indepth description of the process. I've had several of their products and they are good units.

For a 6MM increase, you would break the webs apart, chuck it in a mill, measure out 1/2 that distance and mill a hole that is off-center of the original hole by 3mm and it would have to be increased by about 3-5mm so you could make a sleeve to reinstall the crank pin. You would then cut a sleeve with an inside bore equal to the crank pin and outside diameter equal to the new hole you just made. Make 2 of those, one for each side.

The tough part would be compressing the rod, or you could special order a rod from Carillo or Falicon. Expect to pay big $$$$$$ for that. You would also have to releave the rod where it would make contact with the correxponding transmission gears... if you didn't make the throw so large that you dip into the rod main bearings, in which case you just ruined your crank webs AND your rod.

It is possible to custom forge the stock rod. I has to be shortened EXACTLY 1/2 the amount you increase the stroke by. So a 6mm stroke increase means you shorten the rod by 3mm. So bottom line, MEASURE twice, cut once!!!!! And be precise when you do it.

http://www.powroll.com/tech_specs_stroker_howitworks.htm

Share this post


Link to post
Share on other sites

Buy a kit for your bike. That way all the design and problems will have been taken care of.

Share this post


Link to post
Share on other sites

The sleeve method is interesting. But if you're making a sleeve, then couldn't you just re-make the entire crank webs? Just mill off the existing webs, cut a new one and weld it in place? Just a thought, but leaving some of the old web and weld the new around it and could be strong enough?

Here is a picture I stole from another site, I believe it's a +6mm yamaha rhino cranks. Looks pretty neat to me. http://i211.photobucket.com/albums/bb175/Pgregrhino/Rhino700StrokerCrank002.jpg

I think that is the way to do it after all, looks relatively simple. Don't know about strong though. Big bore engines, in this case over 600cc in supermoto and such take enormous stresses, especially when rotating masses are small and there is no dampening on the power train so all the impacts and traction is directly transferred to the crank.

Rod is a slight problem since shorter rods are no longer available. Using a spacer is possible but it cylinder does become rather tall. Some have used special pistons to compensate the increased stroke. Folks at ktmtalk have made custom rods for rebuilding factory strokers, that would be the easiest way. But there are plenty of ways when the time comes.

Share this post


Link to post
Share on other sites

Just buy Travis's (thumper racing) piston and use the stock length rod. The shorter rod is makes for a bad rod ratio.

78mm rfs crank:SXS02580020.jpg

notice the big end has been relived

Share this post


Link to post
Share on other sites
Does he have 100mm or 102mm stroker pistons?

99mm and 102mm

Share this post


Link to post
Share on other sites
99mm and 102mm
Thanks, I didn't know about the 102mm. So that is solved, no problems there then, excellent.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...