Truing and Stroking

crawl9 · Jan 28, 2003

What metal is a crank made of?

I have a 2001 Rm 125 and was wondering what metal the crank if made of? is it 4041?
Thanks

crawl9 · Jan 28, 2003

What is the Best way of Truing a crank? do you have to buy one of thoese expesive Truing stands? Why can's it be done on prcision V-blocks using 2 indicators? , does anyone know How far you can Stroke a 2001 Rm 125 Crank? Also, does any compay make a shorter rod for this year? or is their another year that had a shorter rod I can use?
Thanks

jmics19067 · Jan 29, 2003

You can use v blocks. best bet though is to use 3 dial indicators. two for making sure the main bearings are on center and one for the crank halves being parallel. you can use 2 and just keep moving them around checking what you need but the three would make it easier to see what you need to do as you do it.

"I" would rather use something that holds the crank ends. I "beleive" that if you use v bocks some of your wobble is going to be on the outside of the v blocks. Shouldn't really matter once you get to "zero" but since the numbers are bigger it makes it easier for me to work with. Supporting the crank by the ends only works when the crank ends are perfect if you mash the end up a little pulling off your flywheel it throws off your readings.

jmics19067 · Jan 29, 2003

oh I am not sure what the alloy is but I doubt if it is a Chrome moly steel since the flywheel threads are rather easy to mash and don't break like a hard steel would. I got an old junk crank in the garage I will see if I can remember to take a grinder to it and watch the sparks. Might be able to narrow the alloy down a bit.

EricGorr · Jan 29, 2003

The stroke can be increased 2mmon the off-set. The 1997 RM125 rod is 2mm shorter which would negate the use of a spacer plate under the cylinder or a deep machine cut into the head.
I agree that using live center points on a crank is less cumbersome than the v-block method, but if the end of the crank has ever been struck with a hammer or the end of the mag side is mashed from a flywheel puller, its impossible to get an accurate measure of deflection. In that case knife-edged bearings will do the trick.
Personally I use an RCE crank jig that allows me to press the crank together straight in the first place. www.rceracing.com The best $800 investment an engine builder can make. :thumb:

jmics19067 · Jan 30, 2003

definately a crank jig is the way to go if your serious. I am only a "tinkerer" . The time I spend in my garage I consider play time . I couldn't see spending big bucks on something I would only use every 5 years so I made my own truing stand. I don't really recomend doing it because even though I rebuild my own cranks I still take the final product to a machine shop to double check. But for all intents and purposes I have been lucky and I am enjoying myself.

crawl9 · Feb 12, 2003

Stroking

I'm Stroking My 2001 Rm125 Myself. I'm moving the Pin 5mm and leaving the Stock Bore. Was planning on using the 97 Rod because it is 2mm Shorter. I've heard you can get a piston made with the pin in a Higher spot. Is this true? If So Who does it? If this is true, Could do this and this would put the piston back in stock Location and wouldn't have to deck the head that much. Or would it be better to use a spacer plate??I'm looking to keep the same characteristic of the stock bike, just try and improve power everywhere. and should I use the 2mm shorter rod and 3mm moved piston pin(or spacer plate) or like a 2mm moved piston pin(or spacer plate to increase compression. This would give me more bottom end right? Which wouldn't be a bad idea on a 125. but what will it do to my top end?? Has anyone else had one stroked? What did they do to your bike?
Thanks

flyinzuki · Feb 13, 2003

5mm?
Assuming you did the 10mm stroker, and you made it work with a combination of shorter rod and lower compression height piston, or spacer. If you maintain the same piston deck hight, and quench clearance you will have a much higher compression ratio than stock. Head machining will be required, just making more room by lowering your piston will kill your squish. Wanting the engine to run in the same RPM range will be dangerous, not only does the increased stroke raise the average piston speed, but the decreased rod ratio increases maximum piston speed. It should be real torquey 125 though.
EG said 2mm, looking for more may be trouble.

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