Eric or ne1 who knows - RM 155 Rod??

[robwbright]

For Eric or anyone who knows:

What year(s) connecting rod can be used to make an 02 RM 144 into the 155? I'm not really considering it at the moment as I am VERY PLEASED with the 144, but I watch that unmentionable auction site :laugh: all the time and would buy shorter rod if I saw one for the right price . . .

Then I could build the 155 at my convenience when I rebuild the bottom end . . .

Should I get hot rods or is stock ok?

Thanks
Rob

 

[Masterphil]

A different legnth rod will not increase the displacement, you will need to have the crank stroked 4mm. (They move the big end pin away from the center of the crank 2mm) You will then need the head recut, a 2mm spacer plate or a rod that is 2mm shorter than the stock one. Your CR will be higher, so the head will have to be recut for that. Your port timing will also be off, so the cyl will need to be modified as well. Stroking a 2-stroke is much more complicated than stroking a thumper.

 

[robwbright]

Masterphil:

Maybe I misunderstood Eric, but that's not what he told me on the phone a couple weeks ago. He made it sound like a quite simple procedure which he had done to a 2002 RM 144.

But like I said, I may have misunderstood.

 

[viking20]

I think you have ,- Masterphil is correct

 

[robwbright]

My understanding from talking to Eric was that I would need the shorter rod. He may have meant that I needed the older crank AND rod. But he said nothing about stroking it or needing to modify the cylinder further . . .

Hmmm. . . This is odd as my discussion with him was about all the options arising out of having the 144 done and it surely did sound like "sure - you can pretty easily convert it over when you rebuild the bottom end."

I suppose he'll set me straight here eventually . . . LOL.

 

[robwbright]

Oh yeah - two quotes from two posts by bclapham around the time Eric was doing this - apparently I wasn't the only one that thought it was easy - these are what I had been reading before posting this thread:

"Last time i spoke with eric, he was building himself a 2002 rm155- 58mm piston with an extra 2mm stroke- its easy to do since the older rm125 (97-99 i think) rod is 2mm shorter so no spacer place is needed."

"from what i understand, if you go any further than 2mm stroke, you run into serious issues with port and ignition timing and your pipe and ignition wont match up with that much increased stroke"

"it is something i will consider when my engine gets its overhaul next season if the stroker kit isnt much more than a replacement crank."

"on the newer rms its easy to build the 2mm stroker and keep the port timing ina good ball park since you can use the 2mm shorter rod from the 97 rm"

Rob

 

[Masterphil]

It isin't something that is very difficult, It just takes $$$.

Using the shorter rod, the CR and port timing and time-area will still be in the ballpark, but hardly optimised.

Bottom line, if you want to stroke it to 155, you will need to have the crank stroked, and the cyl and head modified in order for it to all work together well.

Phil at Crankworks.com would be an excellent choice to do the crank work.

If it is possible to recess the combustion chamber in the head 2mm or to use a spacer plate, I'd think that using the longer (stock) rod would be a better option than using a shorter rod. The longer rod will have more dwell at TDC.

 

[bclapham]

yes, you stroke the crank 2mm and then use the shorter rod from the 99???? or something bike for a direct bolt up. or, you can use the stock rod and a 2mm spacer, or you can go without the spacer and re-cut the head. or, you can be a real hero and run the longer rod (04-06) and put a really thick spacer in there. you might need to relieve the inner cases tho.

as for the port timing, just get the 2-stroke tuning book and crunch the numbers, it isnt far off.

however, as for trawling e-bay for cheap parts to send to the tuner, its bad form really- they need to make their money from parts as well as service and its a pain in the ass for the tuner when the saod parts arent the right ones, LOL.