Converting RM100 to 125 (77/78 models)

[Loydee]

I know the stroke is different (and bore) but by what method is this achieved? In the big end pin positioning on the crank webs or is it just the length of the rod. If it's just rod length could ya just buy a 125 con-rod kit and rebuild crank. Thing is I have loads of cheap,shagged RM100 engines and a 125 top-end don't wanna rebuild as a 100 which nobody wants (including me.....!) 

[TM BILL]

The Rm 100 has a different crank pin offset 

[Lozza]

Loydee,  I am pretty certain that they both have a 50mm stroke(56 X 50mm for the 125 and 50 X 50mm for the 100),IIRC in 81 they went to the 54 X 54mm square dimensions.Rod length(that does other things) doesn't change stroke only crankpin offset does, that's a big job changing offset.From looking at my 100 the main difference is the thickness of the liner, which means just bore to 56mm and away you go.

[Loydee]

Lozza, yeah OF COURSE rod length don't govern stroke, jeez what an eejit.....! Anyway according to my net research the 77 RM125 had the 54 x 54 engine. Thing is one of my engines No. 21xxx (1977) above the gbox sprocket -no stroke info, the No.27 xxx (1978) has stamped "Piston Stroke RM100-50mm,RM125-54mm".
So looks like would need an 54mm RM125 crank to convert? 

[Lozza]

Ah yes the 76 was the last of the 56mm bores, by the time you spent the time and money going to 54 X 54mm(it all depends on crankpin diameter) it would be easier and cheaper just to find a 125! I would be just boring the cylinder to 56mm, you would just have to compare compression heights , if you can't get the same as a 100 get a higher one and gasket the barrel up to match.

[Loydee]

 The more I think about it -yep just get a 125! Over here RM125s cost a lot even just engines 'cos they are used in the Twinshock championships. No class for the RM100 so they're unloved and unwanted, shame really as I've got two and two spare engines :-(   Thanks for your time fellas

[Tim754]

Loydee , if you are light in stature you could practice and improve your riding/racing skills and fitness. I bet a competent rider on the 100 (which you have and can get others cheap) would beat many others that are useless flab like me............ and boy does it hurt to be beaten by a 100/90cc in the 125cc class

[crs-and-rms]

the rm  125 s,m and a all have 56 bore and stroke 50
the rm 100a has a 50 bore and stroke 50 and this is the same for all rm100s up to the x model
the rm125b has a 54 bore and stroke 54 and this is the same for all rm125s up to the d model

[Doc]

The RM100A was a unique critter and very attractive to someone like myself. '78 is about my cutoff and pre'75 are my fav's. Suzuki was in the transition phase. The '76 RM100A is based heavily and is more or less a 'debored' lightweight version of the '75 RM125M. It does not follow the 'newer' case/reed design of the engines the other A model RM's had. It had a downpipe and was styled like an RM125M and I wish I had one sitting in the garage. Even more desirable then a 125A in my eyes. A 'real' RM100B followed suit the next year as did the first of the RM80's that were a far cry from the '76 TM75A they replaced  



while we are on the subject of RM100's I still have that NOS '77/'78 RM100B/C cylinder sitting on my shelf. $100 will give it a new home

[Loydee]

Well certainly food for thought, I guess if I just use an RM100 to hone me, er, skills as Tim754 says it won't be going to waste. I 've got an RM100X sitting there nearly ready,done top-end on it,and I'm rewinding stator coils at the minute. It'll save me using TM125 in the enduros (which won't start after I've dropped it anyway-gasssssses up). Thanks Doc for the pic of RM100A never seen one b4 - what a looker! 

[BAHNZY]

Aside from the items that have been noted, the crankshft and cases are completely different.
The 100 has smaller webs and as a result the area around the crank web where it spins in the cases is a smaller diameter.
ie: you cant physically fit a 125 crankshaft into a 100cc case set, and in reverse, if you fit a 100cc crank into a set of 125cc case's there will be (roughly) a 4mm gap between the crank and the cases.
If anyone needs a set of 100cc cases let me know. I was an ill informed bunny that bought a 100cc engine under the impression that the cases were the same as a 125. This is what i was told by people in the "RM Know", at least the gearbox was useable and this ended up in the 83 RM175 project.

[Doc]

Bahnsy, was it me who told you that? Sorry if it was 

Loydee, unlike the RM100B/C/N/T/X, I have never in my life seen an RM100A except in photo's  jeez I'd even love a TM100 but they are just as rare here

[BAHNZY]

Doc,
Not a chance. Even if it was (and it wasn't) the good advice provided by you would far outway an engine that donated a gearbox in the end anyways.

Bahnsy

[Loydee]

Hey Bahnsy thanks for the invaluable info, crazy that Suzuki made entirely different cases and crank for the 100 to the 125, why?!! Anyway saves me wasting cash on a 125 crank.Cheers