RM 100N/T

[rocketfrog]

Just giving the young blokes lil RM a top end freshen-up when I noticed the rod seems to have quite a lot of play at both the gudgeon end and the big end. So much so that the piston can be rotated in the bore about 2mm. Is there any-one out there who has any specifications for the serviceable limits of conrod freeplay for this model? Should this be a problem for an old air-cooled donk- ie do these clearances take up when the engine is at running temp or is a leg out of bed iminent?

[Doc]

Personally I think it sounds a little sus Rocket.
Sideways movement yes but to allow the piston to twist to me says there's something wrong. The piston ring locating pins run close to the ports on the RM100's so any twisting could allow the ring gap to hang up on a port edge. The specs should be similar if not the same as the RM125N/T. Unfortunately I don't have crank specifications for this model. The wear that is allowing this twisting could also be from a totally rooted gudgeon pin and brg. Does the rod twist too or just the piston. If just the piston twisting be careful when you remove the piston pin or the bearing being so stuffed may fall apart and drop needle rollers into the crankcase   A bit of lateral or sideways movement of the rod in the crank is normal but the allowance to twist the piston isn't. There's a general RM manual at the link below and the crank info starts on about page 72. Might be of some help

http://cid-86267c09a3452a71.skydrive.live.com/browse.aspx/1975%20-1979%20Suzuki%20RM%20Service%20Manual
 

[rocketfrog]

Thanks Doc,
                I have twisting movement from both the crank (big end) and the gudgeon end. ie prolly the movement from each contributing to a net rotation of about 2mm. Just looking at alpha sports, I notice there is not a rod kit listed, is that odd-dont most manufacturers offer the replacement rod as a kit? Anyway it looks like they have all the bits for the job. I was looking at some piccies of old RM 125's recently and stumbled onto a pic of a 75 model at Buladelah - is that yours? whassat, an S or M, whats the diff?

[Doc]

ummm..can't say I've ever been to Buladelah so chances are it's not me. I don't own an RM125M or S, the ones you may see my on or near are Dodgee's beasties. Both an M and an S Suzuki never did the rod kit deal, if they did it was quite rare and few and far between. Another place in the US I've been told are a little cheaper again than Alphasports is http://www.prosourceoutlet.com/pages/parts/viewbybrand/1/Suzuki.aspx I've not utilized them to know but sooner or later I surely will you might want to check you local dealer too, with the $ the way it is and shipping costs it may not work out to be much more buying locally.

[Lozza]

Some movement is always there if you try to rotate the piston in the bore.But the piston goes up and down and it's the crank that goes round and round. With rods/pistons for the 100 a KSI KT100 J or S rod will be lighter, stronger, cheaper and more readily available. Ditto for pistons.

[rocketfrog]

hey Lozza,
              are you saying that a yammy go-kart eng rod, bearings, crank pin, stroke etc (kt100 s/j ) will fit a 1980 Suzuki RM 100? If so, that sure would be cool, since the genuine item is NLA in Oz. Also, since taking some measurements, I have discovered that the bore in the 100 has been oversized to accomodate a 125 piston , but retains the 50mm stroke. Perhaps overloading the crank and its associated components has lead to the rod bearings being in such poor shape. I also notice the barrel casting has 123cc cast-into the LHS, are the 100 & 125 barrels the same part - just bored out?

[Doc]

Rocketfrog these rods are definately still available ex Suzuki Australia $71.10 retail (they're actually showing 5 in stock). Compared to $45ish US + the postage it brings it inline with alphasport prices. Only thing is you can order the rest of the stuff from alphasports and get it thrown in the same parcel for the same flat rate which does make things considerably cheaper in the long run. This rod fits about 50 different 100-125cc models some of which such as the TF125 (mudbug in NZ) were made right up to the mid to late 90's. Part number is 12161-39300. As for the 123cc on the cylinder it is a 125 cylinder no doubt about it. 100's had 98cc cast on the side

[rocketfrog]

Okay Doc,
              I'm surprised that the 12161-12002 number didn't sub to the revised number (12161-39300) when my local dealer plugged it into the system, however alpha has all the bits for the job and the other crowd you hooked me up with are generally cheaper but do not show up as having a rod or crank pin unfortunately. When you order from Alpha Sports, do you get charged a flat shipping rate upto a given size/weight? I presume you have ordered through thse guys before.

[Doc]

Yes, a flat rate for shipping Rocket and I too am surprised the number didn't sub, that old number has been out of use for many many years. Get a quote for the parts you want and simply ask how much weight you're allowed. You'll find they are pretty helpfull.

[Lozza]

Not sure if it's a 20mm or 22mm crank pin, if it's a 20mm it goes straight in(rod kit is about $80). I think you have a 125 also

[rocketfrog]

Lozza, I have RM 100 as a VIN prefix on the cases, a barrel with 123cc cast into it, a piston with 74mm Dia and the stroke measures 50mm. the replacement bigend bearing is 19x25x16 so I guess the big end diameter is 19mm the conrod b/end bore is 25mm and the bearing width is 16mm? Perhaps it is possible to machine the crank halves to accept an o/s pin but me thinks this is going to bit much trouble for this bike. Still, a complete rod kit for around $80 is reasonable if I can find a machine shop which could incorporate the mod and fit the new components.

[Lozza]

Did you actually measure the crankpin (and bore)size?As 19mm is a very oddball size, 18 and 20 are common, as is a 74mm bore witha 50mm stroke.To enlarge a crankpin bore properly involves EDM wire cutting.So to maintain the correct interference fit on the pin and to ensure it's parralel to the crank axis.Sounds complicated and expensive but isn't.

[rocketfrog]

correction on the bore size 54mm, not 74mm. I have not yet disassembled the crank to verify the dimensions, however the replacement bearing dimensions come up as described.

[Lozza]

If those dimensions are lifted from Alpha sports I would say they measured the bearing cage.A 125 with a 50mm stroke will have a 56mm bore   

[rocketfrog]

The stroke for the RM100 is 50mm and the bore is 50mm (std). The RM125 has a 54mm bore and 54mm stroke. The RM100 Engine that I have here has been fitted with a 125 barrel, so in essence the stroke has remained the same (actual piston travel measures 50mm), the bore has been increased by 4mm and the squish incorrect because the length of the barrel is correct for the 125 - naturally. So here is the conundrum, do I replace the rod with the standard item to remedy the sloppy bottom end and seek out an RM 100 T Barrel to return the engine to std specs (ie; restore the correct squish) or modify / replace the 100 crank so as to increase the stroke to 54mm and simply convert the 100 into a 125? I suppose the obvious answer is upgrade to 125, but I may be best putting it out there for either a 125 crank or a 100 barrel and head and seeing which comes up cheaper. At the end of the day the cost of the repair and the availability of spares will be the driving factors. After all it is only a fun bike, but preventative maintenance now could be the difference between keeping this little RM going for another 25 years or junking it when it expires because of the repair bill out weighing the practicality of a major repair job.