My Wife's RD125

[sav0r]

Thanks for the moral support gentleman, it's always appreciated.

I ran into some trouble getting the crank cases split, basically I got them about 1/4" apart and now they won't budge. I have a clymer manual, one of those catch all types, but I can't find it anywhere. So I googles and Yamabits has this puller (pusher?). I won't be buying it, instead I'll find some junk out of the scrap bin and make one.

https://yambits.co.uk/rd125-crankcase-splitter-p-43610.html

The picture isn't very good, but the labyrinth seal o-rings are looking pretty shot. Maybe this is normal, but they look pretty gruesome to me. It doesn't look to me like there's a snowballs chance in hell they were actually sealing.

[sav0r]

I made this piece of junk pusher out of a piece of junk. It worked fine until I got a rod jammed up and then the pusher bent. I realized my error, moved the rod, and it all came apart. Unfortunately I don't own a press to press the crank out of the other case half, so I ended up driving to my fathers to do that. I took my boy with me, we hit the hot tub after working in the shop. We also tossed some helicoils in the perches so that the mirrors no longer fall off from looking at them. Speaking of mirrors, I only have one. I can't remember which side, I'll have to check. The seat cover is almost on too. Not ready for pics, but I am pretty happy with it so far.

[NoRiders]

Those internal seals do look nasty...when I say seals, I mean a gobfull of tigerseal or similar. The central seal is critical to good combustion chamber dynamics....I wonder how it ran?

Kudos for taking on the engine rebuild btw.

[sav0r]

After closer inspection, it looks like one o-ring was probably sealing. The other definitely was not. The spring was out of the right side crank seal and just floating on the crank. That probably wasn't helping things.

The bike ran as recently as last summer, but it certainly had issues. The one carb was rotten when I got the bike and i never got that side to run with that carb. So I replaced the intakes with DT intakes and fitted Mikuni knockoffs, the right side then would fire, but it never ran consistently. See the video below. If after a rebuild I still can't get it tuned consistently I'll likely buy real Mikuni's, but I'll cross that bridge when I come to it.

I'm considering building the crank myself. How hard can it be (famous last words)?  We used to build our own karting engines and had great success doing so, but we never pressed our own cranks, a friend of ours always did it for us. I'm just not too keen on waiting 6 weeks (or worse) for somebody else to do the crank for me, and maybe it will be a learning experience.

https://www.youtube.com/watch?v=i_FZBOtluvE

[m in sc]

do it, ive done mine as you know. not too hard, just be patient

man i -hate- vertically split twin cases. main reason i gave away that ds6 i had.

[sav0r]

Alright, I'm going to do the crank myself. Should be fun! I'd say I'm not a fan of vertical split cases after this, they never bothered me with the karting stuff. Of course those engines didn't require press fitting everything.

I was looking for this book tonight, I found it obviously. It's from 1972, but seemingly the information is pretty on the nose. Lots of neat information on Yamaha twen 125's in there. Apparently in race trim they revved 3k more than in street trim. I think it's good for 10k or something in stock form. 



Inside the book I found this memorandum, I must have stashed in there back before we moved as I found the book in one of few remaining moving boxes. Vaughn was one of my best buddies, we raced together from about 12 years old up until his death (we are very similar in age). He jokingly liked to be called Ace, always had it stenciled on his race cars, and he was one hell of a wheel man. He was the smoothest driver I have ever seen, I have never figured out how he managed to look so effortless on the race track. He passed away from complications due to a motorcycle crash... Godspeed, Ace.



Sorry for the buzz kill. Anyways, here's the crank. It's pretty cute.



Here's the transmission. Gears looking pretty typical for Yamaha transmissions of the era. The teeth looking grainy likely a product of hobbing. They wouldn't stand up to any modern inspection, but with the 8hp this bike will produce I'm sure they have quite a bit of life in them.



And one half of the case, it's got some sludge in there. Not much else of interest to see really.

[rodneya]

Is there just a piece that slides in between the cases on top of the lab seal? Is all the black goo on top of the lab seal to seal the piece that slides in there?
Weird design. Makes sense for a single cylinder engine, but for a two cyl is looks a bit hokey. Still a pretty cool little motor though.

I dont mind vertically split cases, but I always had problems getting a YZ250 gearbox back in place. Need extra fingers and a bag of elastic bands.

[SoCal250]

The lab seal has an O-ring on the outside and it is housed inside a part called the Crank Cover. The photo shows the top of the crank cover.  The crank cover has a 2-pc seal on the outside. That black stuff almost looks like the cover seal has started to break down or dissolve. There's also a "filler" part that drops in on top of the cover on those 2 pins.

[SoCal250]

Apparently in race trim they revved 3k more than in street trim. I think it's good for 10k or something in stock form. 

FYI, the redline on a RD125 is 10.000 RPM. While the US version did not come with a tach, some other regions did come equipped with a 12,000 RPM tach from the factory. Part # 396-83540-00-00.  I've always dreamed of retrofitting mine. The tach boss on the front right corner of the case is present on the US RD125B but would need to be drilled. Sourcing the parts would be tough because the gauge is hard to come by, especially in nice shape.

[sav0r]

The seals are breaking down alright, basically crumbling.

Of course the gasket set from Yamabits went out of stock sometime in the last few days. So I'll have to search around for one.

[SoCal250]

Not sure if you've sourced seals yet but here is some recent info that I compiled in my notes. The prices are from large vendors and just for reference, and not the Yamaha MSRP. The left crank seal is NLA but there are alternates according to info I found, 93102-20447-00 is supposed the be the same and is currently available.

RIGHT
Crank, R      SW-28-40-8  Yamaha 93103-28011-00, ~$8.00

Primary gear O-ring   Yamaha 93210-180A6-00  (93210-18023-00) , ~$2.35

Kick start   SDO 15-26-6  Yamaha 93109-15001-00 OIL SEAL, ~$4.60

LEFT
Crank, L       SD-20-40-8  Yamaha 93102-20117-00 OIL SEAL, SD-TYPE, Not Available
Seal markings: ARS SD 20 40 8 - 1 GS
   Interchangeable Parts:
     93102-20010-00
     93102-20117-00
     93102-20231-00
     93102-20447-00, ~$6.10

Sprocket      SD-28-44-7  Yamaha 93106-28043-00 OIL SEAL, SD-TYPE, ~$7.00

Shift Shaft    SD 12-22-5 HS  Yamaha 93102-12321-00 OIL SEAL, $6.15

[sav0r]

Thanks, SoCal. I'll have to check those seals out.

I got the seat more or less done tonight. I'm okay with it. The seat cover quality is decent, fit ended up a bit asymmetrical (hardly visible in photos but after wrestling it for an hour it's obvious to me), and the material just isn't all that stout. It's a huge improvement over the old cover though. So i'm calling it a win!

[NoRiders]

That'll do ...nicely. And, considering your overall approach and restoration level you hope to achieve is a realistic outcome. Good progress...sav.

[85RZwade]

That seat cover job looks great to me; I'd let you do a cover for me in a heartbeat! And now I know I'm not the only one who uses old socks as shop rags!

[sav0r]

Thanks for the compliments dudes. I appreciate it!

I got the parts from Yambits yesterday. 5 days from the UK to Pennsylvania. Incredible service!

The frame painting is under way (sorry for the awful photos). Crank pressing tools are in design. I have a huge load of shocks for customers right now, I have to get them finished, then I'll machine the crank tools. I worked over our Lisle hone and will be honing the cylinders in house. I reached out to Chuck as well as others, these tiny cylinders don't have the luxury of getting serviced at any old shop. Luckily the bores are straight and fairly clean, honing them up .25mm should be a breeze.




Also, I machined this absolutely ridiculous shift lever for my 350 (I dont have a thread for it and wont start one for a shift lever). I could have fabricated something much simpler, for way less expense, but I wanted a learning experience. So I present the worlds most expensive RD350 shift lever. 97% removal rate and each billet is like $30 in material, and I fucked it up 4 times before getting it right. I've been testing a new machine control software and that is in part responsible for me just throwing cash down the toilet as I need to test it out. Things went wrong at times, but I am happy ultimately. The result isn't perfect but things are looking promising.