Here is my 83 yz125 K project.
I've owned the bike for 20 odd years, it was my first 2 stroke MX bike and as a kid growing up on a farm surrounded by bushland it bought hours of endless fun with friends.
I absolutely abused the sh!t out of it as a kid and kept bodging it up to keep it going. Eventually the water pump seal gave up and the gearbox oil went milky. I kept riding it for a while like this before throwing it in the shed.
Recently I decided to dig it out and to my surprise it was in worse shape than I remember. Maybe my dad threw parts out from cleaning the shed out over the years or my brother got his fat fingers onto it.
My plans for this bike is to build it to a near as possible OEM standard to pass down to my son when he's older.
Anyways here's what it looked like after I dug it out of the shed after all these years.
To be continued...
So after digging it out of the shed the first thing was to tear it all down to a bare frame. Like many other yz125k's it had the dreaded chain wear on the inner right hand side of the frame, missing weld nuts where plastics bolt on and as an added bonus the chain roller was dead.
To be continued....
After stripping the frame down I went searching for a cylinder for the engine as mine was completely stuffed. During my search for a replacement 24X cylinder I crossed paths with a guy named Alex aka dick trickle on these forums.
Alex is a walking enclopedia when it comes to the yz125k as he raced them in his younger years and also has two in his inventory. Alex gave me alot of good advice which has helped me alot during this process.
Anyways first up was to sand blast the frame, inspect it for cracks, straightness and replace all the weld on threaded weld nuts.
This is probably where things got out of control for me... Having an engineering background I took a route that was probably over kill but the results were worth it.
I had the frame 3D scanned at a local friends workshop where I could input the model into Solidworks and get to work redesigning the rear 'subframe' diagonals. These were later CNC bent from 4130 chromoly and Tig welded into place.
After all the repairs to the frame were complete it was sent back to the powder coaters for another sand blast and gloss black powder coating of the frame.
To be continued...
that is bad ass, nice work!
Thanks mate. I was really pleased with the results of the frame. Alot more to post when I have some spare time..
The Shop
So with the frame sorted out and whilst I was waiting for my used cylinder / jug to arrive from the states I decided to tare into the engine.
I was hesitant at what I might find inside the engine after all these years and after what I saw on my original cylinder. The previous owner had drilled and tapped a hole from underneath the power valve bore and fitted a socket head cap screw to hold the power valve shut / in place..
Once I got inside the motor my fears came to life. Wrong bearings fitted on the crank, damaged crank case, rusted and pitted gears amongst a heap of other issues.
Here are some pics of the opened up engine. She's not pretty.
Socket head cap screw holding the power valve...seriously wtf...
Old and replacement cylinder:
Inside the motor... R.I.P:
And yet when I parked it up some 20 years ago it still ran...
To be continued...
holy hell! best of luck, sir
Now that I know the motor is in need for a full rebuilt top to bottom I've started collecting parts.
In the meantime I've decided to focus on the frame. First thing on the list was the rear shock. Mine has always been 'sloppy'. I believe this is because the bearings were flogged, the internals worn and it probably has no gas charge left.
I spoke with Alex about this and he told me the internal bore was never hard anodised from the factory and once worn, the bore usually gets 'scratched up'. Alex told me he had to buy a handful of used shocks before he found one that wasn't beyond repair.
After looking at used rear shocks and factoring in the possibility it could be beyond economical repair and having to roll the dice again I decided I would start the hunt for a new old stock (NOS) shock.
Eventually I found one in the Netherlands:
To be continued..
With the rear shock sorted and finding a crack in my original rear swing arm I went searching again for a replacement. I stumbled across a NOS rear swing arm, relay arm and connecting rod so I snapped them up. I Also replaced all the swing arm, relay arm and connecting rod bearings, seals, bushes, shims etc etc...
NOS swing arm:
NOS relay arm, lower chain guide and connecting rod:
NOS swing arm pivot bolt (have the nut too)
84 yz125 upper chain guide fits the 83 and is slightly longer and provides better protection to the frame to prevent chain wear.
Bearings and shims
To be continued...
Just some behind the scenes stuff..
I originally tried to repair my original swing arm. It was badly pitted and after stripping the anodising off and during the sanding of it I came across a crack near a weld.
The original relay arm by this stage I had hand polished and was looking smick but something in my brain was telling me it looks out of place (not how Yamaha intended)
Original swing arm in an acidic bath to strip clear anodising off :
Original relay arm before and after hand polishing:
Relay arm is for sale if anyone wants it pm me.
Next up was to tackle the front forks.
One side was bent from a accident I had one afternoon on the way home from out bush were I had a head on crash with a girl riding a banshee.
They inner tubes were also pitted/ rusted so another full overhaul was in order.
After stripping them all down and cleaning everything down once again I went searching for replacement parts. Eventually I gathered together all new bushes, springs, valves, seals, stop tubes, circlips etc etc.
After this the inner fork tubes were sent to a specialist workshop on the otherside of Australia where they were straightened and re hard chromed.
Here was the result: (they came up better than new seriously)
After this I decided to once again start polishing the lower fork tubes but once again I didn't like the finish as it wasnt original.
Eventually I came across a set of NOS lower fork sanctions so I snapped them up, rebuilt the forks with all new and reconditioned parts and filled them up with Motul full synthetic fork oil.
To be continued...
You missed an oppotunity to hard anodise the fork sliders. It's a worthwhile thing to do as it gives NLA parts a fighting chance against wear and it isn't expensive. Company local to me did our last pair for £40/AU$75. They need to be painted or polish the finish off the outside, as we don't know if it's the silicon or magnesium content in the alloy Yamaha used from the 70s-80s, but they come out blue. The platers say it's only these parts from us that turn out like this.
The NOS fork sliders are clear coated from the factory. I would not want them to turn out black/blue....
So with the front forks rebuilt it was time to get them back onto the bike. Before this could happen i replaced the steering bearings and seals.
The gaiters were somewhat a pain in the arse. I found a set of NOS gaiters however one was damaged from the previous owners storage methods. I spent hours trying to fix it without any luck and to be honest they would of be fine but to me I knew it wasn't perfect. I ended up buying and trying about 3-4 sets of aftermarket gaiters. The best quality ones are the polysports and an Italian brand called ariete, which are in the bike currently. Maybe in the future I'll come across a better set of NOS gaiters and replace them but for now they'll do.
After this I got to work on sourcing a new triple tree and clamp. I eventually found a NOS set that I snapped up and got everything mounted back up.
And to finish off the front end a set of NOS handle bars, bar clamps, levers and grips were added.
To be continued...
Next up was to sort out the rims and hubs.
Upon closer inspection of the original rims I discovered cracking in the front rim and the rear had a decent flat spot. The hubs appeared to be ok so they were removed, stripped of bearings and seals and sent to the powder coaters for blasting and recoating in satin black.
The hubs were then rebuilt with all new bearings and seals
After this I sent the reconditioned hubs off to a local motorcycle rim builder to have them reassembled and trued up using custom stainless steel 316 spokes, excel rims and anodised nipples. I opted for the excel takasago style rim without the writing / logo on the side so that they would look more OEM.
Very happy how they came up:
Once the rims were finished they were fitted up with Bridgestone rubber (x30 from memory)
To be continued...
Great work!
Works been really busy lately so haven't had much time to touch the bike. Anyways with the frame, rear shock, rear swing arm, front forks and rims sorted it was time to hunt down some plastics.
Everyone says use DC plastics and I did buy their radiator shrouds and to put it bluntly their sh!t. Whether I use them or not in the short term is to be seen whilst I hunt down some NOS originals.
I eventually came across a guy on the opposite side of Australia who had a NOS seat, fuel tank, rear mud guard and side panels which I jumped on. I then bought a OEM front from partzilla as they still make them or had stock.
To be continued...
Wow! Great work. Following closely.
Awesome work! Need to see the spreadsheet of all the expense
Pit Row
Spread sheet...
I'm not keeping track of the cost haha I don't want to know!
Well the frame is almost finished now just some finishing touches to add and it's onto the engine.
Recently I came across a guy in Sweden with two NOS radiator shrouds which are currently enroute to me.
Speaking of radiators... What an absolute pain in the arse they are. I've been searching for months on end for a set of NOS radiators to not avail. I can find the non filler side but the filler side I can't find anywhere. (IF YOU KNOW SOMEONE WITH A NOS FILLER SIDE LET ME KNOW!)
I spoke to Alex (the guru) and he told me he's running the Chinese replicas on his bike with good results. I looked into them and for the price I thought why not give them a shot....
Whist the quality was ok and the fitment ok with some fiddling around, I just couldn't live with the fact they're not originals so I went hunting for used sets that I'll hopefully be able to recon back to near new standard.
Chinese rad: made by gpi
My hoard of OEM used ones I've been gathering:
The two that might make the cut for Resto;
Also whilst looking for radiators I've managed to collect a complete NOS set of radiator hoses, radiator cap, all new mounting hardware etc...
To be continued..
Very impressed!! And your ability to find parts is amazing.
Finding Parts isn't as easy as it appears. I spend hours and hours at night when laying in bed searching around.
It does show to me however, the stuff is out there you just have to find it.
Small update.. My NOS radiator shrouds arrived today which basically conclude all the parts needed to finish the frame side of things off.
I also have all the parts needed now to have the engine built. Stay tuned for more pics and info to come regarding the engine. Ive got some real cool and rare goodies
Just another small update. Life and work has been really busy lately again but I managed to get a few hours in.
Working on the radiators first up was to have them vapour blasted.
Next up was to straighten the fins out as best as possible on each radiator as it had a few bent ones.
It's a very tedious job but they're looking alot better. Next up is to clean them all down and give them a good flush before painting.
I also cleaned up the kick start lever off the original motor. Here's a before and after:
To be continued...
Here are some pics of the progress of the engine:
Replacement crank cases vapour blasted:
Crankcases after abit of sanding to remove imperfections in the ally ready for painting:
Cylinder after some sanding (still working on it) has had a new sleeve fitted and honed up to suit the NOS standard size piston and ring
NOS clutch cover, power valve linkage cover, magneto cover, water pump housing and cylinder head:
To be continued...
Here's a few more goodies for the motor:
NOS clutch basket, hub and pressure plate. I also have all new springs, fasteners, plates and discs.
NOS crank assembly with genuine rod and bearings. Also have NOS standard size piston and ring for it
NOS power valve:
NOS flywheel:
To be continued...
Your ability to find NOS Yamaha parts for that bad boy is amazing! Nice job on the restoration; my first ever bike was a 1983 YZ250...always drooled over the red/white ones (ours were yellow/black until they finally switched to red/white in 1985)
Yes I always wondered why Yamaha made the US version yellow and black and the rest of the world white and red.
The front number plate on mine is off a USA bike and the yellow as I just can't find a genuine white and black one anywhere.
Outstanding work!
There are 1982/83 YZ490/250 front plates available that are fairly cheap, about US$35. Not quite the same style but close. I found quite a few bits are available for the bigger bikes by changing the last digit on the part number from a 0 to 1, as it gives you the part number for the white panels.
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