Rebuild

In a way this is my favorite part of the restoration. Making old parts look like new is a lot of fun.

Make sure that every effort is made to ensure that the parts are functionally correct. When assembling the smaller sub-assemblies refer to the parts manual often. A great restoration is in the details. So make sure that every nut, bolt, screw, bushing, bearing and seal is where it is supposed to be.

One of the most important ingredients of a great restoration is patience and determination. You are working on a very old piece of machinery. It may have been sitting in a barn or even outside for many years. It is not going to be easy to take apart. You will face many frustrations and difficulties during this process so please take your time.

When you start to tear down the bike you’ll discover rusted or stripped nuts and bolts, bent, broken parts and stuff welded to places it should never have been. These parts can be very difficult to remove and it’s easy to damage parts that are hard to replace.

This is a time to use your ingenuity, creativity and patience. And please use the proper tools so you don’t break anything or injure yourself!

Frame

The project started by stripping the paint off the frame. First using paint stripper to remove as much of the three layers of paint that was possible. The top coat was a nice black, but there were a lot of bumps and chips. Under that first black layer were a few coats of pink!! Finally the base layer was the original Yamaha black paint.

After the paint stripper, I used one of those inexpensive sandblasters in an attempt to remove the paint from some of the harder to reach areas. I used a small compressor and I did this during the very hot and humid summer. Not the best time to sandblast outside, the sand stuck to every bit of exposed skin.

It took forever to get it all off. Then I used a wire brush attachment on my drill. When the paint was all off it was wiped down with lacquer thinner to make sure there was no grease, loose paint or dirt anywhere.

A kickstand is not needed on a motocross bike. Most motocross bikes have the stand removed before the first race. So why leave the kickstand bracket on the bike? While cutting it off will only save a few ounces I wanted my bike to be a race bike not a trail bike. Hack that thing off. Then use a grinder to roughly shape it and use a file to smooth out all the rough spots.

On the 1980 YZ125G the CDI black box is mounted on a bracket welded to the steering stem. On the 1981 YZ125H they moved it under the seat. With the black box mounted under the seat it is out of harms way and the frame looks cleaner.

I used a tap to clean the threads on all the tabs on the frame. This cleans out any rust, straightens the threads and makes assembly that much easier. It’s a good idea to do this before you paint. Then again after the frame is painted.

I painted my frame using spray cans of PJ1 Gloss Black epoxy paint. It came out really good considering the use of rattle cans.

The preparation work took a really long time and was frustrating. If I ever restore another bike I’ll bring the frame to a professional to have it sandblasted and painted.

The paint cured for 2 weeks before I started the assembly.

I installed new bearings and races in the steering stem. Most of the rubber grommets were replaced with NOS Yamaha parts and the old ones were cleaned with lacquer thinner and sprayed with Silicone spray.

Suspension

The YZ125G was designed for a rider that weighed between 140 and 155lbs. I may have weighed that much when I was 16 years old but at 47 I weigh significantly more. The standard front forks were 36mm in diameter which were okay for a light rider, but would flex badly with a heavier and/or faster racer on board.

I located a great set of forks from a 1982 YZ490 which are 43mm. This is a bolt on modification when you use the YZ490 triple clamps. The heavier YZ490 fork springs seem to be just right for a 180lb racer.

The fork leg above is before and the lower one is after. A scotch brite pad and some elbow grease got the fork leg cleaned up and then some Simi-Chrome metal polish gave the legs a nice shine.

The paint was stripped off of the triple clamps and then re-painted gloss black. The steering head bearings and every nut and bolt on the triples were replaced with new hardware.

Brian Rivers recomended that I use the swing arm from the 1981 YZ125H which is approximately one inch longer than stock. This should help to restore the geometry of the bike . The YZ490 triple clamps bring the front wheel slightly closer to the motor. The added length of the swing arm should compensate for this.

When you use the 1981 swing arm you should also change the brake stay arm and brake rod to the 1981 parts, because these are longer also.

Originally I was going to strip the paint from the swing arm and polish it. After spending 2 or 3 hours attempting to remove all the paint from the swing arm I decided that it would be better and easier just to paint it silver.

Once the paint cured Ken install all new bearings and seals. On a vintage bike this is an area that usually has been neglected for many years. Worn bearings will make the bike handle very poorly.

I cleaned and polished the swing arm pivot bolt,nut and NOS chain pullers. The chain buffers were replaced with new ones.

I used a 1981 mono shock with a shock spring from a 1980 YZ465. The heavier spring was designed for a heavier racer and should keep the suspension balanced front to back. Of course shock body was polished and the lower shock bearing and clevis pin were replaced with new parts. This is another area where neglect causes handling problems.

Wheels

This is something that some people will not put much thought into. But wheels are one of the most important parts of a motorcycle. If you are going to race your bike put some effort into the wheels. Most vintage bike spokes are rusted or seized which make it difficult or impossible to adjust or tighten them when they loosen. If you do nothing else with your wheels at least make sure that you can adjust the spokes.

I wanted to use the Gold rims for my bike because I like the look. If you can afford to buy Excel rims great. My hope was to restore my bike on a budget so used wheels were found on E-bay.

I forgot to take photos at times while working on my bike, so some of the step by step pictures alternate from the front to the rear wheels.

The wheels were in pretty good shape. But I wanted the wheels to work excellently and look great, They were disassembled completely. Then paint remover was applied to the hubs using a spray can .

Once most of the paint was removed from the hubs they were cleaned with a scotch brite pad to remove the loose paint. A small metal file was used to remove any small imperfections on the hub. The bearings and seals were removed from the hub before painting. Then cleaned with lacquer thinner and painted with PJ1 flat black.

It is easiest to install the new wheel bearings and seals at this time. Don’t skimp on replacing the wheel bearings, they are the cheapest hop up modification you can make to your vintage bike. It takes a lot of power to turn a bearing that is seized, frozen or full of dirt and grime. New bearings work so much better and they are cheap, much less than a hot pipe or porting.

A set of front and rear NOS spoke sets was purchased and then polished individually by hand using Simi chrome polish. The anodized rims were cleaned with lacquer thinner and steel wool to remove all the stains and paint spots on the used wheels. I laced the hubs carefully using an assembled wheel as an guide.

On the front of the YZ be aware that the rim is supposed to be mounted only one way. This is because the spoke holes in the rim are drilled for the conical hub. Unfortunately I didn’t realize it until after they were laced wrong.

As you see in the photo I used small twist ties at the cross points of the spokes, this helped a great deal to align the rim and install the spoke nipples.

I screwed the nipples in until the edge of the nipple aligned with the last thread of the spoke. This centers the wheel in the hub, then the spokes were slowly tightened a quarter turn each as I went around the wheel. Be patient with this part it will save you time later.

I don’t have a truing stand so the forks were used for the front wheel and the swing arm for the rear wheel. Just install the wheel on the axle and spin it. Use sharpie to mark the high spots and adjust the spokes until the rim spins true. I did this many, many times until they were just right.

As you can see from the photos the finished wheels were well worth the time and effort.

Motor

The motor of a vintage motocross bike has usually been used, abused or neglected from storage. It is the motive power that makes your bike go. This is an area that I spent a considerable about of time, effort and money.

I had all the lower end work done by Ken Napolitano ( mx135rider@aol.com ). He offered excellent advice and was willing to do what ever it took to get the job done correctly. He suggested that every bearing and seal in the lower end be replaced. So all the bottom end seals and bearings were purchased from Hunter Motorcycle ( http://www.huntermotorcycle.com/ ). All of them are still available from Yamaha.

An NOS right center case and an NOS Crank were purchased on E-bay. I had a decent left center case that was carefully stripped and painted. During the assembly Ken found that the tabs for the shift drum were broken off! So it was off to the Speed & Sport Yamaha web page to buy an NOS left center case.

Ken inspected the gearbox carefully and then assembled the entire lower end. He was nice enough to let me hang around and watch him work. Do not attempt to rebuild the bottom end unless you have some experience and the proper tools required.

I was fortunate to find a brand new clutch basket in one of many lower ends lying around. This is one of the rare parts that are next to impossible to find. To match this an NOS inner basket was installed at the same time. The metal clutch plates were within tolerances so they were reused and new fiber clutch plates and springs were installed.

Another area of concern on the 1980 YZ125G is the long front cylinder stud. These would come loose over time and the threads would become elongated. The result was an air leak which would lead to an engine seizure. Make sure that the threads are not “loose” and loctite the stud into the cases.

With the NOS clutch cover found on E-bay I had virtually a brand new lower end. I was lucky to find these NOS parts, but remember they were found over the course of three years. Most people would not have the patience to wait that long to build their bike.

I found a really good used cylinder. It had been ported by Eric Gorr at some point in time. It needed bore job so it was sent back to Eric Gorr (Forward Motion ). Wiseco piston kit was purchased along with the machine work and they charge reasonable prices for their work and parts.

You can still find used cylinders and they are not expensive. It is not necessary for them to be on the stock bore for a racer. The cylinders have a steel sleeve and these can be replaced at a nominal cost, but please have this done by the most reputable place possible. I have seen replacement sleeves pressed into the cylinder incorrectly and this could cause problems.

Make sure that the bracket for the clutch cable is still intact. This is cast into the cylinder and can be broken.

Cylinder heads are another area that you need to be careful about. Since these bikes had a few issues that caused engine seizures the heads received a great deal of abuse. I purchased a used head from Bike Boneyard ( www.bikeboneyard.com ) which turned out to be from a 1982 YZ100. This was also sent to Eric Gorr for modification to fit the YZ125. He also offers welding and reshaping services for the combustion chamber for damaged heads.

The machine work they do is first class and I would highly recommend them for any work.

The only mistake I made was in asking Forward Motion to sandblast the cylinder and head. I was not clear in stating exactly what I was looking for, they did a very good job of sandblasting if you were to paint it black. Unfortunately I was looking for the early Yamaha factory bike look. So I had a local machine shop sandblast all the paint off of the cylinder and head. The result was exactly what I was looking for.

Once it was totally sandblasted the cylinder and head were painted with high temperature clear paint. I was told that the bare aluminum would start to turn white with age.

I used a 1982 YZ125 Kickstart lever on the bike. It is made out of aluminum and is much lighter than stock. It also received a clean up and polish. It was re-assembled with a new clevis pin, spring, ball and dust cover.

The front motor mounts also recieved the clean and polish routine.

Exhaust Pipe

It is difficult to find an OEM exhaust pipe in good shape. They tend to rust quite badly and get dented on the header pipe.

A Retro Rocket Pipe from Vintage Iron ( http://www.vintageiron.com/ ) was purchased for the bike. This thing looks just like a Yamaha Factory pipe! Unfortunately my pipe was in the garage during a damp two week period of rain and it began to rust. So a scotch brite pad was used to clean all the rust off then it was wiped down with lacquer thinner and painted with high temperature clear paint.

Many racers spray their natural steel pipes with WD 40 and scrub them with a Scotch brite pad after every ride in an attempt to keep them from rusting. I hope that the clear paint will free me from this maintenance chore.