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walkahz

Yamaha 1980 Monoshock Upgrades/Mods

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Hi All
As some might be aware Ive got both a 125 and 465 G model Yamaha with the original non linkage yamaha monoshock.

After A few rides on each bike like many older machines the suspension is......lacking. Especially when carrying my 95kg behind.

Forks are relatively straight forward and I will likely throw some cartridge emulators and suitable sized springs which should bring them to an acceptable level.

Rear shock is more difficult.
For those unfamilar with these setups the shock is mounted up the spine of the bike under the tank and there is a remote res mounted in the airflow under the tank.
Im currently pulling apart a spare 125 shock to have a look at and see what im in for. A lot of the period parts for these shocks really focused on finned reservoirs to keep the oil temps down. These run a piston for nitrogen separation.

What I am thinking is to make up a custom res that is finned (if I can be bothered for bling factor) and possibly for bonus points add a compression adjuster from a more late model bike to get some sort of adjustment there as currently only rebound is adjustable. (not sure if this is worth the extra stuff around though). Rubber res hose to be replaced with braided line. I have a lathe and mill at home as well as contacts with CNC gear so really doing this as more of an interest project more than anything. 

Rear springs are progressive stock but genuine stiff springs are near impossible to get so i will likely get a twin spring setup to suit my weight (465 already runs a twin spring that is just too soft so that might make its way to the 125 and the 465 will get a beefier unit.) Although i have found that tecnik suspension in sydney sell progressive springs that may be suitable so i will look into this as well.

 

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Got stuck in after I had made to make a set of soft jaws to suit.

Pulled the spring off and let out the nitrogen and removed the res.


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There was no bumper left and fell out of the plastic shield in pieces!

These early shocks have issues with wear as they were not hard anodized stock. I will look into getting them hard anodized depending on the price i get, i know of a few places here in Melbourne who could do this.
 

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A mates been rebuilding a YZ125 like yours & i know he is ocd about everything so i'll get intouch with him, he lives in Brighton.

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Been working on stripping these down some more. Decided to pull apart the 465 shock at the same time as it is like riding a pogo stick.

Seal head removal was covered by making some spacers to fit on each side of the shaft. The whole lot was put in the press and pressed down squishing the rubber gland in the middle enough to get the circlip out with some wrestling.

This was the state of the oil that came out manky and black no good!

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The internals of the shock are also very different to a modern bike.

Only about 3 shims on the compression side and a deflection limiter and none on the rebound, Rebound seems to be taken care of by the holes on the step on the shaft with restriction provided by the pushrod runs through shaft to the adjuster on the end.

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If any one has tips for doing some basic valving on these old bangers let me know.

The body on the 125 shock and 465 shock are the same. Which is good because the body is shagged on the 465. There is a lip big enough to feel and see which will no doubt leak oil past the piston. The 465 shock also had no gas charge and hardly any oil left which is why it was such a pogostick. (probably also why its so worn but this bike has had a very hard life in the past)

The 125 body is in pretty good nick. Measured with a bore gauge and total change in dia is within 0.1mm and runout also 0.1mm which i think is pretty good for a 39 year old part.

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So plan at this stage is to rebuild 1 good shock for the 465 out of the parts of the 2. The worn out body I am looking at turning up a new sleeve and welding onto the head but need to get my head around how to bore a 46mm hole 300mm odd deep and keeping it straight round and smooth (need to make up a beefy boring bar!)

Initially will keep the body raw but once ive got a bit of a batch of them i might get all 3 Hard anodized at once (price for 1 was at minimum charge so more economical to get a few done)

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Love your work!

I'm curious what diameter shaft and piston? Did they use a bladder or piston in the reservoir?

Takes a bit of research but I have had good luck with matching up Honda genuine parts (they list and sell  individual components and not just the assemblies for their bikes) for bumpers, piston rings and bladders for my KXs and DR. For seal heads I've mail ordered the KYB subcomponents from the states to rebuild the seal head assemblies (bushings, seals).

BTW, how did you end up going with sourcing shims? Did you find someone local?

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Pretty sure they used a piston, remember a mate pulling 1 apart & it had a piston in it.

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49 minutes ago, BearMX said:

Love your work!

I'm curious what diameter shaft and piston? Did they use a bladder or piston in the reservoir?

Takes a bit of research but I have had good luck with matching up Honda genuine parts (they list and sell  individual components and not just the assemblies for their bikes) for bumpers, piston rings and bladders for my KXs and DR. For seal heads I've mail ordered the KYB subcomponents from the states to rebuild the seal head assemblies (bushings, seals).

BTW, how did you end up going with sourcing shims? Did you find someone local?

14mm shaft compared to 16 on most modern bike. 46mm pistons.

it is a piston reservoir, Im thinking of converting to a bladder but not fixed on it at this point.

 

Thanks for the honda tip, Ill likely only need it for the bump stop as I can buy aftermarket 1 piece seal heads which i will use in place of the stock multi piece setup.

 

For shims Technik in Sydney list on the website lots of shims so i will probably give them a go!

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14mm kyb should be the early CR500, XR600 etc. I've got a new one i bought for the DR650 that I haven't fitted yet if you want to check it out.  They're cheap as well for OEM (typical Honda good value) at under $20.

46mm piston is big for that time, a lot of the early Honda and Kawasaki single shocks were 40mm. Its good in theory as the bigger piston can have bigger flow for the sharp hits and the shims don't fatigue as much as they don't need to deflect as much. I think around 92 the cr125 ran kyb with that size but I'd need to check. If it is there's a good chance piston ring will be the same. The rings last for ages on the kyb but if you can get one at Honda parts ease and price its worth it.

I'd stick with the piston reservoir unless the piston or bore is damaged. I personally prefer the bladder as its easier to service but some say they can blead gas into the oil over time.

 

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