I have recently found a 175 barrel in reasonable condition and as such I have started to put together a 175 conversion.

I understand they are more or less the same except that the 175 does have a few differences.

My main concern is the oil delivery system.I think that the 175 pump has different gearing inside it compared to the 125 pump. I assume this is to deliver more oil to a bigger cylinder.
I can't seem to find anything more than one forum post about this on the entire internet, and it looks to be correct.
Therefore I currently assume that the 125 pump is not adequate and that I need to find a 175 pump assembly to not destroy the top end.

I also understand it needs an extra clutch plate and an extra friction plate. 5 plates will work for a while, but it really needs 6.
However I cannot source another rubber cushion for the extra plates.
Although I now believe that the existing cushions might be the cause of the clutch drag that I can't seem to get adjusted out.
I am thinking of leaving them out to see what happens. Would this be a bad idea?

As always I appreciate your input.
-Stoyan

I have both a DT100 and a 175mx. Both pumps have exactly the same gear reduction ratio of 40:1. Both have the same min pump stroke / output and both have the same spec output at max pump stroke.

Your 125 will be no different.  It may have a slightly different pump cable setting, but setting the cable to put the pulley lift cam slightly closer to the lift pin at idle should fix that.

Edit.  A look at the good book confirms that the 125 pump is the same.  Only the cable pulleys are different.  They have different marks.  You are golden.  Just advance the pulley timing a little.

Junk those square section anti rattle o rings in the clutch.  They cause more problems than they are worth.

The 125 had a shorter clutch basket than the 175.  It's not simply a matter of adding extra discs. 

I've also seen some later 125's that appear to come with the 175 clutch from the factory. 

Thanks.

I was a little unsure as I assumed they were the same but CMS has the pump and worm shaft as being different part numbers.
If they are more or less the same thing I will make the slight adjustment and go with it.
2A7 cylinders are not so easy to find recently, so the less seizures the better.

O-rings are coming out. Saw another forum where someone was complaining that neutral is almost impossible to find when the engine is running. It sounded familiar.
Recently had issues with 1st dislodging its circlip causing all sorts of violent lurching and general fun, so its apart anyway at the moment.

I have a October 82 built Model of 81.
It appears to be a 2A8 frame number with a few 4J3 parts that seem perfectly compatible.
This looks like it might hold one more set of plates, but having never seen a 175 clutch assembly I am not certain.

Have a look at my 175 rebuild thread.  It is very long, but contains a lot of pics, many of which involve the transmission.  You may find a lot of info there.
www.yamaha-enduros.com/index.php/forums/...-a-grand-tale-of-woe

 

It does look like you might have the 175 basket.    I've seen that on the later models. 

Agree with tinkicker and MarkT, straight swap, you need the carburetor jetting from the 175, done this countless times, all the stuff they said about the clutch, yep, use the full width fiber clutch plates from YZ 125 or IT 175 , getting the stack hieght right you may need thin and thick driven plates to get it right

Clutch hub 'might be longer' on the 175 before you go trying to stack another fiber & steel in.
Check these partzilla links as numbers are different.
125 clutch would handle the power if the clutch is in good condition.
The basket numbers look the same on both bikes--just saying!!.
www.partzilla.com/catalog/yamaha/motorcycle/1981/dt125h/clutch
www.partzilla.com/catalog/yamaha/motorcycle/1981/dt175h/clutch

At the moment the assumption is that the 2A6 part is a simplified version of the 1W1.
I was aware that the baskets are the same. Which is why I thought it was worth a try.
Worst case I have some spare clutch plates +1 extra.
 
It appears the cylinder has become an amusing amount of guesswork given that it did not have a piston with it.
Its cleaned up reasonably well but there is no telling what size it is other than its not standard.
The rings I ordered didn't fit.1mm end gap.
A micrometer and T gauge makes it about 66.4, which is either a looser 0.25 or a tight 0.5 at a guess. (If anyone knows of a list of bore measurements)
I guess the only option is to buy more sets of rings to tell for sure...

The carb is a 24mm unit that I have a selection box of jets for, so that is ready to go.