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The adapter I bought was M26x1.5, but I reckon you have a different fork assembly. My stanchions measured up at 31.5mm, so I think you have the earlier type. According to my Mick Walker book these were only fitted to 100, 125 and 160 but I know this book has some inaccuracies.

Thanks for that info Ian, it does seem that I've probably got the correct early forks.

The bike was dated as a 1959 model by Mick Walker and Nigel Lacey's list the 30mm fork seals on his website as fitment for early 125/175 models.

Incidentally, some measuring and searching on eBay showed up suitable seals for £8 delivered, as they appear to be a standard 40x30x7mm size. The damper O-rings took a bit more searching for, but seem readily available (nearest size is 24x17x3.5mm), a pack of 10 again sourced from The Bay of Evil. Grand total for everything about £13 all in

I'll keep everyone updated on progress once I have all the parts to hand.

Cheers, Jason.

I don't recall any O-rings inside my forks. 30mm forks must be a different arrangement but I've never seen a picture of what's inside them.
Ian

I found some info in the 160 Monza jr. spares catalog. This bike obviously has fork with 30mm stanchions, and indeed no O-rings inside. May be it is similar to the early 125/175 type. I can send the pdf (~9MB) by email, on request.

chers Hans

Here's some pics of the internals of my 30mm forks. The O-rings is the one at the end of the damper rod.

The fork internals look pretty much the same as the 31.5mm forks with the exception of the groove cut into the damper piston. I’ve seen that on Japanese forks but it’s not meant for an o-ring rather a hard plastic split piston ring. I’m not sure about your forks, weather that groove is stock or not but I can’t believe that an o-ring there is a good idea.

Check the tubes for straightness by placing them side by side and rolling them together, if you see a gap anywhere they are not straight.

Aaron

It does look pretty similar to my 31.5mm forks, but my damper pistons are plain with no seals.
The parts illustration Hans posted is rather different, but matches the description in my Mick Walker book, which says there is a removable bush in the top of the slider. Item 470 in the picture I think.
Was there another 30mm fork?

Hi Aaron, interested in your reply and as to why you think that an O-ring is not a good idea? There is currently one fitted and the dimensions appear to match a common size?

I had assumed the purpose was to provide an oil seal to control the flow of any oil which made it's way up the inside of the rod and guide it back down, as well as providing a soft contact surface between the damper and inside if the stanchion? However, I'm no suspension expert that's for sure and am happy to stand corrected.

Seems peculiar that the groove would have been created on a whim and fitted with an O-ring if it wasn't for some purpose?

Incidentally, the top fork caps also have a similar groove in them, although neither has O-rings fitted currently. However, this is normal on all modern Ducati forks I've worked on in the past, so it maybe that they've been removed and just not replaced?

O-rings are not meant to provide a sliding seal that’s what oil seals are for, they are a static seal. A soft rubber sliding surface contacting a hard metal surface which is not even ground and polished and also soaked in oil is a bad idea. If that groove is standard which I believe there is a good chance it isn’t I doubt it was meant for an o-ring rather a hard plastic split ring.

There are many forks that use a piston ring on the damper rod but I’ve never seen an o-ring there. Google “damper piston ring”

Just because something is a certain way doesn’t make it correct, especially when working on a 60 year old motorcycle.

Good luck with sorting out your forks, hopefully someone on here has worked on those particular forks before and can chime in.

Aaron

Hi Jason,
You mention a groove in the fork top nuts, but no "O" ring. In fact I think there should be an "O" ring there. On my 31.5 mm forks one top nut "O" ring was damaged, you have to feed them in carefully when replacing the top nuts. Oil was appearing around the fork top nut.