Bought the cheapest M1 on the net - help with ID and specs?

[rentedmule]

Updates!

I threw the parts cannon at the brakes, but finally figured out the correct combination of adapters, rotor sizes, and grinding to make things work:

I used an A2Z adapter I bought 20+ years ago for the Manitou on my old Kona Scab and needed a Hope 225 mm rotor
20250129_195910 by Rented Mule, on Flickr

I needed to modify a Hope +20 mm adapter on the rear with a 203 mm rotor
20250129_195926 by Rented Mule, on Flickr

I'm getting close to wrapping up the mock-up phase now:
20250129_200138 by Rented Mule, on Flickr

I do need help with chain guides though - I've never used one. I'm hoping to run my XTR DH cranks
20250129_200256 by Rented Mule, on Flickr

I acquired this MRP guide, but my frame doesn't have the universal chain guide mounting provisions. The back of the MRP guide looks like this:
20250129_200356 by Rented Mule, on Flickr

I'm assuming I can't use this without some sort of adapter. Any suggestions? I think there are ISCG adapters that can be sandwiched between the BB and frame - is that what I need?

Just for kicks, I weighed the setup as-is and it's well under 40 lbs. The tires are mounted with standard tubes at the moment but I'll be remounting them tubeless once I build up the patience and arm strength required to wrestle with dismounting and mounting them again. I'm searching for local painters now too and will finalize a colour choice shortly.

 

[Cloverleaf]

That chain device is the one for BB mounting rather than ISCG which has three bolt holes. The bit on the back that you're looking at there is just two grub screws to bite into the outside of the BB shell and prevent it rotating then the lower arm inevitably hits the ground. I think you're missing the posts though for mounting the C-plate to the boomerang though by the looks of it, as there should be three countersunk bolts going into these posts which then slide through the machined stubs on the back of the boomerang which you then fix in the correct place with the grub screws.

 

[Cloverleaf]

Found some pics of what the stubs look like:

https://www.retrobike.co.uk/threads/mrp-chain-device-tensioner-world-cup-pro.101808/

 

[rentedmule]

I was hoping you'd chime in, thanks! It looks like I'm definitely missing some bits. A while ago when I tried mounting the plate with the rollers, I must have had it in the wrong position because that raised bit the grub screws you referred to was hitting the frame. I'm going to pull the crank again and mess around with it. I wonder if MRP still sells though posts... I bought replacement rollers from them, so maybe they still have spares.

Appreciate the help!

 

[Cloverleaf]

I was hoping you'd chime in, thanks! It looks like I'm definitely missing some bits. A while ago when I tried mounting the plate with the rollers, I must have had it in the wrong position because that raised bit the grub screws you referred to was hitting the frame. I'm going to pull the crank again and mess around with it. I wonder if MRP still sells though posts... I bought replacement rollers from them, so maybe they still have spares.

Appreciate the help!

Ha, no worries, I'm glad my useless information is useful to someone! The posts are just bits of internally threaded aluminium bar so fairly easy to replicate. I could really do with a World Cup type 2 device like you have there for mine as it's what I originally had but the Slalom 1 just clears with a 118mm ISIS BB so I've left it for now. But if I had a type 2 I'd measure the post diameter for you. Instead you'll need to reverse engineer it and measure the size of the holes in the boomerang with some verniers. If you get the right size aluminium rod (even if it's a little under the grub screw will still hold it all in place), it would be easy enough to cut the rod it to length (about 20mm IIRC) and then drill and tap some holes in one end for the countersunk M5 bolts (possibly M6, but I'm pretty sure they were different to the bolts holding the rollers together). As always it's worth asking about the spares but I suspect they're long gone as I seem to recall they were fairly difficult to track down even back when they were contemporary parts.