1981 Yamaha SR250 Cafe Racer

Despite it being 42 degree's, I had a day off and I was determined to get some work done on the bike!

This is the standard chain guard. It is lame. I thought I could do something with it, though I do intend to make one out of ally sheet later. The white mark is where it will be cut.



Once cut and shut, and with a coat of bumper paint it looks much better. I drilled the rivets out and gave the steel support a trim with the grinder so it didn't hang over the edge and then gave it a coat of gloss black.



On the bike it looks pretty neat.



I also mounted a little ally box in front of the battery for the fuse box, relays, etc.



I finally settled on a position for the regulator to go where it's in the airflow but out of sight. I will make up a better triangulated bracket later to mount it, but for now it's okay sitting on a bit of flatbar.



The biggest job I did though was to start making the upholstery pan. To start, I drilled the holes to attach it in the seat pan, and layed some chop mat on top, roughly trimmed it to size.
I then pushed some m6 x 15 cap head bolts (that I'd ground the heads off of) through with a washer under each.



Then once the first layer of chop mat was impregnated with the resin, I stuck extra layers of chop mat over where the bolts sit and stuck them in with resin. Once this layer is dry I will glass another layer on to finish the piece, as I ran out of resin today.



This will then have foam stuck to it, which will be shaped, then I will upholster it. Yay!

Here's the upholstery pan lifted from the seat. After it's trimmed to shape, it will have foam glued to it which will be shaped and then upholstered. It attaches to the seat via 5 embedded studs.



So there's some updates for you! I don't get a lot of time to update this thread thesedays but I keep a build diary going on my local forum (Perth Street Bikes) and try and get back here to update you all when I can.

Did some more work on the seat today.

This is the upholstery pan trimmed to fit.



Now to foam the upholstery pan. Places like Clark rubber will sell you squares of seating foam, but at ridiculous prices. Those blue foam camping mats make great seat foam if you cut them up and layer them (about 3-4 layers) and are pretty comfy.



Then lay one piece of foam over the pan, coat liberally in contact gel (bostik contact adhesive is good for this) line it up, and clamp it down as tightly as you can.. Clothes pegs help with the edges. Once this dries completely you can add more layers in much the same way, then sand and bevel the edges with a sander and start on upholstery!

This morning I glassed in some aluminium plates on the underside of the seat to reinforce the mounting points.



Now .. I shaped the foam on the upholstery pan. Here's the seat with the shaped upholstery pan on. Yeah! I think it looks pretty awesome. The seats nearly finished.



Dumped it on the bike to have a look, it's not bolted up so it looks a bit wonky.



I'm happy with it .. so I can start painting the tail and upholstering the seat pan now!

I've also found an 18" rim for the rear so I'll be getting onto that soon.

mna i love this!!! your attention to detail and "the little things" is just awesome! you know i love it! nice work bud!!! nice work!

Looking great man. I think it'll help a ton to lift the rear of the tank 1/2"-1" to level out the bottom. This will kill the mini-chopper/standard look of the bike and help give a flowing line from the tank into the seat.

Great work brother!

I had the day off and got stuck into the bike. I had the magic red card in the letter box yesterday, so I picked up my rearsets this morning!

Here they are as they arrived..



I quickly dismantled them so I could modify them. I don't need the heel guards in my application and they look out of place, so off they come.



I cut the heel guards off and smoothed and radiused the edges, then reassembled them and looked at how to mount them on the bike. The bottom bolt hole on the hangers lined up on both sides with the stud hole for the original footrest hangers from the SR, so I bolted this on and used this as a reference for finding the right position for the pegs to suit me.
330mm off the deck felt right and everything cleared and the linkage for the gear change would line up so it was simply a matter of designing a bracket to triangulate the holes on the rearsets with the holes on the frame.



To design the brackets to reconcile those holes with each other, I used CAD (cardboard aided design) and bolted a piece of cardboard behind the rearsets and traced around it, to work out what shape it needed to be.



Here's the brake side, surprisingly needing only a bit of work to get it all going .. A pedal stop (the master cylinder used to do that), a return spring and a linkage rod is all it needs as the lever already pulls rather than pushes, score! The extra bolt hole in the footrest hanger will attach the pedal stop and return spring.



And on the gearchange side ..

To attach the gearshift linkage rod end to the gearchange shaft, I simply cut down an old gear lever, leaving just the splined hole and a stub to drill a 6mm hole and pop an m6x25 through.



I need to shorten the linkage rod and recut the thread on the end to accept the rod end .. I made the brackets out of some 3mm ally plate I had lying around, but will get the final ones laser cut in 6mm duraluminum .. These are just on there to help me work out the linkages. Even 3mm ally holds my weight though, surprisingly.



I also got my pod filter today. Posh are the same brand that made my speedo, nice quality stuff from Japan.



It's going to be a bit of a squeeze so I may need to move the ignition key barrel a bit further back but it'll get in there.

Also .. I am changing the tank to an RD400 tank soon!

Cheers,
Ben

More to come soon!

WOW!!! You are killin' it man! Very nice work. Super clean. Looking forward to see your finished product!

Great work, appart from the rear mudguard behind the engine, i think it looks like is not part of the bike. If it was me i would mount it closer towards the rear wheel. Just my opinion, lol.

A little update..
I made a little bracket to hold the return spring from 3mm ally. This uses up the extra bolt hole from the master cylinder that I couldn't remove.




Have done a bit more head scratching on the back brake (the front magnesium TLS drum from an early 80's Yamaha IT250 enduro bike). With a bit of advice from my tame engineer, I've worked out how to make it all work in it's new orientation on the back of my SR250 (another one is on it's way from the US to go on the front!).



I need to add a plate to the back of the actuation lever to mount the clevis fork for the linkage, to keep the linkage in line with the second pivot so it moves up and down in the same plane.

Simple .. but it took a bit of headscratching to figure out.

Despite feeling pretty crook today, I had a day off and I'm determined to get this bike done! I must perservere..

This cut down fender is really just a stop-gap until I can afford a nitroheads front fender, but even so it has to look right, and it was sitting a bit too high from the tyre, so I measured the height off the tyre and worked out that I needed to redrill the mounting holes 20mm higher.



Looks better already ..



This is how it sits on the forks now.



This is the new instrument panel I fabricated after relocating the ignition key (again) due to clearance issues. I like it more than the original .. Why didn't I just leave the key where it was and do this from the beginning?!



The view from the side, I was worried about it looking funny because its a bit loger to make room for the key barrel but with the bend in it the difference in length is neglibile.



More to come tomorrow if I feel better ..

Looks amazing lunar. Really cool stuff.

Well I got a bit of work done today with mixed feelings.

First up I got out the vernier caliper and started measuring up the SR 250 engine cases and XT 350 barrel and head I got from debaser (cheers mate), when the stud spacing was found to be too far off and the cam gears too far out of line, I abandoned any notion of being able to use the top end, and instead looked at whether I could use the sleeve to put it an SR 250 barrel and get an 86mm bore (up from 76.5mm!) but that was a no go as well.. unless I turn the outside diameter down at least 2mm (it's 100mm OD).

I'd rather go the 350 Warrior sleeve (84mm bore) in SR 250 barrel route that DuggyT from ThumperTalk devised to bore out an XT 250 to 312cc here : Vintage Frankenbike project!!! YZ/XT - ThumperTalk

It's a tight fit but it works. I think I would like to turn down the outside diameter of the sleeve by about a millimetre to leave a little more meat between the oil galleries and stud holes and the liner. I can't see that effecting the strength of the liner by much and I'd feel more comfortable.





This also needs the piston modified to accept the SR's 20mm gudgeon pin and the engine crankcases relieved to accept the larger cylinder liner into the cases.



Damn that's a thumper!

It's a lot of work but as I'm currently learning to mill and machine through a family friend in his workshop, it'd be a great project for me and I have a spare engine that I can fiddle with over time and the rebuilt 250 that's already in the bike so why not.

I have been laboriously drilling out the mounting brackets for the rearsets from 12mm aluminium. These will be drilled and countersunk so they slide onto the threaded studs that originally mounted the foot rest hangers, as well as bolting on by two M8 bolts. Solid as a rock.



I re cut the thread on the end of the gearshift linkage after shortening it and spaced it out off of the gear change shaft to keep the linkage running straighter .. still needs a few tweaks as it's not working properly on downshifts yet but it's getting there.



I also got my friend to help me machine out the axle hole in the IT 250 brake plate to the right size to accept the rear axle from the SR 250. Here's a pic of it on the back of the bike .. needs a modified swingarm brake torque stay and a modified linkage to be completed but it sure looks cool on the back ..

Okay a better day was had by all today and more work done on the bike .. though I realised today I am basically undoing the past couple of months work mounting and upholstering the seat. Oops.

As I posted earlier, the mounts on the frame had to be changed to suit the RD tank. I cut up an old SR tank and drilled out the spot welds to keep the mounts to attach to the frame, but first I had to drill out the spot welds on the frame an remove the old mounts. Drilling out spot welds is pretty easy. Centre punch the depressions where the spot welds are and drill a small hole, then counter sink the hole to roughly the diameter of the spot weld, and with a wiggle they should pop off.



And off they come! The holes will be welded up and the paint touched up (thankfully I didn't get it powder coated after all) and cut back so it shouldn't be noticeable.



I cut up an old SR tank today. Here are the mounts I salvaged from the SR tank stuck on the rubber bungs on the RD tank for mock up.



I also cut the rear mounts off the rd tank as I'm fabricating my own to suit the SR frame.



This is the profile and the look I was after! The bottom of the tank lines up with the bottom of the seat and follows all the way to the back of the bike. I
had to cut back the seat a bit (but not as much as I added to it before) The front of the upholstery pan will be changed as well as I'm going to add a lip at the front to cover the back of the tank where there is a gap.



The frame is marked where I need to weld the mounts on .. I really need to buy a welder! Does anyone have a portable set up?



It looks great, but there's still a lot of work to go!

Looks great man!!! That tank/seat combo is killer!

man that looks awesome!!!! just amazing work bud!

That tank, seat combo is bad ass! I had serious doubts that you'd ever get an SR250 to look like a cafe racer but you've proven me wrong every step of the way. ;D This thread is what first brought me to DTT in the first place when I got my 1980 SR250 a couple of years ago. I long ago gave up trying to cafe it and restored it to mostly stock. Now I'm inspired to revisit the SR250 next winter and turn it into a bobber. I'm definitely borrowing several of your ideas from this build though. The front fender and speedo panel for sure.

lunar_c said:

.......I think I would like to turn down the outside diameter of the sleeve by about a millimetre to leave a little more meat between the oil galleries and stud holes and the liner. I can't see that effecting the strength of the liner by much and I'd feel more comfortable..............

Click to expand...

If the picture above is what you're referring to in regards to distance to the stud galleys - it is fine. You are much better off keeping as much thickness in your sleeve rather than the barrel in this instance. Turning down the outside of your sleeve increases the risk of it cracking.

And I LOVE the RD tank on there - makes this bike look really good now

Spot on, hillsy. And lunar_c, remember that its the actual cast liner that is copping the pressure of the combustion - you want it as thick as possible (within reason).

This is a brilliant build, the bike will be a real pocket rocket once you get it on the road. Might take some fiddling with the sprocket sizes to match the final drive to the new engine characteristics, but that shouldn't be too hard. Looking very, very good man.

Cheers - boingk