Restoring compression and where to get bits these days?
Posted: Sat Jan 24, 2026 7:20 pm
Hello all, long time no see. I'll post my progress on this whether you like it or not, but hopefully some of you can help me out along the way. Back in September I was happily riding around the Eifel region of Germany when I started to hear quite a loud tap being bounced back at me off the road when cornering, then a few miles later while doing about 50MPH in 4th gear uphill the bike started to slow down and I'm told quite a lot of smoke came out the exhaust. I pulled into a petrol station a few miles later and the bike sounded terrible running on one cylinder. I limped it back to the hotel and found that it seemed a lot better above 4K RPM, and at 5.5K it seemed relatively smooth. Thankfully it was the end of the holiday and, after it coughed into life on Saturday morning, I rode it all the way home to Worksop in the UK on motorways where it seemed somewhat happy. The Eurotunnel was an interesting experience but at least it got up the ramp at the exit!
I hoped it would be something simple like a hole in a carb diaphragm but they seem fine, although the plastic slides certainly rattle around in the carbs now. Finally today it was warm and dry enough to take the head off and.. well..
This is a replacement head from a TDM850 and I'm not sure what miles it has done, but I still find this a very weird failure. I've never heard of this happening on these engines before? I still have the original head with soft inlets, I may choose to replace all the inlets in that but at £60 each they aren't cheap at all. I may remove this valve and see what the seat looks like. I may just get another TDM head and use that. Not sure yet.
Thankfully the bores seem to be fine though she really chews through oil on the motorway:
The hone marks are still mostly visible though the thrust side is glazed with some small vertical scratches on the left bore. I would like to fit OEM 1.0mm oversize pistons and rings but I see these are basically unobtainable nowadays, only 0.5mm items remain and that's at CMSNL so just a few boxes of new old stock. Does anyone have any leads on finding 1.0mm items? Anywhere in the world is fine, I can wait for shipping. I'm not really interested in aftermarket high compression slappy forged jobbies, this is a road machine.
So yeah, this is where I am with it now. I'll try and measure the bores to see if 0.5mm oversize is enough, and keep an eye out for some reasonably priced inlet valves but I doubt I'll find them any cheaper. The cam caps are also a little torn up on this head so I'm not sad to give up on it. Also if anyone has a personal recommendation for getting a rebore done in the UK then let me know, I've never had to do that so far. The closer to Worksop the better, obviously!
I hoped it would be something simple like a hole in a carb diaphragm but they seem fine, although the plastic slides certainly rattle around in the carbs now. Finally today it was warm and dry enough to take the head off and.. well..
This is a replacement head from a TDM850 and I'm not sure what miles it has done, but I still find this a very weird failure. I've never heard of this happening on these engines before? I still have the original head with soft inlets, I may choose to replace all the inlets in that but at £60 each they aren't cheap at all. I may remove this valve and see what the seat looks like. I may just get another TDM head and use that. Not sure yet.
Thankfully the bores seem to be fine though she really chews through oil on the motorway:
The hone marks are still mostly visible though the thrust side is glazed with some small vertical scratches on the left bore. I would like to fit OEM 1.0mm oversize pistons and rings but I see these are basically unobtainable nowadays, only 0.5mm items remain and that's at CMSNL so just a few boxes of new old stock. Does anyone have any leads on finding 1.0mm items? Anywhere in the world is fine, I can wait for shipping. I'm not really interested in aftermarket high compression slappy forged jobbies, this is a road machine.
So yeah, this is where I am with it now. I'll try and measure the bores to see if 0.5mm oversize is enough, and keep an eye out for some reasonably priced inlet valves but I doubt I'll find them any cheaper. The cam caps are also a little torn up on this head so I'm not sad to give up on it. Also if anyone has a personal recommendation for getting a rebore done in the UK then let me know, I've never had to do that so far. The closer to Worksop the better, obviously!
