35mm exhaust on an R100T

[Maarten]

I recently bought a fairly cheap not running 1980 R100T.
Now that I have it running I noticed that smoke and black liquid was spitting out towards the front.
The previous owner has put a nice aftermarket exhaust on there, but it seems to be the wrong size. The outer diameter of the tubes that go into the cylinder are 35mm and from what I can find it should be 38mm.

2025-05-30 11_13_50.jpg (138.66 KiB) Viewed 2020 times

2025-05-30 11_14_26.jpg (71.97 KiB) Viewed 2020 times

For budget reasons I would prefer to stay away from buying a new exhaust system. I know that it may have an effect on performance, but I doubt it would be much since it only looks to be a short piece at the start of the exhaust that is narrower.
Now I’m looking for the best way forward and could use your help with that.
The options that I can think of are the following, but I’m open to suggestions.

A. There are Airhead models with 35mm exhaust if I’m not mistaken. Would a clamp ring and compression ring of one of those models fit in my R100 exhaust nut? Or do I need an exhaust nut from that model as well and would that fit my cylinder?
I think an R65 has 35mm pipes, but not too sure. When looking that model up on the realoem website I noticed it only has a clamp ring and no compression ring. Also the part number of the nut is not the same as the R100 one.
If something like this would be possible it looks like the easiest solution.

B. Get some stainless steel ID35mm OD38mm tube, cut off a certain length and heat it up with a torch and try to shrink fit in on the start of the exhaust. Would this be a proper and airtight solution and would this be even achievable?

C. Find a 35mm clamp ring and have a special compression ring made to fit.

D. Cut off the 35mm ends of the exhaust and weld 38mm tubes on there, but my weld will never be as pretty as the other ones.

Any advise on how to approach this best without breaking the bank?

[barryh]

I think an R65 has 35mm pipes, but not too sure.

Yes it does but the spacing between the Exhaust ports would be different because the R65 cylinders are shorter so it's to hard see how this exhaust was made for an R65.

[Maarten]

That’s a good question. They seem to line up pretty well. Maybe because the headers are mounted to the splitter with a spring so a little bit of rotation might make up for it. I’ll keep a good eye on in when I get my exhaust nut tool in and unmount the stuff.

[Kurt in S.A.]

Not sure if you have experience in removing those exhaust nuts. But if the previous person didn't use a good amount of anti-seize, taking them off can be a problem. If you start to remove them, and then it becomes harder to turn the wrench, stop and cut the exhaust nuts off. Hopefully you will not have galled the threads much.

Kurt

[SteveD]

Option D. Have a crack yourself if you can. If it's not what you'd want, pay someone to do it better.

Or, sell the exhaust and buy something good and proper.

Check that you haven't got the left valve cover on the right.

[Rob Frankham]

On a R100 engine, the exhaust ports on the head will be 38mm or 40mm depending on the age of the engine and the model it was fitted to. a 35mm header will not seal unless there is some sort of additional packing in place. Only the R65/R45 models have 35mm headers.

The seal at the header is formed by a pair of tapered cast iron rings, one with a internal taper, the other with an external taper. The externally tapered ring is split. These rings are sandwiched between in rose nut and the head so that, as the nut is tightened, the split ring is compressed tightly around the header pipe.

On possible solution to your dilemma would be to source split sealing rings and rose nuts from a R65. I'm not sure how well this would work as the R65 models only have one ring, the externally tapered split ring, and the internal taper is in the rose nuts so you may just achieve a seal that way.

As a 'helpful comment... no offence intended' I wouldn't run with an exhaust like that. It may look good or it may not but the length of the header up to the silencer and the provision of crossovers is critical for the carburettor jetting and the running of the engine in general and this is very different to a stock exhaust. Even worse, in this case, the two headers look to be very different lengths which will play havoc with engine balancing.

Another 'helpful' comment... I think you should check the carbs. The bike has flat top carbs (as opposed to 'dome top' carbs which have a raised dome and a central boss). As far as I'm aware, flat tops were only ever used on R45, R65 and R80 engines. I could be wrong... but... it's well worth getting the carb model numbers and checking them against the listings to see what model the carbs actually came from. I would also wsnt to be sure that it really is a 1000cc engine (with 1000cc cylinders). The actual capacity should be cast onto the cylinder flange near the crankcase. 10 for 1000cc, 8 for 800cc etc...

Oh and, since it was mentioned above, the rocker covers are definitely on the wrong sides... or to be fair, the one we can see is... the fins should be pretty much parallel to the ground although it makes no difference to the working of the engine.

Sorry to seem a bit of a wet blanket. No doubt it will make a perfectly good bike but I think there's some work to be done.

Rob

[Kurt in S.A.]

The Bing manual shows the flat top carbs as model 64-2. In their table, it shows they were used on R45, R65US, R80, R80US, R90/6, and R100/7 engines. The R45s had throats of 26 and 28mm while the rest had 32mm throats.

Kurt

[Rob Frankham]

The Bing manual shows the flat top carbs as model 64-2. In their table, it shows they were used on R45, R65US, R80, R80US, R90/6, and R100/7 engines. The R45s had throats of 26 and 28mm while the rest had 32mm throats.

Kurt

Thanks for that. I think it may be a 'US thing. I've never seen a R100 with flat tops in the UK and the lists I have (which are mainly Euro listings) don't show it. Come to that, I've never seen a /6 with flat tops... I was under the impression that the first use was on the R45/R65 range in 1978. Just goes to show, there's always something new to learn...

Rob