The finished article in place

Problem

Whilst rebuilding my 1973 Triumph Tiger 750 I was in search of a solution for the carb to airbox manifold rubber, which the previous owner had… not fitted.

Worse than that (i’ll ignore the chromed engine covers for now grumble grumble…) the airbox was originally designed for a straight twin carb arrangement, hence the T140 engine and frame numbers, but these had been blanked off and a large, unsightly hole had been cut in alignment with nothing in the centre of the airbox covers. This left a nice hole that didn’t line up with the now One Concentric Mk1 carb, and was too large to fit a standard Tiger 750 manifold rubber boot. Great…

The first attempt just to get the bike on the road used an old gaiter glued to a lump of rubber sheet. The results were less than satisfactory to say the least.

TPU?

One of many attempts

I was always hesitant to try printing in TPU since there’s approximately half of the internet that say “that’ll never work!”, “you can’t do that with a bowden extruder!”, “there’s no hope on an Ender 3!”.

Well clearly that was all rubbish and TPU apparently works absolutely fine on the standard settings found under Generic/TPU in Cura. The only modification I experimented with was to decrease the print temperature to I think 230°C. I think this decreased the stringing a little. I never had any problems with adhesion either, printing on blue painters tape because I found the textured build plate on the Ender 3 V2 original bed to be too sticky for when I print in PETG.

On of many test fits

It was a complete paint to measure this system since the airbox sits at an angle to the carb, and both holes are different sizes, and are misaligned, so it took a few attempts to get something I was happy with. Also the airbox wall thickness varies from top to bottom because it’s not a section that was ever meant to have a hole in it…

The part was designed to be a tight fit onto both sides of the airbox for a nice seal, and is clamped to the carb with a nice little hose clamp. I left the extension ring thing I don’t know the actual purpose of on the carb to give it something more to clamp to. It seems to work quite well.

le cross-section

At the very least, by 3D printing the part, it let me smooth out the very strange transition between the angled airbox and the carb as much as possible. I’m sure this makes next to no difference in terms of power or performance, but it makes me feel good so sod it.

In place

TPU and Fuel?

Yupp. That's a rubber grommet from TPU sat in some fuel

As far as I could find online, TPU will absorb some level of ethanol, up to 6%, but I couldn’t find any mention as to how this affects the material’s physical properties at all.

I’ve placed a couple of rubber grommets that I also printed in TPU for replacing a few on my CX500 (the material works very well for this as well by the way, strongly recommend!), in a jar of E10 fuel to see what happens. At the time of writing its been about 4 months and I haven’t noticed any deterioration. Looks hopeful that this should work as a long term solution.

Fin

Well that’s about it. I would strongly recommend considering printing hard to find replacement grommets from TPU. Especially for things like mounting grommets for plastic panels where there’s no load concerns.

Also printed a grommet for the side panel as the ones I bought online were way too big to fit

Hoenstly my only comment about using it for this is that the TPU ends up with a plastic like surface but with the flexibility of rubber. So you wouldn’t be able to use it for foot pegs or anything else relying on grip sadly. I don’t know if there are any other filaments that would suit this purpose better?