With the "allen key method" to check belt tension, what kind of force should i apply to the allen key trying to pass it between belt and roller? Wild swearing and sweating? Mild profanities? Firm polite pressure? Light touch? Is the convention 5mm passes/ 6mm doesnt with used belts ? (900ie) Changed my belts 1-2000km ago and im not all confident i got the tension right ;-)
I would aim for firm polite pressure, you'd be surprised how much more tension there is when the engine is up to temperature.
Ok, thanks! That gives me a basis to work with. I'll use restraint, moderate force and minimal swearing, although the plumbers crack may still cause a PG13.
I always noticed that when the engine was hot, the vertical cylinder belt was tighter than the horizontal one; obviously owing to expansion. So, I always did the vertical one just a little looser than the horiz one.
Heard that before and to be honest I used to do it on my Desmoquattro, but have never bothered with my air cooled 2vs and never had a problem.
I always use the 5mm on the horizontal and 6mm on the vertical cylinder belts. With the minimal mileage that I do I also run the belts for 2 years or a tad more.
Not disputing that one expands more than the other it seems to be the accepted wisdom, but why would one expand more than the other?, they are pretty much the same aren’t they?. (Is it something to do with the crank case architecture maybe? Or the vertical cylinder runs hotter due to less airflow?)
The rear cylinder runs hotter because of less air cooling and expands more hence stretching the belt more.
I use all three methods. 5mm, 45 deg twist, & 110 htz. I turn the engine over a couple of rotations and do it all again 2 or 3 times. I then go and have a cup of tea and a bite to eat. Come back and do it all again so that when I put it back together again I have no doubt that it’s all tensioned up correctly.
When all dots line up, the horizontal cylinder is at TDC and tension can be checked. I understand, that you should have the vertical cylinder at TDC to check belt tension. I have myself, like you, simply checked the vertical cylinder belt tension several times each time rotating the engine a bit, not caring about TDC. I don’t know how much pre-tension is on the belt when the cylinder is not at TDC, but with the Allen key method you do apply some force which, in my completely undocumented theory, easily will overcome any pre-tension from not being at TDC. Maybe TDC and no pre-tension is important if you use the frequency method, which is why Ducati recommends it, as they use the frequency method for checking belt tension.
A 10 degree centigrade difference only gives a thermal expansion change of 0.002" over the length of the belt
I guessed at a 10 degree difference, maybe more, but could be a lot less - so you wouldn't think so...
So what is your opinion on having the cylinders at TDC when using the Allen key method? Does it matter or not?
Not a clue - but I know Chris Kelly says there is too much emphasis put on the whole process as his belts are not going to break These days with free apps and cheap mics for your phone I would always recommend using Hz though. Our 2 Valve belts nee 99Hz nominal due to less stretch than OEM - and if set higher can emit a noise until bedded in, partly due to the additional coating on the teeth that the OEM belts don't have. For the time it takes there's no harm following a set procedure in my view.
For me the primary worry of belts too tight is not belt breakage but shaft loads and shaft bearings. But I’m not a skilled mechanic, so I might be making things up
Replacing bearings must surely be the lesser evil. The last thing you want to do is drop a valve which will certainly damage valve/s, guide, piston, con rod? and crank? Not to mention the rings, small and big end bearings. Every time I replace the belts I'm surprised how loose the old ones are.
