I'll start by thanking Exige for the VERY useful info he gave me on tensioner bearings - superb. I know there have been a million posts on the subject of timing belt tension, but this is a simple (I think) question and I do NOT intend to offer any theories on belt tension.... This is on a 1985 2v 650 engine. The workshop manual shows the special spring balance tool - just put it on the tensioner rollers, pull to show 6.5kg, then tighten the bolts. Sounds simple. I got a (pricey) version of the tool from Mdina. This has a little can (cylinder) that fits over the rollers with a bracket for the spring balance to pull on, however when you have adjusted the tension and you remove the can, its wall thickness (1.5mm) effectively being removed means that the belt seems very loose. The attached jpg might help explain what I mean. The Ducati (Cagiva) special tool seems to have a section of the can removed so you presumably don't get that metal wall between the belt and the roller. Anyone else had this problem? I thought I would ask first, rather than just hassle Mdina for a refund as "not fit for purpose". Either that, or are belts on these machines are supposed to be looser than Widow Twankey's washing line? .
Has anyone used a spring balance tensioner? Surely someone must have - it was the official Ducati method of tensioning cam belts back in the day..
When I first got a 900SS with cam belts back in 2000 I used a spring balance to set the tension. I just used a bit of bent fence wire to hook behind the belt as close to the tensioner as possible and attached the spring balance to it. I subsequently gave that up and used the deflection method like everybody else. Nowadays I use a frequency measuring app although that wouldn't be much use for a Pantah engine as the required frequency is unknown. I've done so many belts now though that I know what the tension should feel like when I push the belts with my fingers and this is what I did for my 500SL.
Thanks Derek, that's all useful info. I'm learning! Now that I am lucky enough to have 2x Ducatis (Monster 2001 and Pantah 1985) I feel the need to understand cam belts more... The tension is supposed to be 6.5kg on the Pantah which is sort of easy enough to get a feel for as you say.
Derek, on the spring balance method, my piece of kit tells me to set it at 6.5 kg. Does that align with the tension you used back in the day? Problem with this piece of kit is that when the little can is removed from the rollers, I think the tension drops to just the odd kg.
After all this time I can't remember the figures for the the 900SS but the Pantah 500 manual says 5mm deflection at the fixed roller with a load on the spring balance of 2kg. The 650 Pantah uses the same belts but your figure of 6.5kg seems very excessive.
aaah - but it kind of makes sense with the tensioner I got from Mdina. Setting it to 6.5kg then removing the can from the roller would knock its back to about 2kg. And that 5mm figure looks right too. I think the problem I have is that after several decades of old British machinery I am used to locking up a V-belt bar-tight to avoid slippage. The cam belts run at what looks like a rather slack tension but then one hears that the main cause of belt failure is over-tightening. I am learning, thanks to excellent support from people in this forum.. Many thanks.
Measuring the 5mm required space at the fixed roller: the recommended method is to use a 5mm allen key. I don't like pushing sharp-edged items in there, but have found that most chopsticks are 5mm across and a lot softer on the belt and roller.
I use it all the time on my 2v bikes, just a slightly modified fishing spring balance though as noted in the Haynes manual. I've changed and tensioned dozens of belts this way over the years and never had any issues. I do also double-check using the old ''twist the belt' and 5 and 6mm Allen key methods though, but it's never a million miles away.
