John feeling down the plug hole for TDC isn't accurate either. To be 100% you first have to get the accurate position of the front cylinder. That can only be achieved on a built engine by using a positive stop for the piston close to TDC. A modified spark plug makes a good tool for this. You attach your degree plate and pointer, rotate the engine in one direction until it contacts the positive stop, take a reading (say 30deg), rotate the engine the opposite direction until it contacts the stop take a reading again, (say 45deg). Add the two =75deg divide by 2 your degree plate then needs rotating without moving the crank to 37.5 deg now when the pointer is at zero you will be at TDC. Then its very accurate to go back 270 degree. But I always like to turn the crank in its normal direction so the belt pull is on the correct side. All this as well assumes the cams are correct as the pulleys rotate independently of the cams, so you could get really brave and do some cam dialling.
Phil Many thanks for the reply. I understand the technique to find exact tdc as you detailed. As I understand things, what I am trying to achieve is to get the belt ready for tensioning, so as there is no torsion being exerted on the camshaft due to the helper springs. Is exact tdc necessary for this, or can the job be done correctly when the camshaft is in a 'close enough to tdc so that the cam lobe is not actually touching'? I would very much like give the cam dialing a go, don't like the reality of a compromise. Lack of proper facilities is my excuse not to get stuck in!
The problem you face is that the rear cyl belt goes on first and you have no reference. This is why it is done on the front cyl and a cam lock tool holds the rear cams in their relative position. Also remember you have two TDC's per cycle one will be compression and fire then all valves will be closed, the other is on the exhaust stroke. So you could get the front cyl on TDC firing and remove the belt, all valves are closed so you can turn the engine without worry. Get your the vert cyl at TDC firing paint mark your wheels and mark the teeth on the old belt. Slip it off replace with new belt after transferring your marks roughly tension the idler anti clock direction. Then turn engine to TDC on font cyl firing and fit that belt. For the tension I use a progamme called cool edit gives a good heads up frequency with a cheap mic. For a sanity check when front cyl is on TDC firing vert cyl is going down on its inlet stroke.
Thanks for the explanation, that all makes sense. I had watched a few instructional videos in preparation, but none of them were completely clear. I will give the cool edit software a go, I already have a microphone. Good tip about the sanity check.
