Timing marks.

[Bob 1967]

I may have mentioned before, that I thought the TDC marks were out a bit.
I was adjusting the tension of the alternator belt today and decided to investigate further.
With the belt cover removed I lined up all the marks.

Including the difficult to see one.


With the cover replaced it can be seen the TDC is 2.5(or there about) degrees off.

I was correct and will take this into consideration when setting timing.
It would only tick over at 15 deg. btdc (vacuum disconnected), now I know why.

[Othen]

Do you think the fault is with the belt pulley on the crankshaft (maybe the keyway or the key) or with the plastic cover Bob? I suppose it has to be one or the other.

[Bob 1967]

The cover is 1 piece cast metal alloy.
I'll just accept that it's out and factor that in any future work, timing or valve clearance.

[Othen]

Originally Posted by Bob 1967

The cover is 1 piece cast metal alloy.
I'll just accept that it's out and factor that in any future work, timing or valve clearance.

It is just down to manufacturing tolerances then, our motors are probably all the same: +/- a few degrees. It will only really affect the ignition timing; perhaps this is a little reminder to all of us that precision mattered a lot less in the pre-digital age so it is best to set things up so they run well than by numbers in the book

PS. I was thinking about tis (as a surveyor) whilst Bobwalkin. It is perhaps not surprising that the timing mark can end up a couple of degrees out. The error balance for the chain of observations is quite large. First the hard to see mark on the crank, the parallax standard error in viewing it from an odd angle through a gap in the drive belts and pulleys must be a few degrees in itself. Then the inexact length of the belt (due to both manufacturing and wear) means the camshaft and auxiliary shafts can only be positioned +/- half the pitch of the belt teeth. Then there are parallax errors for observing both those marks. On top of that there is the positioning of the cover that contains the ignition timing marks.

Some of the standard errors will end up coincidentally cancelling each other out - but overall perhaps we shouldn't be surprised the visible error comes out to be a degree or two out.