A10 timing gears

[Goldy]

I am building the engine for my A10 project and I seem to have a problem. I fit the crankshaft pinion and line up the dot with the idler gear. I then fit the camshaft gear and set the line on the gear to the line on the idler. No problem so far until I rotate the crank and the marks are no longer in line. The pinion has 22 teeth and the cam gear 44 teeth but the idler has 49 which seems odd. The pinion is part No 67-339 the idler 67-336 and the cam gear 67-338. According to the parts book they remained unchanged from 1950 to 62. My engine is 1955 cast iron head swinging arm.   Any suggestions please.

[groily]

If you turn the engine quite a few times the lines should align again as if by magic.
The number of teeth on an idler doesn't matter (in the sense that it makes no odds to the overall gearing, which is 2:1 as it should be in this case). The idler just has to fill the gap between driver (crank) and driven (cam) gears which I assume yours does (I have no idea how many teeth there are meant to be on the thing!) The dots and lines won't align very often (although if you hold a straight edge to the crank and cam pinions, they will be aligned at each second revolution of the crankshaft).
After one clockwise turn of the crank, the crank mark will be where it started, the cam mark will be 180 degrees opposite where it started having also turned clockwise, and the idler marks will have moved 22/49ths of a turn anticlockwise - all as you look at it. After another turn, the crank and cam marks will be where you started, but the idler gear and its marks will have turned 44/49ths of a turn. Etc! Provided you're confident that the crank to camshaft relationship is correct when the marks are aligned, all should be fine. If you're in doubt, you could put a degree disc on the crankshaft and note when the valves open and shut in relation to top dead centre and bottom dead centre, but there should be no problem.

[bsa-bill]

 Groily's expaination is full and worded so that even I could understand it.
I remember turning my crank to see how many turns it took to get them all lined up again, my memory is not what it used to be and it was never good at anytime but I think it was around 80 turns ( 83 rings a bell ) would that be right Groily?
Could be I missed  several counts though

All the best - Bill

[groily]

I was rather hoping no-one would ask the 'how many turns' question . . . . There's a simple formula I am sure, but not known to me. Being curious, I'm looking - and if I find an answer I'll post it. It has to relate to the fact that after 2 turns of the crank and one of the camshaft, the idler teeth are 5 teeth out. But as 5 doesn't divide into any of the 22 44 or 49, it's probably a question of factorising or something I forgot before I 'learnt' it ha ha. The engineers and scientists on here will know . . .
However, the philosophy of the 'hunting tooth' - whereby an idler is nearly always a weird number of teeth - is standard practice. What we don't want is the same too/eeth on the idler always taking the load every time a valve opens. So, by giving that gear a prime number's worth of teeth (or at least, like 49, a number not divisible by any of the numbers that divide into the number of teeth of the driver and driven gear (in this case 2 and 11 and 22)), we get to a situation where the load is shared equally between all the teeth of the idler and it, theoretically, lasts a very long time. With consequent benefit to the other pinions in the train, which are not having to mesh with the same ever more worn teeth. Amazing the stuff that goes into making even the apparently simple bits of a machine, n'est-ce-pas?

[unclemeat]

Good work Groily !!!

[Goldy]

Thanks very much Guys, much appreciated.

[RichardL]

Risking ridicule, I believe that the crank must turn 98 times to realign the timing marks. Using Groily's 22/49ths figure and multiplying by turns of the crank, the first whole number for turns of the idler occurs at 49 turns of the crank. However, this results in 24-1/2 turns of the cam. At 98 turns, the cam will have turned 49 times and all the marks should align. Any objections?

Richard L.

[bsa-bill]

No argument from me Manosound - I thought 83 from memory,  but  for my memory 15 turns out is not bad

All the best - Bill