Sq4 Valve Timing

[chris.shearwood]

I've been looking at Sq4 valve timing and, like others before me, I find the subject a bit confusing. Bruce Longman's Valve Timing Procedure, which can be found on Draganfly's web site, is very helpful but still leads to some head scratching for me.

The diagram of the crankshaft simplex sprocket shown on Page 4 of his procedure shows 123º between the keyway at 6 o'clock and the other two keyways. If one rotated the sprocket 123º clockwise on the crankshaft and used the keyway shown adjacent to the datum timing mark on his diagram wouldn't the result be a 3º retarding of the simplex timing and not the 5.5º indicated on his diagram? Any comments?

[rschurr]

Yes, I also found that the words duplex and simplex on this drawing were mixed up a bit may be when editing the original of Bruce Longman.

[chris.shearwood]

I've been tossing and turning in bed unable to sleep thinking about how Bruce arrived at the 5.55 degree figure and I've finally figured it out but it is almost 4 AM so now back to bed!

[rschurr]

I'm curious about the solution, I couldn't find out, despite sleepless nights too.

[chris.shearwood]

Hi Rudolf,
To understand Bruce's figures it is important to note that the keyway at the 6 o'clock position in the above diagram is aligned with the point of the "bottom" tooth of the sprocket. If somebody machined fourteen keyways aligned with the points of each tooth of the sprocket, they could slip any one of them over the crankshaft key and the valve timing would not change at all. To get some slight valve timing adjustment a keyway has to be machined such that there is a difference in angle between a line going from the centre of the sprocket through the keyway and a line going from the centre of the sprocket through the point of the adjacent tooth. This keyway could be machined adjacent to any of the other teeth (besides the bottom tooth in the diagram). I guess to make it roughly symmetrical, Ariel chose to put their keyways about 120° apart. So, again referring to the above diagram, the point of the fifth tooth up to the right from the bottom is at 128.55°(5 x 360°/14 teeth) (or 5 x 25.71°= 128.55°,roughly 2 o'clock). To create the offset needed for timing adjustment, they machined a keyway at 123° or 5.5°offset from the point of the tooth (128.55° - 123° = 5.5°). When the sprocket is placed on the crankshaft using this 123° keyway, ALL the teeth are offset 5.5° vis-à-vis the original (or bottom or 0°) keyway. To get the tooth adjacent to the 123° keyway into the same link of the timing chain as the 0° one was in, one would have to turn the crankshaft 5.5° in its normal clockwise direction therefore putting it "ahead" of the camshaft or, in other words, retarding the valve timing 5.5°. Whew!
Regards, Chris

[rschurr]

Wow Chris you are the Champion

So as a conclusion, however, the simplex sprocket has 5.5 ° advance/retard

and the duplex sprocket, as it has the timing mark on the tip of 6th tooth is (123°- (6x20°)) = 3° advance/retard

as shown correctly on the drawing.

[chris.shearwood]

Wow Chris you are the Champion !

Hi Rudolf,

I'm definitely not the champion! In fact, I'm embarrassed that I didn't figure it out right away because it is quite simple and straight forward looking at it all with hindsight.

Thank you for taking the time to reply to my original message and good luck with your Sq4,

Chris

[KevinOToole]

I spent many happy hours getting my Sq 4 valve timing spot on. In the end I cut extra keyways in the cam sprocket. I can't quite claim it's an original idea... As Triumph used this idea for many years. I could have done lots of calculations and used a dividing head but... After working for days getting my head around the timing, I decided I could just "eye it". I cut a keyway just before a tooth and a keyway just after a tooth. This gave me the adjustment to get the timing as good as I could get it with a timing disc. Since doing this job a couple of years ago, I have moved on from a timing disc to a digital inclinometer.
This is the type of inclinometer I bought but not the exact one. https://www.ebay.co.uk/itm/Digital-Incl ... SwmLpgFR1W
I hope this is of some help
Kevin

[chris.shearwood]

Hello Kevin,
Thank you very much for again sharing your valve timing adjustment method with us. Have you found a difference in performance between the timing being "spot on" and the timing being a few degrees different from specifications, which is presumably what you had before machining those additional keyways?
I am wondering if Ariel did not provide for exact valve timing because they thought any small difference from the specs would not be noticeably apparent in its effect on performance. After all, aren't the specs a trade-off between low and high speed performance i.e. one keyway of the crankshaft sprocket for general all-round use, another keyway for those who prefer the timing slightly advanced and another keyway for those who like their timing slightly retarded?
I'll admit that I have been too lazy to experiment with valve timing alterations!
As always, any comments would be greatly appreciated,
regards, Chris