Good timing

[nigeldtr]

Sorry if a silly question, but how do you know where 0 degrees is, as the "swing" of the crank pin over T.D.C. could make quite a few degrees of difference?

Regards

Nigel

[a101960]

Sorry if a silly question, but how do you know where 0 degrees is, as the "swing" of the crank pin over T.D.C. could make quite a few degrees of difference?

Not a silly question at all. There is (despite what anyone else might say) only one absolutely guaranteed way to find T.D.C. You will need a timing disc and a piston stop. You will also need to fix a pointer in in position so that you can read off the degrees.Mount the disc on the crank shaft and then turn the engine until the piston touches the piston stop. Note the reading in degrees indicated. then turn the engine in the opposite direction until the piston again touches the stop. Note the degrees indicated. Now divide the difference. That is to say the half way mark between the two indicated degrees (B.T.D.C. and A.T.D.C.) as determined by you piston stop. This is your true accurate T.D.C. There is no other way of finding T.D.C. with any accuracy. You are right in your assumption that there will be an inaccurate indication at T.D.C. due to crank pin swing using any other method. Many people will be of the opinion that near enough is good enough. It isn't.

John

[lawnmowerman]

I wondered about that too. Did you make your piston stop from an old spark plug or can you buy them. Is there an easy way to mount a timing disc on the drive side or do you have to pull the drive side cover off?

Jim

[Alan @Ncl]

Hello Nigel
I have seen some good answers to this on this Forum but can't recall exactly where.  As I understand it, you proceed as  follows, having rigged up some form of crank angle indicator first.

First, you turn the engine until the piston is about beyond about half way up its stroke (say 1.5"" BTDC is a good position, measured down the plug hole) and mark the angle somehow.  Then keep going until you have gone over TDC.  Mark it again on the way down the other side, when you are back to the same position (say 1.5"").  Now you know TDC is is exactly half way between the two marks on your timing disc.  Doing it this way, the geometry is in your favour.

The only problem I guess is for those of us (like me) who do not have timing discs or don't want to take the timing cover or chaincase off.  I guess you could do it on the magneto CB housing end with marks on the fibrecam and the housing but it will be less accurate (lower resolution  since smaller diameter and only half a crank revolution).  Others may have a better suggestion about this.

ps. I put this up without seeing the two previous posts.  I think John is right about turning the engine back and up to the same position ATDC, rather than going over the top as I suggested.  They should give the same result in theory but If there is any backlash anywhere, may not do so.  It would be interesting to see if, and by how much, the two methods differ?  Do any of you 'professionals' with timing discs have an answer to this?

Alan

[lawnmowerman]

Muskrat gave me a good tip on another thread. Once you have exactly set the timing using whatever method, make a piston stop from an old spark plug without the washer. Remove the centre and drill and tap and fit a longish bolt. Screw it in the plughole and screw the bolt in until it touches the piston and lock it with a nut or some Araldite. You then have a tool you can use to set the timing without taking any cases off by gently turning the engine until the piston touches the end of the bolt.

Jim

[A10Boy]

John is correct, that is the only way to do it.

Measuring down plug holes is a sure way to get it wrong, so is trying to set it using the fiber wheel, theres too much backlash in the gears to be accurate.

[Alan @Ncl]

I go along with all that.  A Muskrat piston stop and timing disc are definitely the best way to get it right.  

However, if you are reduced to measuring down the hole then if measuring when around the 1.5" (approx 40mm) point, I reckon the resolution is about 0.75mm per degree which is quite readily detectable with a vernier or similar so its not a bad approximation. To put that statement in perspective, you would need a move of about 9 or 10 degrees to give that amount of linear travel at TDC so by doing the measurements further down the stroke, resolution is arguably about 10 times better than trying to do it at TDC.  This, as with all my statements is, of course speculative, in the absence of peer review by those who are more experienced in these matters.

All goes to show how sensible Nigel's original question was (as John said earlier).

Alan

[lawnmowerman]

Found a complete timing kit at SRM, timing wheel, cush drive nut to mount it on and piston stop. I know you should not have to set the timing too often but when you do it needs to be spot on so worth spending the money for the right kit.
Bit of a drag having to pull off the drive side cover but creates a chance to have a nose round in there and see if all is well and change the oil.

Jim

[nigeldtr]

Guys,

Super answers yet again!  John I see the sense of the stop nice idea. Recently I bought a timing set with a "dummy" plug, drilled to hold and clamp the supplied dial indicator for 35 Euros, came with a few extensions for various head thicknesses. Once I have worked out the geometry for the plug angle and vertical travel, I will give it a go.

I can see the benefit of the use of a strobe light and timing disk. I seriously thought about turning off the mounting lugs on my mag and making up a clamp ring so it can be turned just like an the old car distributor, bit non standard though!

Interesting that the earlier soft tuned engines like my GF were timed with a cigarette paper! Has the passing of time and advances in technology made our lives simpler or just “faster” and more complicated

Regards

Nigel

[a101960]

For the record I bought a T.D.C. finder (see picture below). It screws into the plug hole and has a calibrated centre piece that rises and falls with the piston. Now you would think that this would give an accurate indication of T.D.C. However when comparing the results against the timing disc and piston stop method in proved to be very inaccurate indeed. The reason for this is because of the crank pin swing. When the piston is indicated as being at T.D.C. using such a device the swing factor can be cause the indication to be out by as much as 5 degrees. You might use such a device and set up T.D.C. thinking that the calibrated graduation mark is correctly indicating T.D.C. in reality you might do this several times and each time there will a different T.D.C. but of course you wont know that. I know this because I did comparison checks with the piston stop and timing disc. The piston stop and timing disc position was constant. I am unsure how you could set T.D.C. using a piston stop to go through T.D.C. to A.T.D.C. with the timing disc. You would need to remove and then refit the piston stop and that would introduce an error as there is no guarantee that the piston stop would be returned to exactly the same position.




A piston stop can indeed be made using an old spark plug. Simply knock out the ceramic insulator and central electrode, and break off the side electrode. The plug can then be tapped to accommodate a suitable bolt, or the bolt could be fixed in place with araldite or something similar.

A timing disc can be down loaded from the internet and printed out. Stick it to a suitable piece of card and or laminate it. A suitable image can be down loaded from here

http://www.eurospares.com/graphics/degreew.jpg

Because I had a lot of hassle with timing I also bought a magneto points opening indicator rather than rely on the vagaries of a cigarette paper (is that the right degree of grip or not?) for instance.

I got mine from Independent Ignition Supplies but it looks as though they no longer stock them. However here is another supplier.

http://www.themagnetoguys.co.uk/page008.html.

One final word. No matter how careful you are on a performance version of an A10 (RGS Super Rocket etc)You will probably not be able to entirely avoid pinking/pinging. Why do I say this? Well, here is a verbatim quote from the Rocket Gold Star road test conducted by Motor Cycle magazine published on November 22nd 1962 "Although 100-octane petrol was used, pinking was audible if the throttle was opened hard on full advance at any engine speed" so there you have it. Even back in the day on high octane leaded petrol pinking was a problem. I personally believe that the original BSA timing spec was to advanced at 3/8" (35 degrees B.T.D.C.). I did initially set my timing to this figure, and especially on gradients it could be provoked into pinking very easily. Having said that it went like S**t off of the proverbial chrome shovel. However I have now timed my engine at 5/16" (32 degrees B.T.D.C) which has more or less eliminated the pinking/pinging problem. Performance is not as sharp as it was but I don't want to knock the bottom end out prematurely. Also be aware that the timing figures published in the technical section on the SRM web site are incorrect. SRM say wrongly that 35 degrees = 5/16"

John

[bsa- bill]

I can agree with all the above having found about 2 degrees variance.
A bit more accuracy can be had using a dial gauge ,when the heads off obviously, although I'm sure a method to use it with the head on could be found , the dial gauge does somehow seem to register much smaller movements of the piston than finger or eye can

[Alan @Ncl]

What John and Bill  are saying makes sense if you use the device to try and directly detect TDC.  My little calclulator predicts that around TDC, 5 degrees is only about 0.1 mm (4 thou) and that takes a pretty high resolution instrument to detect (Bill will get close with his clock gauge, hence his 2 degrees which I reckon is about 1 thou).  

However, once you get further down the stroke, the piston stop gives much better resolution and in the critical region of 5/16 to 3/8 inch should be fine as a datum.  But for actual TDC, you can only use the 'bisection' principle which seems to imply a timing disc.  

Was also thinking about Nigel's point about his gauge and translating readings through the geometry.  I measured the angle of the plug hole as about 33 degrees to the vertical (how does this compare with what others think?). This means that the vertical travel is about 84% (cosine of the angle) of the distance measured by the instrument.  However, strictly speaking, this will only work accurately for a flat top piston.  If it is domed, the geometry will change a bit (although this may be a negligible secondary effect if we are only considering movements up in the 3/8 to 5/16 BTDC region).

Also reflecting Nigel's comments, I bet when some of us were poking 'sticks down holes' nearly 50 years ago, we would never have predicted our current level of interest in such things.

Alan

[kiwipom]

hi guys/Alan, yes the technical detail that we now delve into to find TDC, never used to be a problem in the old days as you say a stick down the hole worked well, obviously the interest in the exact is highlighted with age something that we would have never contemplated as a teenager,cheers

[iansoady]

I can agree with all the above having found about 2 degrees variance.
A bit more accuracy can be had using a dial gauge ,when the heads off obviously, although I'm sure a method to use it with the head on could be found , the dial gauge does somehow seem to register much smaller movements of the piston than finger or eye can

I did mine using a home made extension for the dial gauge and with the magnetic base on the fins beside the plug hole (iron head). The clock gauge did respond to the slightest movement even around TDC, although I agree the only really accurate way is with a piston stop and degree wheel (John's method). However, given I'm not after the ultimate I think the clock gauge is accurate enough for my needs.