AN sells something called an "IGNITION TIMING PLATE CHECK TOOL" part # 13.1769

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I know, I know, I’m a heretic ….

Just IMHO… yes, at TDC there is a few degrees of lack of movement. But its very easy to see exactly how many degeees that is if using a simple degree disc, half it, and you’ll be within half a degree of a knots cock from perfect.

This is NOT to say that people should not do the hard stop method who want to, but sometimes I feel we accidentally create the impression that EVERYONE MUST do this otherwise their bike will explode.

For 99.9% of people in 99.9% of circumstances, finding TDC with a degree disc and a careful eye, as mentioned above, is perfectly good enough.

IMHO of course.
 
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Fast Eddie said:
I know, I know, I’m a heretic ….

Just IMHO… yes, at TDC there is a few degrees of lack of movement. But its very easy to see exactly how many degeees that is if using a simply degree disc, half it, and you’ll be within half a degree of a knots cock from perfect.

This is NOT to say that people should not do the hard stop method who want to, but sometimes I feel we accidentally create the impression that EVERYONE MUST do this otherwise their bike will explode.

For 99.9% of people in 99.9% of circumstances, finding TDC with a degree disc and a careful eye, as mentioned above, is perfectly good enough.

IMHO of course.
Click to expand...
I do hope you are not suggesting that people should get their engine up to normal working temperature and advance the timing until it pinks "pings" "knocks" (use whatever term fits your country of origin) and then back it off a little until it doesn't
And ride off happily into the sunset??
 
baz said:
I do hope you are not suggesting that people should get their engine up to normal working temperature and advance the timing until it pinks "pings" "knocks" (use whatever term fits your country of origin) and then back it off a little until it doesn't
And ride off happily into the sunset??
Click to expand...
There are worse methods!

As I’ve suggested before, IMHO a few degrees either side of nominal makes zero detectable difference on a Dyno (in my experience at least), with these crude old lumps.

Degens taught me this too, when I’d be chasing the enth degree he‘d say ‘so long as you're with X there’s no point’. Being a young know-it-all I would say ‘yeah but…’ to which he’d simply reiterate ‘do it if you want, there’s no harm done, I’m just telling you there’s no point’ !

PLUS… all the EI‘s on the market today have vastly different advance curves (different to stock and different to each other), with many not fully advanced until after 5,000rpm.

SO… unless you’re above 5,000rpm and / or you have precisely studied the advance curve of your particular EI, then you actually have NO IDEA of the ign timing irrespective of how accurately it’s been set!

But I must stress, I’m NOT against being precise per se, those that want to, and can do, should do.

But for most riders, most of the time, it is not a necessity.
 
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Fast Eddie said:
There are worse methods!

As I’ve suggested before, IMHO a few degrees either side of nominal makes zero detectable difference on a Dyno (in my experience at least), with these crude old lumps.

Degens taught me this too, when I’d be chasing the enth degree he‘d say ‘so long as you're with X there’s no point’. Being a young know-it-all I would say ‘yeah but…’ to which he’d simply reiterate ‘do it if you want, there’s no harm done, I’m just telling you there’s no point’ !

PLUS… all the EI‘s on the market today have vastly different advance curves (different to stock and different to each other), with many not fully advanced until after 5,000rpm.

SO… unless you’re above 5,000rpm and / or you have precisely studied the advance curve of your particular EI, then you actually have NO IDEA of the ign timing irrespective of how acurately it’s been set!

But I must stress, I’m NOT against being precise per se, those that want to, and can do, should do.

But for most riders, most of the time, it is not a necessity.
Click to expand...
I agree with Nigel but :-
Until you check your degree plate how do you know how far off it is? I used a positive stop and degree wheel and found the plate to be several degrees retarded. This coupled with the fact I'd already set the timing a couple of degrees retarded "to be on the safe side" meant performance was definitely sub optimal and the bike ran hot. Part of the reason I checked it was exhaust pipes discolouring.
Doing the same exercise with a '68 Bonneville proved the timing notch in the crank flywheel and the alternator rotor markings were spot on!
 
wavey_davey said:
I agree with Nigel but :-
Until you check your degree plate how do you know how far off it is? I used a positive stop and degree wheel and found the plate to be several degrees retarded. This coupled with the fact I'd already set the timing a couple of degrees retarded "to be on the safe side" meant performance was definitely sub optimal and the bike ran hot. Part of the reason I checked it was exhaust pipes discolouring.
Doing the same exercise with a '68 Bonneville proved the timing notch in the crank flywheel and the alternator rotor markings were spot on!
Click to expand...

I don’t believe I ever suggested folk shouldn’t check the degree plate.

My point was only that you do not need to do the positive stop method to get within a knots cock of of correct TDC.

And for most people running road bikes, that’s just fine. Even if you are still 12-1 degree ‘out’.

When I built my last motor I did the positive stop method (mainly cos I wanted the cam timing cock on), but I also did the ‘by eye’ method with a degree disc just to see how close I was, and honestly, the difference was well within a fraction of a degree.
 
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Not to sound ignorant but what is a piston stop and could someone post a picture?
Thanks,
Mike
 
YING said:
Not to sound ignorant but what is a piston stop and could someone post a picture?
Thanks,
Mike
Click to expand...
A piston stop is a method of bolting something to the top of the barrel, ideally something rigid and that protrudes down into the bore.

You also put a degree disc on the crank

You then rotate the crank in one direction until the piston comes up hard against your stop. You record the degrees at which this happens.

You then wind the crank backwards until the piston comes up again the same stop, but from the other direction. And you record the degrees at which this happens.

Dividing this by two will give you the accurate BDC point. Subtract 180 degrees and you have the accurate TDC point.

Set a marker up at this figure and reset your degree disc to the TDC point and you‘re good to check cam timing and ign timing very accurately.

Note, even this is not truly perfectly accurate as the pistons are not fully central to the crank centre line on Nortons. But this is really splitting hairs.
 
YING said:
Not to sound ignorant but what is a piston stop and could someone post a picture?
Thanks,
Mike
Click to expand...
Take a spark plug and gut it
Then jam or weld a piece of rod into the plug say an 1" long
Fit a degree disc to the crank
Fit the spark plug
Wind the engine forward until it stops
Make a note of the degree
Wind the motor back until it stops then split the difference
You will then have a true tdc
 
baz said:
Take a spark plug and gut it
Then jam or weld a piece of rod into the plug say an 1" long
Fit a degree disc to the crank
Fit the spark plug
Wind the engine forward until it stops
Make a note of the degree
Wind the motor back until it stops then split the difference
You will then have a true tdc
Click to expand...
I haven’t got a stock cam, so dunno if it applies, but don’t you run the risk of interfering with the valves with this method?
 
AN sells something called an "IGNITION TIMING PLATE CHECK TOOL" part # 13.1769


Initially I used the positive stop method to set the timing while the head was still off during a complete rebuild, but later, having bought one of the AN tools to check the scale in the primary cover, I ground down one of those spark plug hole jobbies to avoid any clashing with the valves incase I turned the engine over the wrong way. (It showed it was one and a half degrees retarded).
 
Fast Eddie said:
I haven’t got a stock cam, so dunno if it applies, but don’t you run the risk of interfering with the valves with this method?
Click to expand...
No not with the stock cam I haven't but I get your point
I'd check down the spark plug hole whilst turning the to see if you have clearence for the valves
 
Fast Eddie said:
I don’t believe I ever suggested folk shouldn’t check the degree plate.

My point was only that you do not need to do the positive stop method to get within a knots cock of of correct TDC.

And for most people running road bikes, that’s just fine. Even if you are still 12-1 degree ‘out’.

When I built my last motor I did the positive stop method (mainly cos I wanted the cam timing cock on), but I also did the ‘by eye’ method with a degree disc just to see how close I was, and honestly, the difference was well within a fraction of a degree.
Click to expand...
I understand how the piston stop method works, but how does the "by eye" method work? Does this mean simply looking down the plug hole to see when the piston is at, or close to, TDC? Please excuse my ignorance.
 
WEM said:
I understand how the piston stop method works, but how does the "by eye" method work? Does this mean simply looking down the plug hole to see when the piston is at, or close to, TDC? Please excuse my ignorance.
Click to expand...
With a degree disc on the crank was what I said. Therefore by using a proprietary TDC finder through the plug hole, or dial gauge with the head off, it’s very easy to see how many degrees the piston is stationary for. Split this and you’ve got TDC.
 
WEM said:
I understand how the piston stop method works, but how does the "by eye" method work? Does this mean simply looking down the plug hole to see when the piston is at, or close to, TDC? Please excuse my ignorance.
Click to expand...
You could stick a screwdriver down the spark plug hole and feel tdc if you wanted to
Best done with a spanner on the crank and you can feel the centre of the "rock",or time the piston is momentarily stationary if you get what I mean
And that would be accurate enough on one of these motors imo
 
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Fast Eddie said:
With a degree disc on the crank was what I said. Therefore by using a proprietary TDC finder through the plug hole, or dial gauge with the head off, it’s very easy to see how many degrees the piston is stationary for. Split this and you’ve got TDC.
Click to expand...
Perfect. Thanks.
 
Fast Eddie said:
With a degree disc on the crank was what I said. Therefore by using a proprietary TDC finder through the plug hole, or dial gauge with the head off, it’s very easy to see how many degrees the piston is stationary for. Split this and you’ve got TDC.
Click to expand...
My degrees aren't in Mechanical Engineering, and I have a hard time understanding how it's possible for the piston to be stationary at TDC. The bearing clearances in the reciprocating assembly are very small. Where does this "play" come from, and is there a mathematical model for it?
 
Don't over think the "stationary at tdc" thing. Obviously mathematically there's a finite point in time when the piston reverses direction. What this means in practice is the actual point of tdc is difficult to detect by eye exactly as it appears stationary over a small proportion of crank rotation at this point. As Nigel has clarified, this inaccuracy probably doesn't matter for the ignition timing on these engines. When dialling in valve timing it absolutely does matter! I have an alloy positive stop that screws into the plug hole, it takes about 20 minutes if you have the primary cover off and stick a degree disc on the alternator rotor with blu tak, magnets or whatever.
 
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Mine is similar to this
.
www.sip-scootershop.com

Piston Stopper SIP, lang

To properly set up the ignition on a Vespa the piston requires immobilising. An especially elegant method of measurement, to provide an accurate dete…
www.sip-scootershop.com www.sip-scootershop.com
 
maylar said:
My degrees aren't in Mechanical Engineering, and I have a hard time understanding how it's possible for the piston to be stationary at TDC. The bearing clearances in the reciprocating assembly are very small. Where does this "play" come from, and is there a mathematical model for it?
Click to expand...
Mathematical model? Dunno Maylar, probably, but it’d be way over my head !

My layman’s understanding is thus:

The conrod connects the reciprocating motion of the crank to the linear motion of the piston. When the piston is at TDC there are a few degrees of rotation of the crank where the big end of the conrod is moving almost linear horizontally. It’s this few degrees we’re talking about.

I don’t know if this is scientifically precise or not? Maybe with adequately sophisticated measuring equipment some linear movement of the piston would still be detectable? I dunno. Nevertheless, for and engine builder it is a few degrees of vagueness that the piston stop method accurately eliminates.
 
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baz said:
I do hope you are not suggesting that people should get their engine up to normal working temperature and advance the timing until it pinks "pings" "knocks" (use whatever term fits your country of origin) and then back it off a little until it doesn't
And ride off happily into the sunset??
Click to expand...
That's exactly what I do. So unscientific and sunny.
 
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