TDC question

acadian · Feb 26, 2013

Just about ready to throw the barrels back on and was wondering, before the head gets re installed, would this be a good time to check TDC on the primary cover indicator? (meaning, with the head off while it's still easy to find TDC without a piston stop?)

pete.v · Feb 26, 2013

Yes, I suppose it would be easier but the procedure would basically be the same, bringing the cylinders up from each side and then splitting the difference.
Also the cast iron surface of the cylinder would offer a surface for the magnetic base of your dial indicator.
I then put a little mark on my stator lined up for the timing mark on the rotor

norbsa48503 · Feb 26, 2013

You could set up a dial indicator but the trick would be to have the piston come to a stop about one inch before TDC. So set up the indicator one inch down low in the hole then load it say .100 now set the degree wheel to say 30 degrees BTDC than back off the piston and coming at it from that way load back to .100 and check the dial for making a correction. Some here would say build a dead stop anyway that bolts down to the jug with a tapped hole for a long set screw that would also keep the piston from rising after one inch from the top. Better than a wobbly indicator. Good time to double check your cam timing too.

Hobot · Feb 26, 2013

Best is to find TDC with just the barrel and pistons installed. I bolt something to barrel top to stop piston or pull up on pistons till TDC with barrel off, then knife edge mark stator @ TDC and 30's BTDC or the proper full advance degree of your particular engine. Its really easy to get confused on which jug and which rotor mark is firing so good to nail polish or scratch mark the proper rotor line. Plus hole stops work but realize the angle mean piston does not press on stop vertically in line with it but sidesways skewed so can bump the stop hard enough to bend or break off say a pencil to float with fuel or oil to fish out. Don't put on a final time mark or even a timing light till diddling timing to best effect, then look to see where to mark for next time. Always a next time I think. Always interesting to see if the factory dial gauge is set right or not skewed from factory or by a loose crank or clutch nut passing by.

acadian · Feb 26, 2013

norbsa48503 said:
You could set up a dial indicator but the trick would be to have the piston come to a stop about one inch before TDC. So set up the indicator one inch down low in the hole then load it say .100 now set the degree wheel to say 30 degrees BTDC than back off the piston and coming at it from that way load back to .100 and check the dial for making a correction. Some here would say build a dead stop anyway that bolts down to the jug with a tapped hole for a long set screw that would also keep the piston from rising after one inch from the top. Better than a wobbly indicator. Good time to double check your cam timing too.

Not going in to the timing cover this time around, have no sense of cam timing issues at this stage. My understanding, with the few brain cells I have left, is all that's required is to set up some sort of piston stop parallel to the piston's travel, attach the timing wheel (which I've printed off from the interweb and glued to a blank CD) to the alternator rotor center, wind a pointer wire to the barrel studs and find the sweet spot between two arbitrary points on the timing disc. That sweet spot should be TDC, from where I just back off 30 degrees... remove wheel and discover how far off the factory gauge in the primary cover really is...

mike996 · Feb 26, 2013

"Best is to find TDC with just the barrel and pistons installed. "

Totally agree. Degree wheel/piston stop is more accurate than what you will be able to achieve with the barrels off. OTOH, it'll probably be OK based on Norton production tolerances!

Hobot · Feb 26, 2013

Yep back and forth crank turns on piston stop then split the difference. In reality alls ya need is close enough to start then diddle best timing, then can mark and measure as needed to ease next time. I have grabbed pistons and lifted them w/o barrel on to hit TDC and double check my prior TDC marks. Another way I found acidently was testing leak down, crank stayed centered with air pressure, until I tried to move off TDC to check another level of ring contact and instantly pushed engine all the way to BDC past rear brake resistance. By this pressure TDC method I discovered my prior measure method on Trixie was one degree off TDC, BFD.

acotrel · Feb 26, 2013

I've got an old spark plug with the electrode removed. I usually make a bubble on it with kitchen detergent then rotate the crank backwards and forwards slightly over TDC until the bubble is maximum size, then set the pointer on the degree wheel. When I set the cam timing, I set the tappets then rotate the crank forwards until the pointer is at the desired exhaust timing, then rotate the cam in the correct direction until the tappet clearence is gone. Then I gently push the sprocket onto the cam and check all the other opening and closing points. My cam sprocket had three key ways broached into it, after I softened it. It is easy to get the opening points exactly where you need them.

acadian · Feb 26, 2013

suggestions for a good starting point to set the piston stop? 1" below cylinder top?

norbsa48503 · Feb 26, 2013

You would like it to be 30 and 30 on each side of 0 so do a little checking and playing with a set screw dead stop and write back. If you used 10 and 10 it would work but not be as accurate.

Matt Spencer · Feb 26, 2013

" attach the timing wheel (which I've printed off from the interweb and glued to a blank CD) "

You need a live 60s one , if you want it phycidelic .

yea. piece of p . but checker both ways , rotationally . clock & anti clock . particularly in regard to cam / ign .

Id throw on T.D.C. , 3/8 before T.D.C. & a 24 & 30 B.T.D.C. in pencil , check then scribe .
Then knock the primary cover , so theyre half a degree out .

:lol: :shock:
as in check nothings loose / moveing , before proceeding , not after . Specially the Rotor .

acadian · Mar 2, 2013

Going to run through the entire procedure again to double check, but the primary gauge is reading 6 degrees off!

mike996 · Mar 2, 2013

"and glued to a blank CD"

A CD is inappropriate for a Commando. Use a 45 RPM record.

When you say the primary gauge is 6 degrees off, are you referring to the mark on the Rotor? It's hard to believe the factory mark is THAT far off. Mine was within 1/2 degree but 6 degrees is a serious amount of error!

acadian · Mar 2, 2013

mike996 said:
"and glued to a blank CD"

A CD is inappropriate for a Commando. Use a 45 RPM record.

When you say the primary gauge is 6 degrees off, are you referring to the mark on the Rotor? It's hard to believe the factory mark is THAT far off. Mine was within 1/2 degree but 6 degrees is a serious amount of error!

Ran through it 3 times, the primary gauge reads 6 degrees off... other variables to consider in gauge movement - primary cover warping, fitment on locating pins, loosening of rivets holding gauge etc... it may not be entirely unreasonable that it would read so far off.

1up3down · Mar 2, 2013

with your assumption that the primary mark is off some degrees and with the usual variables of one's own screw ups:

just set it where you think it should be and start the damn bike

advance the timing until with your usual kickstart prod it kicks back, you know you are now too advanced

retard timing until no "real" kickback

poof, your timing is now acceptably close enough for 50 year old British twin adventure and she start and run fine

and you will likely not get it any much better no matter how much agonizing and fiddling

rpatton · Mar 2, 2013

acadian said:
Ran through it 3 times, the primary gauge reads 6 degrees off... other variables to consider in gauge movement - primary cover warping, fitment on locating pins, loosening of rivets holding gauge etc... it may not be entirely unreasonable that it would read so far off.

I think one variable near the top of the list is that the wheel you are using is off. Don't take it personally, it's just that it would be a miracle if it were otherwise. There are a few reasons why they make wheels this big, the main ones are that the markings are true and that you can see them.

You can get one of these for $18.00. 9" diameter.

acadian · Mar 2, 2013

rpatton said:
acadian said:

Ran through it 3 times, the primary gauge reads 6 degrees off... other variables to consider in gauge movement - primary cover warping, fitment on locating pins, loosening of rivets holding gauge etc... it may not be entirely unreasonable that it would read so far off.

Click to expand...

I think one variable near the top of the list is that the wheel you are using is off. Don't take it personally, it's just that it would be a miracle if it were otherwise. There are a few reasons why they make wheels this big, the main ones are that the markings are true and that you can see them.

You can get one of these for $18.00. 9" diameter.

Don't think so, ended up using one of these. It's just a mark on tin gauge, I'll know for certain when I fire it up

TDC question

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