Top dead centre accuracy

[Greenmachine]

Another stupid question from the land of oz. I just installed a pamco ignition and HHB pma. I have read threads how to find TDC and I was wondering how accurate it needs to be with the set up I just installed. I have read how to make use a degree wheel and piston stop. I don't have access to a welder at the moment to make a stop out of a spark plug but I do have a dial indicator which I found to be of little use on an assembled engine due to the angle of the spark plug entry. I can see through the spark plug entry when the piston gets to the top and goes down. Would I be able to roughly guess tdc by looking at the position of the piston and then mark it on the pma or would I have to be more accurate? And before anyone says I should have marked it from my old generator,The engine is freshly rebuilt and I never had the old generator.

 

[I am Carbon]

I don't think you could have marked it from the old gen. you have to mark it after it is on.

 

[hein weijers]

I can see through the spark plug entry when the piston gets to the top and goes down. Would I be able to roughly guess tdc by looking at the position of the piston and then mark it on the pma or would I have to be more accurate?

This method is far too inaccurate! The error in "reading" the TDC can be more than 10 degree this way.

To make a stop out of a sparkplug: ruin an old plug by crushing the insulating tip around the elektrode. Make a piece of hard wood of +/- 8,5 mm diameter, drill a hole in the center of it and slam it (with glue) into the sparkplug. There you are!

Hein

 

[Greenmachine]

Thanks hein that might just work.

 

[peanut]

you don't need to make anything special to mark TDC thats just making a simple job difficult. what is more important is to be meticulous .

I have used a pencil or screwdriver on cars bikes and scooters for 50 years and its never failed yet.

What you also need to appreciate is that there is considerable slack in the drive system, cam chain gear lash etc so 1 degree out on the crank can become several degrees at your points and advance.

Remove the stator and both spark plugs.
Turn the engine round anti clockwise using a socket or ring spanner on the crankshaft whilst keeping a thumb over the spark plug hole until you can feel compression building under your thumb.
Stick a pencil or screwdriver into the plug hole and slowly advance the crankshaft until the screwdriver will rise no more . That is TDC

Some propose that you mark a point equidistant either side of TDC and then devide that to find exact TDC but personally I feel it is more logical to use the highest point anti clockwise on the compression stroke as TDC because that is where TDC will be when the engine is running

 

[hein weijers]

Sorry Peanut, but I don't agree here. You need a hawkeye to see the rising of the piston during the last 3 or 4 degrees. The movement is less than 0,1 mm (.0039 ")

The slack in the drive system has nothing to do with the setting of the TDC position. When the crank rotates, the piston moves, clockwise or counterclockwise, thas doesn't matter. Only when you set the ignition timing, with the points or pickups on the camshaft, it is important to do that turning the crank counterclockwise, seen from the LH side ( dynamo side) of the bike.

Hein

 

[peanut]

the slack or lash in the transmission system doesn't have anything to do with setting up TDC but.....

if you have 2-3 degrees slack in your chain and points and advance etc that will have a big effect on your spark timing which is what this exercise is all about . making sure that you get your spark at the right degree of BTDC !

The point that I'm trying to make here is that the tiny percent of a degree of varience when setting TDC is not that critical ...like you are suggesting. You need to keep this in some sort of reasonable perspective.

A combustion engine is not a precision clock its a very crude and sloppy mechanical device with lots of built in tolerance to reduce friction heat and natural wear and tear.

 

[DogBunny]

I have to agree with Hein. The only way to accurately find TDC is with a piston stop, making two marks, and then marking the midway point. The same thing can be done with a dial indicator, but because of the angle of the spark plug entry this takes finesse. It's easier for the average person with a piston stop.

 

[5twins]

I have the dial indicator "timing" gauge I bought years ago for use on two strokes. I've tried it on the 650 and it works fine. Yes, the plug hole is at an angle but the trick is to find T.D.C. visually, by eye or with a straw or wood dowel, first, then install the gauge. That way, up and down movement of the piston is very slight, maybe 1/4", during your readings and use of the gauge. I use the gauge mainly for verifying that the timing plate on the '80 and newer models is correctly positioned. I have checked some earlier models but never found the factory marks to be off.

 

[peanut]

I just think we shouldn't make our advice and recommendations unecessarily complicated for owners, especially when they may have limited knowledge or experience with these motors.

Intellectually I agree with exacting engineering but being practical ... these are crude 70's motorcycles some of which are now 35+ years old

 

[TwoManyXS1Bs]

Here's a snippit from a simulation run for XS650 long rods. These numbers will closely match stock rods.
Note that the piston position values go to tenths of a thousandts of an inch (about 2.5 microns).
For reference, the threshold for human touch is about 0.0010", thickness of a human hair is about 0.0030", paper about 0.0040".

 

[grizld1]

You don't need a welder to make a usable piston stop from a spark plug. All you need is a spark plug shell with a 1/4" ID nylon bushing pressed into it and a 1/4" bolt with a long smooth shank. Cut the threads off the end of the bolt, round the tip with a drill press and file, polish until the bolt slides freely in the bushing. Re. plug angle, just relieve the plunger by hand while the piston is on the rise.

Don't try to get a clean result by manually locating the high spot. The more accurate way to go about the job is to read the degree wheel at equal drops before and after TDC. TDC will be at the midpoint between the readings. Work until you get repeating results.

 

[Gcraay]

Sorry Peanut, but I don't agree here. You need a hawkeye to see the rising of the piston during the last 3 or 4 degrees. The movement is less than 0,1 mm (.0039 ")

The slack in the drive system has nothing to do with the setting of the TDC position. When the crank rotates, the piston moves, clockwise or counterclockwise, thas doesn't matter. Only when you set the ignition timing, with the points or pickups on the camshaft, it is important to do that turning the crank counterclockwise, seen from the LH side ( dynamo side) of the bike.

Hein[/QUOTE

]Question, I'm using Hughs technique of piston stop tool and degree wheel. He says that after installing the stop turn the crank and note the degree difference between the left and right turns, devide by 2 and adjust the wheel the difference. After the math my difference is like .5. Yes .5 so...I'm to adjust only. 25 degrees. Alright that makes sense. But to maximize the stroke I would want the piston to be at its highest point? I can bring it higher and my wheel reads 12 degrees. Even though the stop is only about an inch and an 1/8th from the end of the plug it's like it's holding the piston lower than maximum stroke. If I visually bring the piston up then install the stop it gives me resistance only a few turns of the plug in.

What am I not understanding?

Wouldn't it be best to back off the stop until your right at that nearest point of piston hight and stop at minimum insertion?

 

[TwoManyXS1Bs]

Wouldn't it be best to back off the stop until your right at that nearest point of piston hight and stop at minimum insertion?

Sure, you could set the piston stop to where it just barely kisses the piston as you swing the crank back-and-forth near TDC, and that'll get you close, maybe within +/- 2° or 3°. Most folks would want their TDC mark a little more precise. Maybe this video can help:

 

[Gcraay]

Awesome, thanks. I've never done an engine before, so I'm a little bit nervous about blowing it up. I appreciate the advice.

 

[blackapache]

Another stupid question from the land of oz. I just installed a pamco ignition and HHB pma. I have read threads how to find TDC and I was wondering how accurate it needs to be with the set up I just installed. I have read how to make use a degree wheel and piston stop. I don't have access to a welder at the moment to make a stop out of a spark plug but I do have a dial indicator which I found to be of little use on an assembled engine due to the angle of the spark plug entry. I can see through the spark plug entry when the piston gets to the top and goes down. Would I be able to roughly guess tdc by looking at the position of the piston and then mark it on the pma or would I have to be more accurate? And before anyone says I should have marked it from my old generator,The engine is freshly rebuilt and I never had the old generator.

mate mid way through a rephase rebuild too with hughs goodies, i have degree wheels piston stops and anything else you need, im down at pottsville - just moved down from currumbin, if you wanna come use any of it feel free, id love to see how your tackling it all. im about to degree my cam and add my pamco etc so itd be sweet to see what you did on yours. my numbers 0410419791. jesse.

 

[chizler62]

This isn't rocket science, it's not even paper airplane science.
1st there shouldn't be a need for finding TDC when installing a PMA or and pick up coil?
Someone please show me a photo of what this is?
2nd No better way to find TDC than a degree wheel with an indexing pointer. 100% accurate if done right.
.....and yes the cam chain slack can be an issue if you're mounting the degree wheel off the cam. Typically not done though.

A PMA on a XS650 has a woodruff key. A aftermarket pick up coil should be indexed off the cam etc
So, please explain why your needing to find TDC
Why are you all doing this

 

[I am Carbon]

This isn't rocket science, it's not even paper airplane science.
1st there shouldn't be a need for finding TDC when installing a PMA or and pick up coil?
Someone please show me a photo of what this is?
2nd No better way to find TDC than a degree wheel with an indexing pointer. 100% accurate if done right.
.....and yes the cam chain slack can be an issue if you're mounting the degree wheel off the cam. Typically not done though.

A PMA on a XS650 has a woodruff key. A aftermarket pick up coil should be indexed off the cam etc
So, please explain why your needing to find TDC
Why are you all doing this

hes not needing the mark for the pma its because of he pma it's for the Pamco.
As you already know having a TDC on rotor is a wonderful thing when a guy wants to time the engine. So that's why he just might want to time the pamco igition.

 

[wherearewe]

This isn't rocket science, it's not even paper airplane science.
1st there shouldn't be a need for finding TDC when installing a PMA or and pick up coil?
Someone please show me a photo of what this is?
2nd No better way to find TDC than a degree wheel with an indexing pointer. 100% accurate if done right.
.....and yes the cam chain slack can be an issue if you're mounting the degree wheel off the cam. Typically not done though.

A PMA on a XS650 has a woodruff key. A aftermarket pick up coil should be indexed off the cam etc
So, please explain why your needing to find TDC
Why are you all doing this

I think people need to do this because, (as discussed a couple of days ago on another thread where i got ripped a new one for saying this), the PMA swap doesn't fit the stock woodruff key. The XS650 uses a 4mm one and the PMA rotor a 5mm one. Even with the key it can slop around. You need to purchase and file down a new key if you want one that works tight. This is fine if you want to do it to ensure no slippage, but many people follow manufacture instructions and dont, relying on the taper to hold it in place.

Most of the PMA rotors out there that I've seen dont have a ref mark for TDC printed on them matched to the keyway since people can effectively install them as they please. You need to find TDC to mark the ref point on the rotor yourself after install.

Yes it would be better if the kits came with a custom key and a ref point calibrated from the keyway, but they dont.

At least that what i think. I also dont know how this old thread came back to life!

 

[retiredgentleman]

This is not an old thread...................3 or 4 months is not very old, threads come back to life after years sometimes.

Its fine for HHB to say just install the PMA rotor and then find/mark TDC. What happens if you need to remove the rotor, say to replace a shaft seal? How will you be sure that its replaced correctly? I suppose you just take the time to find TDC with the crank/pistons again.

If you sell the bike, the next owner may pop off the rotor at some time and not even be aware that that he would need to find TDC each time.

Using a proper size woodruff key ensures that no rotary index can change, and that any present or future owner will not have the ignition timing go out of spec.