Best method for finding TDC

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LTGTR

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Could you help my understanding of which is the best way to find TDC.
I have read all/most of the posts about the different ways to find TDC on our XS650's on this forum, but I cant really say one of those methods ended up being the effective way to do it.
For example, the plug angle is too much for the piston to push up a dial indicator or plunger bored through an old spark plug and the piston stop method wont work because it will foul on the valves when you reverse the rotation (approx 360). Maybe my understanding of the Piston Stop method is not right - the theory of this method is certainly accurate.
There are some other methods on this forum but they seem extremely technical and involved - no disrespect to those that constructed them.
Of course my question assumes you dont have the TDC mark on the Rotor and you have to find it from scratch.
Thanks for any help,
Regards Ray.
 
I've never had any issues using a dial indicator but I bring the piston up very close to TDC before installing the gauge. I just watch the piston through the spark plug hole and find a rough, visual TDC. Then I back it down maybe 1/4" to 1/2" and install the gauge. I never tried doing a 360° rotation of the engine with the gauge in place, maybe that would present issues like you mentioned, but the way I do it there's no need to.
 
LTGTR said:
For example, the plug angle is too much for the piston to push up a dial indicator or plunger bored through an old spark plug and the piston stop method wont work because it will foul on the valves when you reverse the rotation (approx 360). Maybe my understanding of the Piston Stop method is not right - the theory of this method is certainly accurate.
Click to expand...
Just to hedge my bets, I remove the valve lash adjusters.... valves stay closed... problem solved.
Using the bump stop method, or any other for that matter is a one time shot, so it's not really that big a deal to have to reset valve lash.
 
Depends on how precise you need your TDC. Old school screwdriver in the spark plug opening and watch for it to peak - obviously make sure you are on your correct stroke. You can take off your points cover on the right side and look for the pin to be at 12 o’clock and 6 o clock. I did this for my 277 to have another mark to utilize so it won’t be the same as 12 and 6.
 

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Spark plug hole angle is only an issue with the piston on the rise. If you want accuracy you need to index anyway, no need to crank the motor over TDC with the dial indicator plunger in contact with the piston. With the piston rising BTDC, pull the plunger up a little, turn the motor a little, find the high spot, zero the indicator, back off the piston, and take a degree reading at a known drop BTDC--say .025". Then pull up the plunger, turn the motor past TDC, and take a second degree reading at the same drop ATDC. True TDC (or as close as we're gonna get to it) is the midpoint of those readings. Rotate the crank to that midpoint, lock it, zero your degree wheel, scribe your marks, done.
 
Did mine with a piston stop and home made degree wheel. Bit of a pita but didn’t mind because it’s not something one would do regularly. Got the timing decal from Hugh’s Hand Built. Don’t pay any attention to the engraved TDC on the rotor, it’s an old mark from the po.

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Last edited:
LTGTR said:
Could you help my understanding of which is the best way to find TDC.
I have read all/most of the posts about the different ways to find TDC on our XS650's on this forum, but I cant really say one of those methods ended up being the effective way to do it.
For example, the plug angle is too much for the piston to push up a dial indicator or plunger bored through an old spark plug and the piston stop method wont work because it will foul on the valves when you reverse the rotation (approx 360). Maybe my understanding of the Piston Stop method is not right - the theory of this method is certainly accurate.
There are some other methods on this forum but they seem extremely technical and involved - no disrespect to those that constructed them.
Of course my question assumes you dont have the TDC mark on the Rotor and you have to find it from scratch.
Thanks for any help,
Regards Ray.
Click to expand...

Welcome to REDNECK-OLOGY-101
#1--Take a brand new pencil, or something similar
#2--add 6 inches of air line
the same diameter as the pencil
#3--shove pencil (eraser end) into air line
then tape to secure*
*This is so pencil dont get lost in cylinder

#4--remove spark plug
#5-incert pencil end into spark plug hole
lowering till it hits top of piston
*air line is on top of pencil
(eraser) and outside of cylinder

WARNING!!!
**You should see eraser/air line sticking out of
spark plug hole, if not, grab air line and remove pencil
and stop going forward with this post**

IF this happens-->
YOU must find something longer than a pencil
to continue!!!


#6--rotate motor by hand
#7 notice pencil rises then lowers?
#8--Rotate engine again till ya
think the pencil has risen to its highest point
#9--stop rotating motor
#10--Congrats!
ya got TDC

**Not responsible if ya crunch a pencil
or any other object in your cylinder
This is a suggestion not an order to perform! LOL
 
Last edited:
BTW the reason for taking readings at equal drop either side of TDC and computing the midpoint is to correct for piston "dwell." For a few degrees before and after TDC the piston moves very little, and taking readings at equal drop on either side of the dwell range enables you to locate true TDC with greater accuracy.
 
Thankyou all very much for the explainations. I understand the technique so much better now.
The motor in my 77 is tired and I want to get another motor and rebuild it. I believe this is the best way for me to learn so I am picking on the things that I dont understand.
I have a Yamaha workshop manual, Hans Paul's motor book, parts diagram and you guys as my resources.
I dont own a Dial Indicator yet, and I imagine it would have to be mounted very securely to get the consistent reading. Does anybody have a picture of how a dial indicator is mounted to take a reading down a plug hole angled like our 650's.
Once again, thanks for your help,
Regards Ray.
p.s. finding a complete motor to rebuild is a bit difficult here in Oz.
 
Here's a YouTube video on making your own .....


But there are plenty for sale. Just Google "two stroke timing dial gauge".
 
I just had a go at finding true TDC, using the piston stopper I made the other day. I backed off both valve adjustment screws all the way, hoping that would ensure no valves fouling on the stopper bolt ( M8 with the threads machined off the last 20 mm of length, so around 6.5 mm OD). My XS still has stock camshaft and valves. I ended up with the piston stopping 19 degrees BTDC/ ATDC. Would that give a sufficiently precise TDC location, or should I wind the stopper more in, to achieve, say 25 degrees before/ after?
 
arcticXS said:
Would that give a sufficiently precise TDC location, or should I wind the stopper more in, to achieve, say 25 degrees before/ after?
Click to expand...
I know this sounds counterintuitive, but your most accurate readings will come from the crank being 90° before and after TDC.
If you think about it, that's where you're gonna get the most piston movement per degree of rotation.
So the further away from your 19°, the better.
 
Jim said:
I know this sounds counterintuitive, but your most accurate readings will come from the crank being 90° before and after TDC.
If you think about it, that's where you're gonna get the most piston movement per degree of rotation.
So the further away from your 19°, the better.
Click to expand...
Yes, I get that, but that's not really possible with a piston stop that screws into the spark plug hole on an XS, due to the angle of it. On for example an RD, using a 10 mm stroke dial gauge, you get quite an angle either side of TDC. I saw that Raymond had made a piston stopper for the RE single he has, and it seems he was happy with just 10 degrees either side (20 degrees total) I just wanted some input on what is considered "good enough"
 
Yeah... after rereading your comment, I see where you're going. Oops.
I don't really have an opinion on "good enough." Maybe someone else will.
 
I've always used a dial gauge, so I've never played with a piston stop. On the RD's the timing specs are given in mm BTDC and, of course, the gauge is 90* to the piston, so direct measurement is possible. On XS's - and others - with angled plug holes, I still use a dial gauge to find true TDC and the rest of the work is done via degree wheel (and the specs given in degrees). True TDC mean finding the mid-point of gauge/piston movement. IDK I've always been able to get perhaps +- 1* : close enough for cam or ignition timing:shrug:
 

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