Is This a Safe and Accurate Tool To Find TDC?

[smg65]

Sitting here at work, thinking of finding TDC to mark my newly discovered PMA. Please don't tell my boss!

I found my ancient Yamaha dial indicator while digging through a tool box yesterday. It has the 14mm threaded adapter for the spark plug. As the spark plug enters the head at quite an angle, would this tool be safe and accurate to use? I've read many threads on this and have seen piston stops being fabricated, and recommendations from using soda straws to grandma's chin whisker. The original purpose, and what I used it for, was to time two strokes.

What think you?? Thanks!!!!!

 

[retiredgentleman]

Yes, that should be very accurate.

 

[grizld1]

You have the best there is! It will be very accurate, but as with any piston locator used in the XS650, you'll need to pull up on the plunger while the piston is on the rise. Otherwise the side load caused by the angle you noted will cause the plunger to lock up, possibly resulting in damage to the tool. Also be sure to use the tool only on the compression and power strokes; otherwise it will foul the valves.

 

[5twins]

Oh, that's a beauty, and genuine Yamaha to boot. I have a Craftsman version which is also very nice. To avoid that binding on the tool, I locate TDC visually by watching for the piston through the spark plug hole, then stick the tool in. Then you only need rock the motor back and forth slightly to find the exact TDC.

By the way, I bought my tool years ago for the exact same reason as you, to time two strokes.

 

[racerdave]

If you can come up with the wheel that screws into the end, like the tips, it ease's some of the side loading grizld is mentioning. I would still pull up on it as he suggests. Like the others mine was bough Many years ago for 2 strokes, and many other uses. It even has a adapter to try to compensate for angles plug holes. Central tool company

 

[smg65]

Wow! Thanks guys!! I just wanted to check with you before I used it... I figured you would know. And thanks for the tip regarding finding TDC (on the power stroke) by locating a reasonable proximity to it.... using grandma's chin whisker! That was my main concern.

Yes... the old two stroke days. I do miss 'em. The poor guy who was following my Bultaco (with his well-oiled jacket!) probably doesn't though!

Much appreciated!!

 

[smg65]

racerdave... I was in my glory, then you had to show up and make my tool look small!!!

Sorry... I do realize this is a "family safe" site, and as such will refrain from such inferences in the future.

 

[racerdave]

Sorry for showing off

 

[jefft]

Thats the problem I have had with TDC tools in the past is the side load causing binding due to the angle of the hole to the piston in the XS.
jefft

 

[XS650DL]

Sitting here at work, thinking of finding TDC to mark my newly discovered PMA. Please don't tell my boss!

I found my ancient Yamaha dial indicator while digging through a tool box yesterday. It has the 14mm threaded adapter for the spark plug. As the spark plug enters the head at quite an angle, would this tool be safe and accurate to use? I've read many threads on this and have seen piston stops being fabricated, and recommendations from using soda straws to grandma's chin whisker. The original purpose, and what I used it for, was to time two strokes.

What think you?? Thanks!!!!!

I worked at a local Yamaha shop in the 70's and also have a metric dial indicator for
timing 2-strokes. The AT1 and CT1 spark plugs were vertical and used the fixture
shown in your photo. The DT1 spark plugs were at an angle and required removing the head and bolting a "two-hole" adapter with one of the head bolts. The second hole in the adapter would hold the indicator in the required vertical position. My point is that you may bend your indicator using it in an angled xs spark plug hole.

 

[smg65]

As 5twins and others suggested, I'll locate TDC visually before even inserting the tool. At that point, I'm hoping the plunger travel needed would be very minute. Both plugs will be out and I'll be rotating very slowly and carefully. I'll pull the indicator out if there's any sign of trouble. While I'm not certain, I assume this, or a similar dial indicator has been used by others in this fashion.

Thank you for the reply! I'll be doubly sure to pay close attention so not to damage either the indicator or engine.

 

[5twins]

Yes, it will work fine that way. I've verified the TDC timing marks on several 650s using mine in this manner. I do it mostly on the '80+ machines that have the moveable timing plate to insure a P.O. hasn't moved it.

 

[smg65]

Thank you, 5twins, for the affirmation through real world experience. I would have guessed you have used this technique on, oh say, at least 5 bikes! I will use caution when I do this as the last thing I want to do is break the indicator or have to pull the head!

Thanks guys!

 

[mrriggs]

I love cool tools as much as the next guy but you're making things harder than they need to be.

I have used an indicator to find TDC when degreeing a cam and it's no more accurate than the piston stop method. It does, however, take a lot more time and fiddling around.

If you are only looking to make [or verify] a TDC mark on the rotor then you don't need anything more than a piston stop, fine tip Sharpie, and tape measure.

Remove spark plug and rotate crank until piston is near TDC on compression stroke [valves closed].

Turn crank counterclockwise, screw in stop, turn clockwise until it stops. Make a mark on the rotor.

Back off stop, rotate past TDC. Screw in stop and turn counterclockwise until it stops. Make another mark.

You now have two temporary marks. TDC is directly between these. Measure between marks, divide by two, mark TDC.

There are plenty of ways to make this process more complicated and time consuming, but none of them will be any more accurate.

 

[smg65]

Gee... I dunno. That sounds like more work to me. One part left out is the making or acquiring of the piston stop. How can those steps be any easier than sighting TDC, installing the indicator, rocking back and forth until the highest level is achieved by looking at the numbers? I suppose there are several ways too do it. Unfortunately, I probably won't get to it this weekend as I'm doing my favorite job.... painting!

 

[mrriggs]

Because of the piston dwell, you can't accurately find TDC based on max reading from a dial indicator. You can do it that way but the results won't be any better than the drinking straw or grandma's whisker.

The accurate procedure for finding TDC with a dial indicator is to pic a value below max, like 0.050".

Setup the indicator, find max and zero the dial. Turn the crank counterclockwise until the piston moves 0.050" then mark the rotor.

Turn the crank clockwise past TDC then stop at 0.050" and make a second mark.

Measure between the marks, divide by two, then make your TDC mark.

 

[retiredgentleman]

mrriggs is correct in what he's saying. Because of the mechanical dwell, the piston has very little vertical motion when near to TDC. As he suggested, you need to pick a reference point that is lower down, say 0.200" or 0.250", and use those points to mark the rotary motion. Then fine the middle.

 

[smg65]

OK.... I understand. I appreciate the info guys. Always learning!

Thanks!!