Topic: Top Dead Center

[Steve F]

'76 CB750F, engine still in the frame and fully assembled, what's the best way to check for TDC without refering to the marks by the crankshaft/points.  I remember someone saying you can place a wood dowel in the plug hole, and feel when the piston reaches the top.  Has anyone tried this and how accurate is it?  I need to check the cam timing to see if I have to pull the engine or not to adjust the cam or replace the cam chain. I want to attach a degree wheel once the TDC is established.

[MRieck]

I have always used a piston stop. You need to use it with a degree wheel. It is more reliable than a dial indicator because of the angle of the sparkplug to the piston crown. APE sells them.

[Gibson]

Never heard tell of a piston stop i have to get one now. You can use a peice of wood but if you can afford it new tool's are away's better.

[MRieck]

Never heard tell of a piston stop i have to get one now. You can use a peice of wood but if you can afford it new tool's are away's better.

It's a very good looking tool too!

[eurban]

This thread reminds me of a stressful moment in my Wiseco 836 build.  I used the handle end of wooden artists brush as a probe to check for top dead center.  Poking it through the spark plug opening and rotating the engine slowly it was as if an evil gremlin was grabbing the brush from me so I pulled it back out.  Still convinced that this was a good idea I tried again and wound up with an inch long piece of the brush handle snapped off and sitting in my combustion chamber.  After much worry and struggling I was able to blow it out the plug hole with my shop vac set in reverse and applied to the intake rubber (intake valve open, exhaust valve closed)  What's the moral here?  I'm not sure but I know I won't be sticking any more objects down into the combsution chamber.  Perhaps my difficulties were due profile/space at the top of the Wiseco pistons?  Be careful!

[Steve F]

OK, did a Goooogle search for piston stop, and saw one on the Crane Cams site.  It's just a piece of threaded brass tubing (basically) that gets screwed into the plug hole.  So, MRieck, how does one use these things?  Seems like piston damage is the greatest of all concerns.  But what is the main objective with the threaded plug?    

[Jonesy]

Another company used to sell the "whistle stop" which was what the name implied- a whistle that screwed into the head in place of the sparkplug. It would only whistle on the compression stroke and when the whistling stopped, you were at TDC. I wouldn't use it for highly-accurate piston position measurements, though...

[Jay B]

  I used the handle end of wooden artists brush as a probe to check for top dead center. 

The best thing to use if you're going to stick something in the cylinder to find TDC is a plain old drinking straw. You can feel piston movement just fine and if the gremlins grab it it will fold over and not break. Always make sure to rinse it thouroghly before you stick it back in your marguerita.

[MRieck]

OK, did a Goooogle search for piston stop, and saw one on the Crane Cams site.  It's just a piece of threaded brass tubing (basically) that gets screwed into the plug hole.  So, MRieck, how does one use these things?  Seems like piston damage is the greatest of all concerns.  But what is the main objective with the threaded plug?    

The basic drill...Bring the piston to TDC using the stock timing marks. Attach a degree wheel ( I put it on the generator side by taking out the generator bolt and space it out with washers ). Attach a piece of clothes hanger with a point ground onto one end to a bolt hole somewhere. Bend the piece of wire to read TDC on the wheel. Rotate the engine counterclockwise about 50 degrees, screw in the stop- slowly...and I mean SLOWLY rotate the crank clockwise until it gentle hits the stop. Write the # down. Now rotate the engine counterclockwise again. Proceed slowly as you get closer to TDC as the piston will make contact with the stop. Write down that number. Subtract the smaller # from the larger # and divide it by 2. Add it to the smaller# and move the pointer to that # on the degree wheel. Example- stop makes contact at 30 degrees BTDC than makes contact 20 degrees ATDC. 30-20=10. 10/2=5  20+5=25  Move pointer to 25. Go through the procedure again and the pointer should point to 25 degrees after turning clockwise and counterclockwise. Remove the stop and when you turn the crank to 0 degrees TDC you will be at true TDC at which point you can time cams, check ignition timing etc.

[Steve F]

OK, did a Goooogle search for piston stop, and saw one on the Crane Cams site.  It's just a piece of threaded brass tubing (basically) that gets screwed into the plug hole.  So, MRieck, how does one use these things?  Seems like piston damage is the greatest of all concerns.  But what is the main objective with the threaded plug?    

The basic drill...Bring the piston to TDC using the stock timing marks. Attach a degree wheel ( I put it on the generator side by taking out the generator bolt and space it out with washers ). Attach a piece of clothes hanger with a point ground onto one end to a bolt hole somewhere. Bend the piece of wire to read TDC on the wheel. Rotate the engine counterclockwise about 50 degrees, screw in the stop- slowly...and I mean SLOWLY rotate the crank clockwise until it gentle hits the stop. Write the # down. Now rotate the engine counterclockwise again. Proceed slowly as you get closer to TDC as the piston will make contact with the stop. Write down that number. Subtract the smaller # from the larger # and divide it by 2. Add it to the smaller# and move the pointer to that # on the degree wheel. Example- stop makes contact at 30 degrees BTDC than makes contact 20 degrees ATDC. 30-20=10. 10/2=5  20+5=25  Move pointer to 25. Go through the procedure again and the pointer should point to 25 degrees after turning clockwise and counterclockwise. Remove the stop and when you turn the crank to 0 degrees TDC you will be at true TDC at which point you can time cams, check ignition timing etc.

Excellent description of the use of the tool.  Many thanks!

[MRieck]

 You're welcome Steve.