Help - Search - Members - Calendar
Full Version: TDC?
914World.com > The 914 Forums > 914World Garage
dumbjim
This may seem like a stupid question but when your distributor rotor is pointed at cylinder 1, would that be tdc on your fan pulley?
Robarabian
Yes, but, the marks are not far off... so you to verify the cylinder is TDC. Put a plastic straw in there. Additionally, it is best if you pop off a valve cover and make sure where it is in the stroke.. intake versus compression.

You do this by turning the motor and watching the intake valve and exhaust valve.
jaredmcginness
Ideally, yes. Your distributor should have a little dot to confirm the the rotor is pointed correctly.
Also verify with the fly wheel - my marks were a bit off, but really close.
ClayPerrine
QUOTE(Robarabian @ Sep 30 2020, 10:51 PM) *

Yes, but, the marks are not far off... so you to verify the cylinder is TDC. Put a plastic straw in there. Additionally, it is best if you pop off a valve cover and make sure where it is in the stroke.. intake versus compression.

You do this by turning the motor and watching the intake valve and exhaust valve.



With the plug angle on a 914 motor, I would not recommend putting anything in the spark plug hole. It is likely to get broken off.

And DEFINITELY don't use a chopstick. av-943.gif http://www.914world.com/bbs2/index.php?showtopic=22782

Mark Henry
Clay beat me to chopsticks wink.gif
TheCabinetmaker
CHOPSTICKS! Wonder what happened to him?
Frankvw
I do not want to hijack this topic, but I have to say I never came cross the chopstick story before. Sorry for the person who had the chopstick trapped, but some of the tips and comments were hilarious. It was an interesting read on many levels ! I know I will use a straw whenever I come into a chopstick situation ! ! (so that is a bit on topic again)
rgalla9146
The most accurate method to determine true TDC is to thread a stop into the #1
spark plug hole. Rotate the crank (slowly, by hand) until the piston contacts the stop.
Mark that spot. Now rotate the crank in the opposite direction until the piston
makes contact again. Mark that spot.
True TDC is the mid- point between those marks.
The ignition rotor is a little better than an approximation.
dumbjim
QUOTE(rgalla9146 @ Oct 2 2020, 07:04 AM) *

The most accurate method to determine true TDC is to thread a stop into the #1
spark plug hole. Rotate the crank (slowly, by hand) until the piston contacts the stop.
Mark that spot. Now rotate the crank in the opposite direction until the piston
makes contact again. Mark that spot.
True TDC is the mid- point between those marks.
The ignition rotor is a little better than an approximation.

Can you elaborate a little on this procedure? What do you mean by a stop? what kind of stop screws into the sparkplug hole? Where do I put my marks? on the fan pulley, on the flywheel? or the stop? I'm trying to learn as much as I can to work on my 914. There's just not a lot of mechanics in St. Louis that work on air-cooled engines.
Thanks
VaccaRabite
QUOTE(rgalla9146 @ Oct 2 2020, 11:04 AM) *

The most accurate method to determine true TDC is to thread a stop into the #1
spark plug hole. Rotate the crank (slowly, by hand) until the piston contacts the stop.
Mark that spot. Now rotate the crank in the opposite direction until the piston
makes contact again. Mark that spot.
True TDC is the mid- point between those marks.
The ignition rotor is a little better than an approximation.



While this would work, its overly complicated.

Just go to top of travel and take a visual inspection of the valves to make sure they are closed for that cylinder. That is TDC for that cylinder. Bonus points for marking this in paint in the timing notch in the doghouse, but not even that is required since you don't need to do this very often.

Zach
barefoot
QUOTE(dumbjim @ Oct 2 2020, 01:46 PM) *

QUOTE(rgalla9146 @ Oct 2 2020, 07:04 AM) *

The most accurate method to determine true TDC is to thread a stop into the #1
spark plug hole. Rotate the crank (slowly, by hand) until the piston contacts the stop.
Mark that spot. Now rotate the crank in the opposite direction until the piston
makes contact again. Mark that spot.
True TDC is the mid- point between those marks.
The ignition rotor is a little better than an approximation.

Can you elaborate a little on this procedure? What do you mean by a stop? what kind of stop screws into the sparkplug hole? Where do I put my marks? on the fan pulley, on the flywheel? or the stop? I'm trying to learn as much as I can to work on my 914. There's just not a lot of mechanics in St. Louis that work on air-cooled engines.
Thanks

I knocked the ceramic out of a spark plug & welded a steel rod to the threaded bit. stuck out a little over an inch. smile.gif
rgalla9146
QUOTE(VaccaRabite @ Oct 2 2020, 03:21 PM) *

QUOTE(rgalla9146 @ Oct 2 2020, 11:04 AM) *

The most accurate method to determine true TDC is to thread a stop into the #1
spark plug hole. Rotate the crank (slowly, by hand) until the piston contacts the stop.
Mark that spot. Now rotate the crank in the opposite direction until the piston
makes contact again. Mark that spot.
True TDC is the mid- point between those marks.
The ignition rotor is a little better than an approximation.



While this would work, its overly complicated.

Just go to top of travel and take a visual inspection of the valves to make sure they are closed for that cylinder. That is TDC for that cylinder. Bonus points for marking this in paint in the timing notch in the doghouse, but not even that is required since you don't need to do this very often.

Zach


It is certainly not an everyday necessity, it is the most accurate way to determine
true TDC. If your fan or flywheel are not marked ( and even if they are ) this will provide an accurate location.
A stop can be a modified spark plug as described that has a probe
that can be threaded in or out to provide a stop in piston stroke.
Summit or Pegasus would probably offer some version.
This is a "lo-fi" version of our main content. To view the full version with more information, formatting and images, please click here.
Invision Power Board © 2001-2024 Invision Power Services, Inc.