Top Dead Center

How do you find TDC?

Year, make, and model of vehicle would be a good place to start.

If you want to do it the hard way, pull the spark plug and stator cover.  Put a long wooden dowel just smaller than the plug hole itself into the spark plug hole.  Rotate the engine so the dowel drops all the way down into the bore and then mark this spot on the dowel against a reference point on the cylinder head, this is approximately BDC.  Continue to turn the engine until the dowel moves all the way upward.  Mark this spot on the dowel against the same reference point on the head, this is close to TDC.  Then turn the engine so the rod drops exactly half an inch back down into the cylinder and mark this point on the flywheel against a reference point on the engine case.  Then rotate the engine the other direction past TDC until the rod drops down into the engine exactly half an inch and mark the flywheel against the same reference point on the engine case.  True top dead center is in the middle of those two marks.

Just make sure you're using a long enough dowel so it won't actually fall into the engine.  Usually ten or twelve inches long should be fine.  Depends on the engine.  A two stroke with a short stroke might only need a few inches of dowel.

1 hour ago, bbix_211 said:

 

How do you find TDC?

 

For what purpose?

Look in the manual?

Google it?

One might suggest if you don't know how to find TDC, you should not get involved in any work that requires setting the motor to TDC.

10 minutes ago, CDNSXV said:

One might suggest if you don't know how to find TDC, you should not get involved in any work that requires setting the motor to TDC.

This is often the answer to a lot of questions on here. 

If you have to ask, you shouldn't be screwing with it.

2 hours ago, CDNSXV said:

One might suggest if you don't know how to find TDC, you should not get involved in any work that requires setting the motor to TDC. 

How does one learn to do such work? I could be wrong, but asking questions is better than getting out the tools first. Forums exist to help fellow riders w/ questions like this. Not sure why so many are resistant to pass along their experience. Why suggest to "google it"? Maybe he thought that he'd get better info from fellow riders? I don't understand why people are on forums if they don't want to share with one another. Makes no sense. Maybe less newer riders are getting into the sport b/c too many with the knowledge find it too much of a burden to pass what they know along?

I agree on getting in over your head, but everyone starts somewhere and asking questions is exactly what he should be doing.

Dial indicator with a spark plug adapter.

There used to be a tool that you could screw into the plug hole that had a rod in the center that would do what was described earlier in here with a wooden dowel but this would measure the distance for you in millimeters  and you could lock it in place then take it out and measure. I don't know if they are still available or not I still have one that I haven't used in probably 20 years.

That's it Creeper Jeeper a dial indicator with plug adaptor

7 minutes ago, Huskydog14 said:

That's it Creeper Jeeper a dial indicator with plug adaptor

And if you don't have the adapter, you can make one out of an old spark plug, just knock the center out and tap for a screw to hold the dial indicator.  You can get a dial indicator at HF for like $15 that works just fine.

Dial indicator works but you still have to split the difference between two points further down in the stroke.  There is a range of several degrees at the crank where the piston is stationary at TDC.  This is the most important aspect of the job.

The marks on my bikes are so hard to see I always use a pencil down the spark plug hole.  I pull the crank inspection plug off and put a 17mm socket on the crank nut with a "T" handle.  You can rotate back and forth and feel TDC close enought for any valve/cam work just with your fingers.  I don't need marks or dial indications, since I can tell with way higher precision than a tooth on the timing chain.  Give it a try and see how accurate you can tell TDC.

For TDC on compression stroke, just make sure your cam lobes are facing away from the buckets.

 

A dial indicator only is accurate through a spark plug hole if the plug hole is square to the piston. It is most of the time but not always. If there is an angle, there will be angular pressure on the tip of the indicator and true TDC will not be found.

Nearly all bikes have marks on the flywheel and a corresponding mark on the cases. The marks on the flywheel (you may find three) typically equate to TDC, idle spark and full advance spark. If I am building an engine with no marks on the flywheel, I make marks on it during the build process (head is off and I can use a dial indicator square to the piston). If needed, I can then use a degree wheel later in the process to make additional marks at key points of rotation.

How do you find TDC?


A lot of people jumping to conclusions...

Are you talking about for starting purposes? I assume you are based on your other threads, as I tend to research before replying.
Slowly push the starter down until you find the point of maximum resistance. Bring the starter back to the top, careful push it just past the resistance point (TDC on the compression stroke). It's only going to take an inch or 2. Bring the starter back up and give it a full kick. It should fire right up.
1 hour ago, wielywilly-g said:

The marks on my bikes are so hard to see I always use a pencil down the spark plug hole.  I pull the crank inspection plug off and put a 17mm socket on the crank nut with a "T" handle.  You can rotate back and forth and feel TDC close enought for any valve/cam work just with your fingers.  I don't need marks or dial indications, since I can tell with way higher precision than a tooth on the timing chain.  Give it a try and see how accurate you can tell TDC.

For TDC on compression stroke, just make sure your cam lobes are facing away from the buckets.

 

This.
Simple, easy to do with basic tools and still very accurate.

Synopsis:

1. Pull plug and put pencil down hole. (carefully)
2. Use socket to turn crank (via opening the plug on side of case)
3. Rotate engine till cam lobes are pointing outwards
4. Put finger on end of pencil and rotate engine back and forth slightly till you find "the middle" between where the pencil stops moving up and before it moves down.

Lastly, what is this for?
For the purpose of timing? Or for a simple vale gap check?
If your gapping valves, you don't need to be at exactly dead center, simply "eyeball" the cam lobes and make sure they're sideways. (i just use the back wheel with transmission in gear)
long as they're "out of the way" you can slip a feeler gauge underneath to take measurements.

8 minutes ago, ohiodrz400sm said:

A lot of people jumping to conclusions...

Ha, this is a very good point.
This poor guy (OP) could have simply been asking how to find TDC for starting the bike :lol:
And here were are giving paragraph long answers!

Ha, this is a very good point.
This poor guy (OP) could have simply been asking how to find TDC for starting the bike
And here were are giving paragraph long answers!

I saw he had another post about dead engine starts and someone brought up TDC. As soon as I read that I just assumed that's what he meant. I may be the one jumping to conclusions. Hopefully we didn't scare him away.

I saw he had another post about dead engine starts and someone brought up TDC. As soon as I read that I just assumed that's what he meant. I may be the one jumping to conclusions. Hopefully we didn't scare him away.

Yeah you're right, I was talking about finding TDC on the starting line.
24 minutes ago, bbix_211 said:


Yeah you're right, I was talking about finding TDC on the starting line.

As you slowly turn the engine over with the kicker, you'll feel it reach a hard spot in the compression as the piston is coming up in the cylinder. Leave pressure on the kicker and as cylinder pressure bleeds off, you feel the kicker drop into a detent with another hard spot. That is TDC for starting purposes. Once there, bring the kicker back to the top and give a good kick.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now

  • Similar Content

    • By doniton2
      anybody running the gut buster 24 this weekend? It looks to be a good ol mud race.  http://www.thegutbuster24.com
    • By Bryan Bosch

      Rockstar Energy Husqvarna Factory Racing Star battles Through All-New Format Hell’s Gate to Finish Second
      February 11, 2018 – (Motor Sports Newswire) – Rockstar Energy Husqvarna Factory Racing’s Graham Jarvis has earned a strong second place at the 2018 Hell’s Gate extreme enduro. With an all-new format for this year, concluding in a 20-minute sprint race, the event posed a fresh challenge for the five-time winner.
      For the first time ever, the event kicked off with a Prologue race on the Friday night. Designed to set the starting order for Saturday’s race, the Prologue was an endurocross style event held close to the hosting Il Ciocco Hotel. Although it provided a great spectacle for the fans, the race results were eventually scrapped due to issues with the course.
      Saturday’s heat race went well for Jarvis. An expert at managing his pace, the Brit did exactly what he had to do over the five-laps, finishing fourth aboard his Husqvarna FE 250 in the cold conditions of the Tuscan hills. The position gave him a good start for the night race, which saw riders set off at 3.30pm for a tough three-hour multi-lap event.
      At the conclusion of the night race, the top-10 riders were given a 15-minute break before contesting the new 20-minute endurocross in the arena area of the venue. In freezing conditions, the riders fought bar-to-bar in front of the record-crowd of spectators to decide the overall winner. With the final results calculated on the sum of the previous legs, Jarvis was announced as runner-up, just a few minutes behind eventual winner Mario Roman.
      Graham Jarvis: “I really enjoyed the racing today, I always do here at Hell’s Gate. It’s been a tough couple of days and I’m still getting used to riding the 4-stroke, but the bike has been absolutely faultless over the entire weekend. It has been pretty close between the top-four all day and it’s all come down to the final 20-minute race off. It would have been nice to take my sixth win at the event, but I’m happy with second.”
      Results – Hells Gate 2018
      1. Mario Roman (Sherco) 3:06:03
      2. Graham Jarvis (Husqvarna) 3:08:57
      3. Travis Teasdale (Beta) 3:12:45
      4. Diego Nicoletti (Husqvarna) 3:32:14
      5. Sonny Goggia (KTM) 3:39:26
      6. Michele Bosi (Beta) 3:46:15
       
      Husqvarna Motorcycles North America, Inc.
      Husqvarna has continued to expand its professional race team to compete in AMA Supercross, AMA Pro Motocross, AMA National Enduro, GNCC, AMA EnduroCross, and AMA National Hare & Hound championships. Husqvarna team riders are competing aboard Husqvarna FC 450, FC 350, FC 250, FE 350, TC 250 and TE 300 models.
      Rockstar Energy Drink
      Rockstar Energy Drink is designed for those who lead active lifestyles – from Athletes to Rockstars. Available in over 20 flavors at convenience and grocery outlets in over 30 countries, Rockstar supports the Rockstar lifestyle across the globe through Action Sports, Motor Sports, and Live Music. For more information visit: www.RockstarEnergy.com
      Source: Husqvarna Motorcycles GmbH

    • By deanmellor
      My top end went on m 98 cr 125, the needle bearing clip came out and broke apart and shrapnel got everywhere and blew it up. I cleaned out the top end and washed out the crank with the bike upside down. the bike starts up great and seems to run fine but im getting a weird di-di-di-di-di-di-di noise coming from the motor. i tried adjust the power valve and tightening everything but the noise is still there. Here is a link to a video where you can hear the sound. It gets louder as rpm's get higher and at really low rpm barely idling it goes away. I am pretty new to bikes and 2 strokes so any help would be appreciated.
      https://youtu.be/msfU8WCXg8E
    • By RidingWithStyle
      In need of Cycra Handguards (probends) for my bike. Need 1 1/8 clamp size for renthal twinwalls. Message me if you have any for sale at a good price! 
    • By Baja Designs
      Hello CRF450R Owners,
      Bailey here with Baja Designs.  We recognize ThumperTalk as one of the go-to sources for information on the CRF450R and we receive emails daily regarding the best inexpensive light setup or purchasing the B1K setup ran by Ox Motorsports.  We would like this thread to be a General Discussion area for members and guests to discuss Dual Sport Kits (DSK), Lights, and Lighting Acessories based upon stock and modified charging systems.  Please Post up if you have any questions or comments CRF450R related! 
      www.bajadesigns.com