Help me find TDC

[Charlie]

Hi All:

I am trying to find TDC on the compression stroke on a Kubota 300 horizontal engine. It is a one cylinder. I am having a problem figuring out which TDC is the exhaust and which is the compression. The engine has lifters on the valves and I can take the head off easily right now to see if it is TDC. I am having a problem determining if it is the exhaust or the compression. Can someone help me interpret this diagram.

I think that the intake and exhaust valves should both be closed when the engine is at TDC for the compression stroke. Is that correct?

[Boatguy30]

calder's marine diesel book has a good description of this. Don't have it committed to memory but should be easy to sort out based on movement of valves and reading the section on valve adjustment. Should be tdc on compression first tdc after intake valve closes......
cheers,
Jeff

[boden36]

You are correct. If they are not closed there can be no compression. The valves will be closed for the upstroke of the piston on compression stroke, and then the downstroke of the piston on power stroke.
If is just for the timing, use the marks they mention.
Regards,

Richard.

[propellanttech]

Originally Posted by Charlie

Hi All:

I am trying to find TDC on the compression stroke on a Kubota 300 horizontal engine. It is a one cylinder. I am having a problem figuring out which TDC is the exhaust and which is the compression. The engine has lifters on the valves and I can take the head off easily right now to see if it is TDC. I am having a problem determining if it is the exhaust or the compression. Can someone help me interpret this diagram.

I think that the intake and exhaust valves should both be closed when the engine is at TDC for the compression stroke. Is that correct?

Yes you are correct. Both valves should be up or closed at TDC on the compression stroke. They should overlap some on the intake/exhaust stroke. It should be enough that one or the other will not be slack, when the piston is up.

The problem is finding the exact TDC without a mark on a flywheel or pulley. If your trying to time a camshaft (or injector pump), that mark is important.

James L

[Barnhard_Marine]

Pull the injector cut a long straight length of coat hanger bar the engine over slowly with your finger in the injector hole till you feel it's on the compression stroke. Stick long straight coat hanger down the injector hole till it hits the crown of the top of the piston if the coat hanger is to short you could lose it in the cylinder. Hold the coat hanger with one hand and bar the engine slowly. The coat hanger will rise with the piston and stop rising when the engine is at TDC both valves will be closed. If you go past lift the coat hanger out bar the engine around and try again Good Luck

[Jd1]

Originally Posted by Barnhard_Marine

Pull the injector cut a long straight length of coat hanger bar the engine over slowly with your finger in the injector hole till you feel it's on the compression stroke. Stick long straight coat hanger down the injector hole till it hits the crown of the top of the piston if the coat hanger is to short you could lose it in the cylinder. Hold the coat hanger with one hand and bar the engine slowly. The coat hanger will rise with the piston and stop rising when the engine is at TDC both valves will be closed. If you go past lift the coat hanger out bar the engine around and try again Good Luck

This will not get you an accurate TDC unless you have the coat hanger taped to a measuring device of some kind. Even then, you need to interpret between the point when the piston no longer moves up to the point when it starts to move down.
That is the method I used last time I set timing on a diesel and it worked well.

[Charlie]

I am going to use a dial indicator but I have to figure out how to get it on the piston. Injector hole is rather tight.

[Jim Cate]

G'Day Charlie,

IF you need great accuracy even a dial indicator can be ambiguous since near TDC there is little piston movement for several degrees of crank rotation. Adding a little slop in the measurement due to using the aforementoned coat hanger (!) further dilutes the accuracy. One way of combating this is to pick some arbitrary distance "down" the stroke (where there is greater piston movement per degree of crank rotation) and measure on each side of TDC, and marking the flywheel at each spot. Then find the midpoint between the two marks and Bob's your uncle...

Cheers,

Jim

[Tellie]

If there are no external marks on the fly wheel then there are surely marks on the crank shaft gear and the cam gear that will line up very accurately at TDC

[perchance]

Originally Posted by Charlie

I am going to use a dial indicator but I have to figure out how to get it on the piston. Injector hole is rather tight.

The timing marks should be on the vibration damper and some kind of pointer on the block.

[propellanttech]

Before we start talking about using a dial indicator......

For what reason are you trying to find TDC?

If you are timing a fuel pump, or a cam shaft, a precision location of TDC is required.

If adjusting the valves it is not. Lobes on a crank shaft are not eccentric. When the rocker arms are loose, or equally slack, you are close enough to TDC to adjust. Cam lobes are the same height until hitting the actuation part of their cycle. Since the compression stroke will have both valves closed, the lifters should not move until into the intake or exhaust potion of the engine cycle.

I'm starting to wonder why TDC is needed at this point. If for a fuel pump, yes precision location of TDC is needed, but most (average) people do not adjust fuel pump timing.

If timing a cam shaft, they yes you must use either a TDC mark or a dial indicator, to find the location for the valves aren't moving. But remember the crankshaft revolves to the top twice for each revolution of the cam. When the piston is at top, the cam must be timed with the relative mark of the crankshaft. Both should have marks (and be in the right position) to insure they are in time. Their timing mark location is based on the engine, and can vary wildly between engine makes/models.

James L

[Barnhard_Marine]

My method works fine as long as it's not a pre combustion engine you don't need a dial indicator to do this if you know what you are looking for. The engine this person was talking about is a single cylinder engine he said he was adjusting the valves not timing the injection pump. I did some work on a 6BT5.9M cummins last week. That engine has a timing pin built in to the gear train on the front of the engine to reference TDC. The problem being is the average mechanic forgets to disengage it after its used and it gets sheared off. As a field service Mechanic you need to learn how to improvise. So before you openly condemn me for a practice that works. Think about where you might be... Be it on the waterway or open ocean with frantic wife, kid's or crew. My tips could and may well save you or someone else's bacon in a pinch. I have years of experience with boats from 8' - 200' feet. I am a former USN F-14 power plants mechanic. I have been Chief Engineer on 5 world class mega yacht's and Chief Engineer at 2 world class ship yards. Here is an article that was written about the service I provided for one of my clients


http://2-cycle.mtu-online.com/2-cycle-stories/nightingale

[propellanttech]

Originally Posted by Barnhard_Marine

My method works fine as long as it's not a pre combustion engine you don't need a dial indicator to do this if you know what you are looking for. The engine this person was talking about is a single cylinder engine he said he was adjusting the valves not timing the injection pump. I did some work on a 6BT5.9M cummins last week. That engine has a timing pin built in to the gear train on the front of the engine to reference TDC. The problem being is the average mechanic forgets to disengage it after its used and it gets sheared off. As a field service Mechanic you need to learn how to improvise. So before you openly condemn me for a practice that works. Think about where you might be... Be it on the waterway or open ocean with frantic wife, kid's or crew. My tips could and may well save you or someone else's bacon in a pinch. I have years of experience with boats from 8' - 200' feet. I am a former USN F-14 power plants mechanic. I have been Chief Engineer on 5 world class mega yacht's and Chief Engineer at 2 world class ship yards. Here is an article that was written about the service I provided for one of my clients


http://2-cycle.mtu-online.com/2-cycle-stories/nightingale

I didn't knock your method at all. But pulling an injector when not needed is a situation that could cause more problems.

But no where can I find he stated he was adjusting valves. He mentions the valves, but never clearly states why he is setting the engine to TDC.

Your previous experience is just not needed. I could list just as much experience, but that is just a pissing contest.

The issue here is why is he looking for TDC.

If just to adjust the valves, TDC is not really needed. Just make sure both valves are at their free position. Beyond that, it's academic. And if you want to argue, you will have to show me a diesel cam that has an eccentric, not a lobe.

The answer to the reason for TDC is the most important right now.

James L

[Charlie]

I am rebuiliding the engine. I want to know where TDC for two reasons. One to adjust the valves and two to make sure the imjection pump is timed correctly. You guys are miles beyond my undetstanding of emgines. This is what I did. Both the glow plug and the onjectors are out of the engine. My dial indicator wouldn't fit down the injector hole but it did fit in the GP hole. But it wasn't long enough. The head of the engine was not torqued yet so I figured out when the compression stroke was by looking for two closed valves. Then I removed the head and used the dial indicator to find out
Exactly when the cylinder was at the apex of its travel. The DI indicated the high point of travel. I tried this three or four times and it was consistent. I made a couple pen marks one on the cranshaft and the other on the case. It is my belief I have found TDC. If so I am going to put some permanent marks on. I can see where you would need to make marks at two points if you did not have access to the cpiston as Jim suggested. I did not know what degree of accuracy was needed to adjust the valves.

[propellanttech]

Originally Posted by Charlie

I am rebuiliding the engine. I want to know where TDC for two reasons. One to adjust the valves and two to make sure the imjection pump is timed correctly. You guys are miles beyond my undetstanding of emgines. This is what I did. Both the glow plug and the onjectors are out of the engine. My dial indicator wouldn't fit down the injector hole but it did fit in the GP hole. But it wasn't long enough. The head of the engine was not torqued yet so I figured out when the compression stroke was by looking for two closed valves. Then I removed the head and used the dial indicator to find out
Exactly when the cylinder was at the apex of its travel. The DI indicated the high point of travel. I tried this three or four times and it was consistent. I made a couple pen marks one on the cranshaft and the other on the case. It is my belief I have found TDC. If so I am going to put some permanent marks on. I can see where you would need to make marks at two points if you did not have access to the cpiston as Jim suggested. I did not know what degree of accuracy was needed to adjust the valves.

Now we have some information to go with:

You will need to find the cam shaft timing mark (s), and make sure they have some kind of reference to TDC. If you didn't have the engine at perfect TDC before the head was taken off, the cam may be out of perfect rotational alignment. If you have a manual, it should tell how those marks are aligned. If you do not have a manual, one or all of us may be able to find the information.

It is very important to make sure the crankshaft, and cam shaft are in time. Not having them in time will bend valves.

Since we do not know where you are with the rebuild, you will have to say if your in the tear down stage or bolt together stage. If in the bolt together stage, someone will definitely need to find the cam indexing information.

James L