top dead center

tmar04 · Jan 30, 2008

I am planning to replace valve stem seals on my YM2500 (3T80) with head on. What is easy way to tell when piston is at, or near, TDC, to preclude valve from dropping into cylinder? can't stick straw into injector hole because of the 'precombustion chamber'. got to be an easy way. thanks to all.

rock2610D · Jan 30, 2008

The best way I found was to rotate engine by hand. When compression is felt (rotating will get hard), Push down one of rockers, for each cylinder, one at a time. 2 of the 3 cylinders will not affect ease of rotation. 1 cylinder will cause rotation to get easier and you may hear air excaping from cylinder either thru exhaust or intake, it depends on which rocker you push on. Then you know that that cylinder's piston is on its way and close to TDC.

On my 3T80 there are marks for 1,2, and 3 TDC, if yours does to, that will help.

Be advised that this proceedure is how you adjust lifters too! And is how I determined I had poor compression in 2 cylinders, prior to my rebuild. While rotating engine it never got difficult to rotate by hand, until #3 cylinder came up to TDC. Implying that 2 of the 3 cylinders had poor compression.

RobJ · Jan 30, 2008

Another easy way is with the valve cover removed, rotate the engine until you see the 2 valves rocking..(exhaust valve will be closing and the intake will start to open, technical term is valve overlap). Then turn the engine another 360 degrees and you are close enough. When the exhaust valve is closing at overlap, the piston is pretty close to TDC, the intake valve starts to open as the piston moves down to suck in more air.

Work your way to the other 2.

Good Luck!

RobJ · Jan 30, 2008

rock2610D said:
Be advised that this proceedure is how you adjust lifters too! And is how I determined I had poor compression in 2 cylinders, prior to my rebuild. While rotating engine it never got difficult to rotate by hand, until #3 cylinder came up to TDC. Implying that 2 of the 3 cylinders had poor compression.

Hey Rock, what lifters are you talking about. All diesels I worked on had basically a solid lifter riding on the camshaft. No adjustments.

If you are talking about adjusting the valves...well then nevermind.

Rob

rock2610D · Jan 30, 2008

Sorry, I ment valves, not lifters.

mark777 · Jan 30, 2008

I use a bicycle spoke through the combustion chamber cone...a gentle probe will indicate the piston travel height.

Once you've pulled the valve keepers it may be necessary to keep compressed air in the cylinder to reassemble the seals, springs and keepers...not sure as I haven't done it that way.

Mark

RobJ · Jan 30, 2008

Air usually not needed on a diesel. Flat top piston and a flat head. This is my Kubota which probably is the same as most small 2-3-4 cylinder diesels.

Rob

tmar04 · Jan 30, 2008

Rock2610D, thanks for tip on TDC on 3T80 eng. Mine has the same three marks on pulley, 1-2-3. If I line up the #1 mark with the pointer, does this mean #1 piston is at TDC? Sure would simplify matters. thanks.

rock2610D · Jan 30, 2008

Its a 4 stroke engine. Up once for compression, down once for power stroke, up once for exhaust stroke, and down once for intake stroke.

I believe that when the cylinder # is aligned with the pointer its either in the TDC of compression stroke or exhaust stroke. Not 100% sure though so if I were you I would double check, because its a 3 cylinder and I could be wrong, as firing order is 120 degrees apart.

California · Jan 30, 2008

...and #1 cylinder is nearest the flywheel.

tmar04 · Jan 31, 2008

just had a thought. when I get piston to TDC, and strike the top of valve spring,keeper, won't this drive the valve down, causing it to hit the top of piston? I don't like this idea. I need to be able to secure valve someway to keep it from going down into the cylinder.

rock2610D · Jan 31, 2008

Not sure why you would "strike" anything. Use a valve spring compressor, compress springs and remove keepers, then release compressor and remove spring. RIGHT??

mark777 · Jan 31, 2008

Hey Rock,

tmar04 is talking about replacing valve stem seals without removing the head.

It will be anything but easy/tricky to keep the valve stem height in place while collapsing the sping, installing the keepers.

I wouldn't hesitate to pull the head, dress the valves and seats (even hand lapping them) and re-assemble the head with a new head gasket. Reset the valve clearance and your good to go.....

Mark

tmar04 · Jan 31, 2008

Mark, I think you are right. I had the head off last week, but did not realize smoke was so bad. no big job to remove. thx

RobJ · Jan 31, 2008

tmar04 said:
just had a thought. when I get piston to TDC, and strike the top of valve spring,keeper, won't this drive the valve down, causing it to hit the top of piston? I don't like this idea. I need to be able to secure valve someway to keep it from going down into the cylinder.

It won't heart anything, just don't hit the valve stem. By hitting the spring the stem doesn't move much anyway. That's why this way of removing the keepers/springs works head on or head off. You don't need anything to back up the valve from the bottom. Even if you use a compressor, the valve may still hit the piston to free the spring retainer and keepers. Remember they've been stuck together for some time.

If you are really worried, just lightly hit the springs to loosen the retainer from the keepers, then just press the spring down by hand or with a wrench, have a helper use a magnet to grab the keepers. It'll still press against the piston...it has to...gravity and all.