Lost my TDC, how do I find it ?-gctid383734

[Mileskb]

Just replaced my points/condenser with Pertroniz Ignitor. Mercruiser 470. I would have done this a long time ago, but for some reason didn't realize they made these kits for large engines. I've always used similar on old motorcycles...

Anyway... my flywheel TDC mark is gone. Corroded, faded, otherwise not visible. The timing marks are fine. Is there any other "trick" to establish TDC. On bikes I have used a straw in the cylinder (via the spark plug hole) and manually turned the engine to locate TDC. Is it possible to manually turn this beast? Is there a better way?

Thanks in advance... excuse the pun. (get it advance... like timing advance...)

Miles

[Guest]

Locate#1 cylinder. Remove the valve cover over that cylinder.

Bump the engine with the starter until you see the intake valve on #1 start to close- then, turn the engine by hand until the timing marks are lined up.

[2850Bounty]

If you have lost the TDC mark, you will need to begin fresh.

You will need to perform a PPS (positive piston stop) procedure to locate true TDC.

TDC comes around twice, but for the purpose of the PPS, it makes no difference.

Do an Internet search on the PPS procedure. Look at several...... some are better at explaining it.

Use at least 30* either side of top of stroke. It will be much more accurate this way.

You will need to make or buy a piston stop.

After the intake valve fully closes, the piston will be traveling upwards. This will be the TDC that you want for ignition timing.

.

[Mileskb]

Can I just use the flywheel nut to turn the engine with a ratchet? Done this on bikes, just not large engines... thought I would ask first.

-mkb

[2850Bounty]

What you are you calling the "flywheel" on your Merc 470?

If you have been a motorcycle enthusiast, you've probably done the PPS before.

[Mileskb]

Yeah, I guess that's not really the flywheel... timing wheel.. . the big wheel on the front of the engine with the timing mark on it... or where the timing mark should be...

Yes, on bikes I just used a wooden dowel... Slide it into the cylinder carefully then slowly and manually rotate engine. As the dowel gets pushed out, find the spot where it's pushed out the farthest.. Sometimes need to make a mark on the stick and rotate the engine a little each way to ensure top really is top. Then mark the timing wheel, at the zero mark, and time the engine as normal.

Is it ok to just use the bolt on that front timing wheel to turn the engine, or is there another/better way.

Do all the of the spark plugs need to be out to rotate engine manually? On some bikes, especially with only 2 cylinders... rotating the engine with the plugs in can be difficult.

-mkb

[2850Bounty]

Mileskb wrote:


Yes, on bikes I just used a wooden dowel... Slide it into the cylinder carefully then slowly and manually rotate engine. As the dowel gets pushed out, find the spot where it's pushed out the farthest.. Sometimes need to make a mark on the stick and rotate the engine a little each way to ensure top really is top. Then mark the timing wheel, at the zero mark, and time the engine as normal.

Is it ok to just use the bolt on that front timing wheel to turn the engine, or is there another/better way.

Do all the of the spark plugs need to be out to rotate engine manually? On some bikes, especially with only 2 cylinders... rotating the engine with the plugs in can be difficult.

-mkb

The stick or dial indicator method can be off by several degrees due to a slight continued crankshaft angle while the piston appears to be at TDC.

I'd not do that.

The PPS will get you as dead ZERO as you need to be.

Marine engine ignition timing is very critical.

Your call on using the front crankshaft bolt...... but if ya snap that sucker, and you'll be cursing yourself.

Yes.... remove all spark plugs.

.

[Alaskanmutt]

Harmonic Balancer.

Plugs out will allow it to spin easier by hand or socket, if using the starter it may spin too easy with the plugs out. Depends on the wear of the engine.

.

[Mileskb]

Thanks Rick... as Always... Makes sense... I'll pick up or make a stop... pull all plugs.. etc...

Gonna make one more careful inspection to see if I can the mark first.. Would make it a lot easier if I could just find it. LOL

Thanks

[2850Bounty]

You can purchase one, or make one. Be sure that the tip is rounded, and be sure that it has a locking jam nut on it.



The ones that you can buy typically DO NOT have a rounded tip, nor the jam nut to lock the "stop" when you are finding the 30* +/- range.



As long as the "stop" stops the piston Equally at 25, 27, 29 or 31 degrees at either side of piston-to-stop travel, you'll be OK.

IOW, the important part will be adjusting the pointer so that an equal amount of degrees are seen at either side of the travel while the piston is being stopped.

Once you remove the "stop", and bring the degree wheel ZERO marking around as to align with the pointer, you will be at true TDC.

This will be when you transfer your new marks over.

.

[Mileskb]

Thanks for the assist everyone...

I pulled the plugs and was able to rotate the engine manually with enough ease to locate the timing mark. It's actually a pretty deep groove, it was just filled with gunk...

As they say a picture (or video) is worth a 1000 words...

http://flickr.com/gp/[email protected]/XrqVd6