Please Explain...

imstillatwork · Mar 17, 2005

...The reason for needing to find TDC and and adjust the TDC indicator on these motors? (1981 70HP) What would possibly cause the flywheel / crank relationship to change? I've never seen anything like this on cars before. Found it very odd, and adds an extra step to an otherwise simple procedure of checking the timing.

CaptainPuget · Mar 17, 2005

Re: Please Explain...

Its the indicator itself that is suspect. It can be bent or loosen up and move.

WillyBWright · Mar 17, 2005

Re: Please Explain...

They twist all the power they can out of outboards in general. They're actually very high performance motors compared to most other applications. TDC on a car is Ballpark at best. But you don't run a car motor at redline for hours at a time. That's the difference. There is an easy way to very accurately determine TDC and to keep from blowing holes in the pistions, timing can be nailed right on the head. It allows you to find factual TDC. With cars, you're hoping all the keyways and timing mechanisms agree with where the piston is. If you were to compare the marks on the flywheel and timing chain cover against where TDC actually was according to the piston, it could be off by a few degrees.

imstillatwork · Mar 17, 2005

Re: Please Explain...

ahhh. Geometricly, the idea of finding TDC by checking a set number of degrees + and - and finding the halfway point makes a lot of sense. There is a few degrees where the piston appears not to move while the crank is rotating as it transitions from up to down. So is a locking dial indicator the only way to find true TDC? or is there another way.. no one i know has a dial inticator or tool that will work for this...

DHPMARINE · Mar 17, 2005

Re: Please Explain...

The problem is not locating top dead center.It.s having the pointer (indicator) showing it,so the timing light will help you correctly do sync & linc. DHP

Basscat 1 · Mar 17, 2005

Re: Please Explain...

Originally posted by imstillatwork: So is a locking dial indicator the only way to find true TDC? or is there another way.. no one i know has a dial inticator or tool that will work for this...

Good question. Does anybody have an answer since most of us dont have a "piston stop tool" No. 384887, either?

CaptainPuget · Mar 17, 2005

Re: Please Explain...

I use a small steel ruler with a cross slide. Ive seen it done with a screwdriver pushed through a piece of plastic. I image you could make a tool out of a threaded rod and some nuts.

ob15 · Mar 17, 2005

Re: Please Explain...

I'd be interseted to find out how to do this also.

Walker · Mar 17, 2005

Re: Please Explain...

You can make a piston stop that screws into the #1 sparkplug hole. Turn the crank clockwise until the piston hits the piston stop. Mark your flywheel at the pointer. Now turn the crank counter-clockwise until the piston comes up to the stop again. Mark your flywheel again. Midway between the 2 marks is TDC.

rabidfish · Mar 17, 2005

Re: Please Explain...

Please don't turn the crank counter-clockwise. OH... for the same reason you turn the impeller the correct way while installing it.

imstillatwork · Mar 17, 2005

Re: Please Explain...

So if you adjust the pointer to the new mark, the numbers on the flywheel marks are relative, not absolute, right?

imstillatwork · Mar 17, 2005

Re: Please Explain...

counter-clockwise is what every manual says to do for this. Otherwise, yeah, don't. by hand, less then on rotation is ok for this, its the only way.

rabidfish · Mar 17, 2005

Re: Please Explain...

In the "old days" (showing my age here)we had a tool that would hold an indicator in the spark plug hole. It would thread into the spark plug hole then the indicator would slide into it and had a set screw that would clamp the indicator in place. This method is still used by boat racers of relatively low technology. Mercury used it for almost all the inline 4s and 6s. (Remember the Fairbanks, and Thunderbolt "magnetos"?) To my knowldge, OMC has always used a "stop tool" You could fashion one from an old plug, tapped out to fit a bolt inside.

rabidfish · Mar 17, 2005

Re: Please Explain...

Actually, it doesn't say turn it counterclockwise. It says: turn the crank(clockwise) so the piston is (just) on the way down. Then bottom out the stop tool on the piston. Then continue clockwise until the piston stops on the tool. You never want to turn a working impeller backwards... Unless you want it to stop working... ( OB "101" )

Basscat 1 · Mar 17, 2005

Re: Please Explain...

Originally posted by Rabidfish: Actually, it doesn't say turn it counterclockwise. It says: turn the crank(clockwise) so the piston is (just) on the way down. Then bottom out the stop tool on the piston. Then continue clockwise until the piston stops on the tool. You never want to turn a working impeller backwards... Unless you want it to stop working... ( OB "101" )

You lost me on that one. You say never turn counterclockwise. My Clymer Evinrude/Johnson book says," rotate the flywheel clockwise until the TDC mark is approximately 1 1/2in. beyond the pointer. Depress the piston stop plunger until it contacts the piston and tighten the locknut. Mark the flywheel rim directly under the pointer. Rotate the flywheel counterclockwise until the piston contacts the piston stop tool again. Mark the flywheel again directly under the pointer.

piniongear · Mar 17, 2005

Re: Please Explain...

Walker has it right on this one guys. The only way to find 'true TDC' is to use the stop in place of a sparkplug. Rotate it forward, mark, then rotate it backward, mark again. This has the piston/rod in compression going both ways. True top dead center is exactly mid-way between the marks. This is the method I have always used to find TDC on racing motorcycles and I have never heard of any other way to find true TDC other than this. I set camshaft/valve timing as well as ignition on BSA and Harley dirt trackers for years with a degree wheel and true TDC. The other methods are involving bearing slack and overlap and are not as accurate. I am not saying they will not work, just that they are not accurate...........piniongear

rabidfish · Mar 17, 2005

Re: Please Explain...

Oh... your Clymer manual says turn it backwards... Well, then it must be OK! A fool and his money... stabilize the economy!

tabbott · Mar 17, 2005

Re: Please Explain...

I guess you have to take a chance in determining TDC. Have a new impeller and manual ready for water pump replacement just in case.

rodbolt · Mar 17, 2005

Re: Please Explain...

I am with rabidfish I dont like turning the impellor backwards even though merc manuals say to do it. I always let the piston start down. install my stop tool. home made usually. screw it in till it touches the piston. pull the piston back up against it to remove any slack in the reciprocating assy. mark the flywheel then rotate it clockwise till I hit the stop again. make another mark then split the two marks. I now know where the piston is at TDC and can adjust my pointer so its at the correct mark indicating tdc on the flywheel mark. a trick is to make the marks on a known fixed point.some times it requires me to make a timing pointer out of coat hanger wire temporarily. its not hoodoo, voodoo, nor black magic. but the mechanical TDC must be found to set the pointer. we did it with our race cars all the time. its amazing how far off some balancer marks are. but once we know the correction factor its simple to remark the pointer.

rabidfish · Mar 17, 2005

Re: Please Explain...

There ya go... Dude, you can work with me anytime!

Please Explain...

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