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#853251 07/07/21 9:05 pm
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mrcarb Offline OP
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Is there a tool or marks that can be lined up that will set an A65 at top-dead-center?

Thank you in advance.

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Not TDC but service tool 60-1859 goes in the hole at the front of the case to set 34 degrees BTDC. With a degree wheel and that you can find TDC.
Or, you put a piston stop in a spark plug hole then with the degree wheel you get it.

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Will both valves be closed at 34 degrees BTDC? I am trying to do a leak-down test.

Thank you for replying

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The cylinder on compression will have.

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There is a hole in the front of the crankcase where I put oil in when priming the sump during an oil change, is this where this tool goes? I cannot see any other place. The model is a 69 A65L.

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You really should download a workshop Manual from CBS or elsewhere.
There should be a bolt on the case centre line just to the timing side of the case joint. That is where the timing tool goes.
If there is no bolt there you will get a lot of oil blowing out.

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Thank you DMadigan

@mrcarb - the hole will be on the timing side case directly in front of the engine

Hole size is 3/8" and should be a coarse thread pitch


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Earlier models have a D-shaped cover over the hole, held on with two PH screws. Later models just have a single bolt in the hole.

Turn the crank and locate the slot in the flywheel. There's a special tool for this, but a suitably sized Allen key will suffice. With the tool or Allen key in the slot, the crank will be locked at 34 degrees BTDC, and yes, both valves will be closed on whichever cylinder is on the compression stroke.

Also, with the crank in this position, if you have a '67 or later model, the timing pointer in the primary cover SHOULD line up with the timing mark on the alternator rotor. (Earlier models do not have a timing pointer in the primary cover or an inspection cover over the alternator.) If the timing mark on the rotor does NOT line up with the needle (this can happen if the alternator rotor has been replaced, or the keyway is hogged out), you should make a timing mark on the rotor that DOES line up with the needle.


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For purposes of doing a leak down test, I don't think you need to be that exact. I would just put the bike in 4th gear and, with my thumb over the plug hole, rotate the rear wheel until I felt compression. Then using a screwdriver in the plug hole and rotating the wheel back and forth, I would bring the piston up as far as it will go. There is a fair amount of crankshaft movement around TDC. I don't think that being a few degrees off will affect leak down test results as long as both valves are fully closed.

Ed from NJ

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Originally Posted by edunham
For purposes of doing a leak down test, I don't think you need to be that exact. I would just put the bike in 4th gear and, with my thumb over the plug hole, rotate the rear wheel until I felt compression. Then using a screwdriver in the plug hole and rotating the wheel back and forth, I would bring the piston up as far as it will go. There is a fair amount of crankshaft movement around TDC. I don't think that being a few degrees off will affect leak down test results as long as both valves are fully closed. Ed from NJ

I agree, although it's easy enough to position the crank via the timing slot, or even easier, via the timing mark on the rotor once it's been verified. And yes, anywhere between there and TDC will do.


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DO NOT perform a leak down test at 34 degrees. You need to have the piston, connecting rod, and crankshaft all vertical, in line, and straight up. Do this as edunham recommends above. If the piston is before top dead or after top dead, as soon as you put air into the cylinder to check for leakdown, the piston will shoot downdward rotating the crankshaft either clockwise or counter clockwise. (with force) It is essential you have all those engine parts vertical. A way to hold the crank from turning is advised, even if it is putting the vehicle in gear, and holding the brake on the rear wheel to do so.


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Originally Posted by nert
DO NOT perform a leak down test at 34 degrees. .
Basic truth here. and I've done many leakdown tests on aircraft engines. Right at TDC on the compression stroke is where bore wear and ring gap tends to be greatest and the valves most likely to both be fully closed.

On aircraft, it is the spot where the propeller is easiest to hold still, but a bike doesn't have a prop to hang on to and that 80 or 100 lbs of air pressure can generate a lot of rotating force. Since there is quite a lot of slack in the drive line, IMO a wrench or breaker bar on the drive side crankshaft nut is a better way to hold the engine from turning. An added advantage is the ability to rotate the engine a few degrees back and forth to help settle the rings closer to their normal running spot. This can make a significant difference in your results. I've seen it many times.

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another long time A&P mechanic here. I agree with Mr Kirk above. when I do cars & bikes I like to use a socket on a 1/2" breaker bar to hold the crank. find TDC, be sure both valves are closed, rock it to find top (I usually use a pencil in the plug hole). attach the tester and bring the air pressure up to about 30-40 psi.... now you rock slightly out of TDC to feel for the "soft spot" that indicates where the true top. you should have no trouble holding it at that pressure if you turn too far. rock slightly out of TDC each way and note that one direction often produces a higher number on the cylinder side gauge. that has to do with where the ring gaps are. anyway, find the best direction of rotation to get the number. turn back to TDC... then have a helper run the pressure up to full. on aircraft we use 80 psi, a lot of automotive tends to use 100

and yeah.... 34 degrees is way too far out for a good test, and you probably won't be able to hold it by hand

Last edited by Mitch; 07/19/21 4:14 am.
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Originally Posted by mrcarb
Is there a tool or marks that can be lined up that will set an A65 at top-dead-center?

Thank you in advance.

Yes. LINK

I use this with a degree wheel to find TDC. It is a piston stop so use it gentily or you will break things.

I also have set of these. LINK

To check my work. With the angle of the spark plugs hole to the piston, this can bind and become a piston stop at odd times so be careful.

Last edited by Semper Gumby; 07/19/21 5:28 pm.

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