Last night, after many years being MIA, I finally found the TDC indicator tool my dad gave me many years ago. Nice tool, screws into the plug hole and has a rod that can be locked in place, with a micrometer adjustable action.
Then I realize, it is NFG for use on the M20 because the spark plug is directly above the valve and not the piston! And that little 1/4 inch bolt is rusted in so firmly I dare not even attempt to get it out.
So how do you usually set the piston at TDC for figuring out where to set the timing, short of removing the head??


Vincent

email (option): vinver#ns.sympatico.ca

Well, if that was me I would undo the little bolt after a ride while it was hot or consider some direct heat & if it snaps off, only THEN consider taking the head off.
Those depth guages are often misleading anyway because the angle of entry effects it & even worse on engines with high comp pistons as the angle means the "nose" of the gauge travels horizontally across the piston crown as the piston is raised or lowered meaning a high domed piston also can give wildly false readings, unless you calibrate the gauge to your specific engine with the head on & off.
I think many many machines have been timed wrong with those over the years due to the reading not relating to actual piston travel.

fred with a domed piston where would the TDC be measured from ie the lower part of the dome or top of the piston

email (option): roger.beck@node6.com

Well TDC is TDC whatever part of the piston you use. But I know where Fred is coming from. Ron

email (option): ronpier@talk21.com

I generally take the head off to set the timing..That way I can use a depth micrometer for more accurate piston positioning...Ian

email (option): ian@wright52.plus.com

Hi Ian,

Shouldn't it be just about more or less 11 mm for an M20?

email (option): ahum@quicknet.nl

Hi Vincent..Most are.. .. ..Ian

email (option): ian@wright52.plus.com

Again the superiority of the 16H comes shining through with it's great big head plug bolt, same thread as the spark plug hole.

I wish you could time B33s with the head off.

Finding TDC from piston position is a rough quess in any event as signifacnt angular movement gives only small piston movement at TDC. Better to set the timing fully advanced at 7/16" BTDC where angular movement actually gives decent piston movment.

email (option): pvlietstra@gmail.com

Hi Vincent,

If you do have to remove the head to reset the timing anyway, take the opportunity to remove the small 1/4" BSF set screw (bolt) in the head, if it snaps off, simply drill it out and re-tap it (or get someone to do it for you if no facilities available). Replace the bolt with a stainless one with a copper washer and copperslip anti-seize grease on the treads to enable it to be removed in the future.
Years ago I made a simple tool which I carry in the toolkit and takes up virtually no room, consisting of a thick gauge scrap spoke about 3 or 4" long (so it fits under the tank – use the bent end rather than the threaded nipple end) which I inserted vertically into the said hole and by rotating the wheel with the bike in gear on the rear stand, found when it was at top dead centre, I filed a mark on it with a needle file level with the top of the cast boss where the bolt fits. Measure 7/16" higher up the spoke and file another mark. By using a spoke it serves 2 purposes - a) The spoke can't fall through the hole into the cylinder, and b) you can only inset the tool in the correct way up!
By rotating the rear wheel slowly with the machine in gear in the forward direction with the tool in the hole, as soon as the higher mark is in line with the top of the cast boss on the firing stroke you know the piston is at the correct position and with ignition fully advanced, the points gap set correctly and the timing can be set. After setting and the magneto pinion tightened, it can be easily checked by rotating through the sequence again.

Steve Plumb

email (option): steveplumb1@sky.com

Get off you.....you're not allowed on here! Why not post some pics of your customised gearbox?

email (option): davmax@ntlworld.com

Terry. I have just timed my B33 with the head off.I rebuilt the engine and fitted the barrel to bottom end on the bench before fitting back into frame. I always use four 3/8" nuts on the head bolts to hold dowm the barrel.This ensures nothing moves about and keeps good barrel to crankcase gasket contact. Once timed up I then fit the head .The B33 is a lovely engine to work on and I am looking forward to tomorrow so I can fill with oil / petrol and fire it up Dave

email (option): vinver@ns.sympatico.ca