MGB & GT Forum

Hi

I carefully photo'd my setup before disassembly and left the clamp on the dizzy to make it easy to put it back correctly. pic of rotor position whilst the dots were lined up on the cam and crank pulleys.

in my haste i have now put the timing cover back on.

doh!

so, my question is, which valve is fully open when the 2 dots are lined up at their shortest please? i believe it means 1 is at TDC on compression stroke?

thanks
G

When the two dots are closest, cylinder number 4 is at TDC on its firing stroke. You need to rotate the crankshaft one revolution to put cylinder number 1 in firing position at TDC before you can install the distributor drive gear into the block. Then install the retainer with its single slotted screw, and then install the distributor. Rotate the rotor shaft while pressing lightly on the distributor body until you feel the drive dog on the bottom of the distributor shaft engage its mate in the distributor drive gear...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

This may not apply to you Gray (because of where you're at) and I confess I don't fully understand and perhaps Dick will comment. When I was assembling my engine (watched by my guru) TDC on 1 compression was set and then I put the distributor drive in and trial fitted the dizzy (without the 'o' ring to make it easier). Once satisfied that the orientation was correct I removed the dizzy leaving the drive correctly set. Now I just need to pop the dizzy in when I'm ready.

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Mk1: CKD 11/66 first registered 8/5/67; owned since 3/77. 18GB +40 balanced. Peter Burgess BVFR head. Piper 285. 123. FidanzaFW. 4synch c/r box. Lots more as I did a nut and bolt rebuild; finished 2015. Tartan Red.

GT: December '78. VW Golf guards, flush fit front and rear valances. Torana XU1 vents, frenched indicators & Mk1 rear lights. 'Worked' Rover V8 with Monsoon ECU for EFI. GM4L60E, Lokar tiptronic & Quick4 controller. Vintage Air A/C. FC IFS. CCE 4 link rear. Salisbury with Quaife. Jaguar Storm.

You don't have to over think this, this is how I do it. Get the engine on the stand at TDC compression on #1 cylinder (meaning both valves are closed on #1), now check your distributor drive and distributor drive gear, they will only fit in one way, test fit it the distributor and drive outside the engine block, use a Sharpie marker to reference both the the slot in the drive gear and on the distributor drive, install in the drive gear in the manner it takes to put the distributor rotor at 1-2 o'clock, and there you go.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

and if the head's off? How to tell #1 valves are closed?

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1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

the manual shows a position of the slot in the drive gear @ #1 TDC,

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1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

By looking at the camshaft lobes for #1.

The key to all of this, including the photo of the orientation of the distributor drive gear in the block, is all contingent on cylinder number 1 being not just at Top Dead Center (TDC), which is the very top of its travel, but also that cylinder number 1 is in firing position at TDC, which means it is just completing its compression stroke.

In a four stroke/cycle engine, which is what nearly every automotive engine is, the crankshaft makes two revolutions for every power stroke.

Stroke/cycle 1: The intake valve opens and the piston travels downward, sucking in the fuel/air mixture.
Stroke/cycle 2: The intake valve closes and the piston travels upward, compressing the fuel/air mixture. Note, specifically, that when the piston reaches the top of its travel (TDC) on this stroke, it is in firing position, ready for the spark plug to light the fire.
Stroke/cycle 3: The spark plug ignites the fuel/air mixture, driving the piston downward.
Stroke/cycle 4: The exhaust valve opens and the piston travels upward, pushing the spent exhaust gases out through the exhaust valve. Important. Note that as the piston reaches the top of its travel, it is technically at Top Dead Center (TDC), but it is not in firing position and is not ready for spark, since it has not yet ingested its next charge of fuel/air mixture.

Then the four strokes/cycles start over again. So you can see that the crankshaft makes two revolutions, during which the piston reaches TDC twice, but in only one of those two TDC events is the cylinder in firing position ready for a spark to ignite the fuel/air mixture.

The confusion arises partly because folks confuse TDC with firing position at TDC, and partly because the manuals are not clear enough in explaining that timing chain sprockets dot-to-dot do not put cylinder number 1 in firing position at TDC. Dot-to-dot puts cylinder number 1 at the top of its exhaust stroke, so it has no fuel/air mixture to ignite. It takes one additional revolution of the crankshaft to put cylinder number 1 in firing position, with a fresh fuel/air charge, at which point it is ready for a spark to light the fire. And it is at this point that the distributor drive gear should be installed into the block with the offset per the illustrations in all the manuals. Once the distributor drive gear is properly installed in the block, along with its retainer and single slotted retaining screw, the distributor itself can only be installed in one way, since the drive dog on the bottom of the distributor shaft is offset to match the slot in the distributor drive gear.

Bottom line? The distributor itself can never be installed 180 degrees off; only the distributor drive gear can be installed 180 degrees off, or off by some other measure depending on the meshing of the drive gear's teeth with those on the camshaft gear.

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Dick

Thanks for your account which is all true, but my photograph was taken whilst the dots were aligned meaning MY snapshot of dizzy position was not at the beginning of the firing stroke (which would normally be used for timing) but of compression stroke and i just need to replicate what I had and hopefully my timing should be within spec and the engine will start.

The dizzy runs of the cam so only has one rotation per 2 rotations of the crank so as far as I can see, if I know the dots are aligned (ie, identifying TDC of compression stroke) then I set the dizzy as my photo by putting the drive dog in appropriately.

If I went to TDC of ignition, then I would need to set it 180 degrees away from what is was.

I have turned my engine over so that the exhaust valve on cyl 1 has just closed and the piston is at TDC, so the dots should now be aligned.

This is correct, yes? please tell me if I am missing something.
G

Graham,

I'm not quite sure how else to detail this, but let me try. I'll suggest you forget about your photos and just set it up so that you know it's correct.

With the valve cover off, turn the engine in its normal direction of travel -- clockwise when viewed from the front of the engine, until the second valve from the front, the intake valve for cylinder number 1, opens then closes. Then continue to turn the engine slowly until the timing mark on the balancer aligns with the zero degree mark on the timing cover. This will assure that cylinder number 1 is in firing position at TDC.

Then install the distributor drive gear with the offset per the illustrations in all the manuals. Then install the retainer and its single slotted screw. Now you can install the distributor with the vacuum advance unit pointing toward the heater control valve, and turn the rotor and its shaft until the drive dog on the bottom of the distributor shaft drops into its mating slot in the distributor drive gear. This should have the rotor pointing at about the 1 o'clock position. Install the distributor cap, insert the spark plug wire for cylinder number 1 at the 1 o'clock position where the rotor is pointing. Then install the rest of the spark plug wires in the proper firing order -- 1-3-4-2 counterclockwise in the distributor cap.

Does this make any better sense?

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)