70vetteinpieces

For years I thought TDC was when the piston was at the top of the cylinder and a spark is about to ignite the mixture.

Changed the cam and drive chain to gear drive. Followed the instructions setting the marks at TDC. Crank gear at 12 o'Clock - crank key at 2 o'clock Cam gear at 6 o'clock - cam pin at 3 o'clock.

After that I rotated the engine with intake removed and the #1 intake valve started to open. confused expected both valves to be closed.

Rotate engine until piston is on top. Now the valves are closed and it's compressed ready to fire. But the cam is 180 degree off.

#1 Looked in the Old Mechanic 101 book (1974) and they state TDC is the beginning of the engine cycle. The next thing the intake should open.

#2 Another book stated TDC is the top of the compression stroke and the next thing should happen is a spark

which one is correct?

I do realize a piston will be at the top twice during 2 strokes of a 4 stoke engine , but only one can be the true TDC for reference

and how does the distributer turning at the crank speed know when to send the spark?

Bugman Jeff

I've always understood it to mean every time the piston is all the way up it's at TDC, either on the compression stroke, or the exhaust stroke.

The distributer turns at cam speed, half the crank speed

SteveG75

Crank at 12, cam at 6 is #6 at TDC. Use that to line up the gears.

Rotate crank 360 degrees and you will have crank at 12, cam at 12 and #1 at TDC and ready to drop the distributor in.

David Mc

What Steve said. When the timing gear marks are lined up, #6 is on the compression stroke, not #1

This is not unusual in automotive repair. A lot of the rubber bands import cars use have #4 on compression stroke when the timing marks are aligned.

Mike Ward

You're probably going to regret that. The whine drives most people crazy.

xCCTer

I was thinking the same thing.

Could always supercharge it so you can't hear the gear noise.

70vetteinpieces

Still suffering from brain dead cells. If the crank is at 12 and the cam at 6 should the distributer be pointing at #1 plug?

The crank gear is 1/2 the size of the cam gear. The distributer gear is 1/2 the size the cam distributer drive gear. Unless i am not thinking correctly, the distributer should turn at the crank speed - unless the fact the gears run at 90 degree makes a difference.

I bought the quiet gear set - typically drive only on warm sunny weekends in Ohio doubt the noise would be that bothersome.

Should have built that V8 model kit when I was a younger man

70BBvert

Some answers below

Jim

7T1vette

Hmmm... The 4-cycle engine rotates two revolutions to obtain one ignition firing for every cylinder. The distributor runs at 1/2 speed to the crank.

70vetteinpieces

I have the crank and cam lined up at TDC Crank @ 12 and Cam at 6. Distributer pointing at #1. Will the engine still run? Even though a few cylinders would not be working.

Always believed cam and crank marks aligned closest together - TDC, distributer at #1.
Even some car magazines show examples of this.

70BBvert

oops 2:1 sorry :o

70vetteinpieces No it won't run if you try to start it in that position. It will back fire pretty good through the carb though

You can rotate the crank 360 deg, one full revolution, or install the distributor with the rotor pointing at #6

Jim

7T1vette

Just lift the distributor so the gears are not in mesh, rotate the engine exactly one full rotation, and reinstall the distributor. (This assumes that you are one revolution out-of-time.)

David Mc

I don't think so.

The rotor should be pointing to the #6 high tension terminal in the cap when the cam marks are close together.

Move the spark plug wires around and I bet it starts.

70vetteinpieces

Thanks everyone for your comments. Just seems the repair manual left out one small detail. Use TDC on compression when installing the distributer with it pointing to #1.

Most books state before you tear down put the engine at TDC. Never explain how to correct this step if it was forgotten. Putting an engine at TDC at the beginning of a tear down is useless on OHC when the belt has broken or skipped a tooth.

Roadking

This web site confirms or agrees what you just posted.
http://www.chevelles.com/techref/ftecref5.html