The Stovebolt Site Forums The Shop Area Engine Shop Finding Nemo (I mean TDC) on a 235 Truck Engine

I have a 1962 235ci Truck Engine (in my '48 Chevrolet 1/2 Ton Truck) 6-Volt Setup. I've recently removed and replaced the distributor and have a pertronics setup, plus new plugs & wires. I'm getting a little bit of back-fire through the Carburetor and engine seems to be running at a fast idle. I have a Carter YF on it. Wanting to do a Tune up and adjust valves. Currently have the BB on the Flywheel aligned to the Pointer on the bell housing window. On my distributor, the rotor is pointing to #6 wire to fire. When I removed the spark plugs on #1 and #6 Cylinders, to my eye, it is really hard to tell which Piston is up the highest (in compression) It sorta appears that #6 Piston is a littler higher up than 1, but if it is, it's not by very much. I'm using a flexible bore light from my guns insert in the cylinder, so I've got a good view and honestly, they both look to be about same height in up compression, maybe 6 is higher up by 1/8" if at all. (Q: is there any other better way to confirm when Cylinder 1 is TDC?? Also, I removed the Valve Cover, and in the current position, it appears that all the Exhaust Valves are open or various states of opening, the valve springs all nearly being pushed down, or valves open. The Intake valve springs all look to be mostly up (not compressed) if it matters.

Appreciate any tips I can use to determine true TDC on #1, I'd like to start adjusting all the valves.

- put a cork in each spark plug hole (#1 & #6)
- rotate the flywheel back and forth near the UDC/TDC mark
- the cork that pops out will indicate the firing cylinder

At TDC both the #1 and #6 pistons will be at the top of their bores. The difference is that the #6 valves will be what is known as "rocking" where one valve is just closing and the other is about to open. The #1 valves will be closed.

A piece of oat straw or some such or brass rod inserted on the spark plug hole will tell TDC, air blowing out on the way there will tell which one is firing.

Ed

Pull all six plugs so the engine rotates easy. Tighten the fan belt really tight so you can rotate the engine with the fan blade.
With the valve cover off, rotate engine while watching the two front rocker arms. Go two or four complete crank rotations till you get a good idea of how the valves open and close. When the exhaust valve is closing, the intake should be starting to open. Keep rotating the crank till the intake valve closes. Drop a long wood dowel rod down the spark plug hole. Rotate crank till the rod quits moving out of the hole. This will be TDC.

Another good way is to hold your thumb over the #1 spark plug hole, ( coil wire removed ) and have a helper bump the starter. When your thumb is blown off the hole by compression, you are getting close to TDC. Drop dowel rod down the hole, turn engine by hand till rod quits moving.

It's a lot simpler just to watch the #6 cylinder valves. The exhaust valve is the last one in line, back by the firewall. Turn the crankshaft clockwise from the front until the exhaust valve spring is fully compressed. Continue turning until that spring relaxes and the spring just ahead of it (intake) just begins to move. The #1 cylinder (not #6) will be at TDC on the compression stroke. You can either adjust the valves on #1 at this time, or set the distributor to fire the spark plug. One turn later with #1 "rocking", #6 will be ready for a valve adjustment. The same procedure applies to cylinders 2 and 5, and 3 and 4. They're called "companion cylinders" and each pair comes to the top at the same time, one turn of the crankshaft out of phase with each other in the operation of the valves.

Back when there were real mechanics instead of parts swappers and black box changers, that principle was common knowledge.
Jerry

Yes, if you are able to turn the crankshaft from above by use of the harmonic balancer (or, by use of a starter button).
And, as in this case, the valve cover is removed.

This is true: all modern engines regardless of the design or number of cylinders (L4, L6, V6, V8) have piston and crankshaft throw in pairs, in which one is @ TDC on compression (both valves closed) and its partner is @ TDC on overlap (both valve open). The other pairs also operate in tandem but leading/following by 720 degrees divided by the number of cylinders: 4 cylinder = 180 degrees, 6 cylinder = 120 degrees, 8 cylinder = 90 degrees.
The only difference among engines is which cylinders are paired.

Now, there are 3 and 5 cylinder engines that don't follow that exact pattern, but finding the overlap point and then turning the crankshaft one full turn is the way to find TDC on the compression stroke. The early GM 90 degree V6''s also had an odd-firing cylinder arrangement where there were no exact TDC matchups. I like turning the engine over for for valve adjustments by putting the transmission in high gear and rolling the vehicle forward gently.

I once rebuilt a Ford 3000 3 cylinder tractor engine with a bunch of gear-driven stuff in the valve train- - - -idler gears, accessory drives, etc. There were about six timing marks to align, idlers etc. I always check timing marks by turning the crankshaft two turns to see if the timing marks re-align. Nope- - - -didn't happen. A couple of marks would align, but not all of them. After several attempts at re-timing, I decided to keep turning the engine to see if the marks would ever line up. 96 turns later, it happened! The gears were set up in a "hunting' pattern like a gear-driven clock by including one idler gear with an odd number of teeth to prevent wear on the same teeth constantly meshing with each other!
Jerry

I will add the "Ford 3000 3 cylinder tractor engine" to the "We-don't-work-on-those" list.
"But we do know of some hill folk in an uncharted part of Tennessee who would love to help you."