timing pin and TDC dont match

kenparker

kenparker

Well-known member
I am in process or rebuilding a damaged engine - #1 rod babbit went bad. Before final buttoning up - putting the head back on I set the timing by using the timing pin and noticed the piston was not at Top Dead Center. Got out the trusty dial indicator, set it up and checked timing by using the dial. The timing pin and dial indicator of TDC are 8 to 10 degrees different. The timing pin has the timing set at 8 to 10 degrees before TDC.

I check several times and get same result each time. Normal????
 

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set piston at tdc.
zero dial indicator.
Place timing pointer on front of block
rotate crank ccw until dial indicator changes -.050.
place a mark on the pulley.
rotate crank cw past tdc until dial indicator changes -.050
place a mark in the pulley.
half way between these marks is true tdc
 
1. The cover on the engine is an A..
2. Your "degree bracket" thing looks like it's modified,..welded on....
3. Who's to say that the mark on the pulley is correct?
4. By looking at your indicator,..you are TDC'ing the engine incorrectly!

I've done this for 45+ years for a living,...trust me..
If you want too talk,...send me your phone number.

Dudley

Note,...number #5 is very close!
 
Modern 87 octane gasoline will tolerate more spark advance, so I would set the timing using the Model B cover. If the engine knocks under load, then retard the spark in small amounts until it no longer knocks. Use the cam in the distributor to retard the spark.

I have been timing my Model As this way for many years. This technique optimizes engine performance.
 
Following up on George Miller's post, the crank and cam gear have 25 and 50 teeth. A single tooth off will result in a cam angularity error of exactly 7.2 degrees.
 
MikeK said:
Following up on George Miller's post, the crank and cam gear have 25 and 50 teeth. A single tooth off will result in a cam angularity error of exactly 7.2 degrees.
Click to expand...

You can check it on the "over lap" (stock A cam) by using a square on top of
valves,...they SHOULD be very close to the same height..
 
All above are good suggestions to check.

Usually ........ human nature causes a person who initially asked a Model A or any other type of question to have a predetermined concept of the problem.

in despair, many feel a few of the suggestions offered herein are idiotic ...... so they selectively only check the suggestions which are determined by them to be non-idiotic.

I find that sometimes, idiotic responses usually solves problems.

From experience, in "addition" to other above good suggestions, always use the non-idiotic "Method No. 5" above to verify the "exact" piston TDC position when the timing pin slips in.

The hand crank and crankshaft can rotate CW & CCW a few almost unnoticeable degrees when the piston is at TDC.

This country is over 200 years old, still run by idiots, and just think, we are still here.
 
On #5...the need for this method of splitting the difference for TDC is because of the offset crank. This introduces a dwell period of maybe 3 degrees at TDC without enough movement for even a dial indicator to sort out.
Easier than a dial indicator is simply a stop...bolt a strap across cylinder with a bolt through it and use that to stop the piston in each direction.
 
A correct piston stop has an adjustable bolt through the center to accommodate for piston dwell time as it can still be a factor if you are not at the apex of the upstroke.

Realistically, unless you are verifying cam specs or altering cam timing the dwell time I'ts a none issue. If tdc is that close, and the parts are correct, installing the cam and gear correctly it will align with the crank in proper order.
 
In not trying to confuse this very simple TDC matter, a dial indicator always works extremely accurately, per the gentleman's suggested no. 5, by "always" rotating the crankshaft CW & CCW .......... or CCW & CW .... and next recording "any" two (2) separate readings that are "alike", when read on one's dial as the piston moves down from a complete halt at the top.

In my humble opinion, the most difficult part of this very accurate test in 2016 is absence of "Arithmetic", where many today are totally incapable of finding TDC with this test because they are not capable of adding two (2) simple, separate numerical readings together and dividing the sum of same by two (2) to locate the "accurate" halfway mark suggested in no. 5 above.

Hope this helps.
 
Hi rail,

Similar things can happen today in a store when one sees (3) apples for $1.50 and arrives at the cash register with (2) apples .............. buzzer goes off to call the manager .......
 
H. L. Chauvin said:
In not trying to confuse this very simple TDC matter, a dial indicator always works extremely accurately, per the gentleman's suggested no. 5, by "always" rotating the crankshaft CW & CCW .......... or CCW & CW .... and next recording "any" two (2) separate readings that are "alike", when read on one's dial as the piston moves down from a complete halt at the top.

In my humble opinion, the most difficult part of this very accurate test in 2016 is absence of "Arithmetic", where many today are totally incapable of finding TDC with this test because they are not capable of adding two (2) simple, separate numerical readings together and dividing the sum of same by two (2) to locate the "accurate" halfway mark suggested in no. 5 above.

Hope this helps.
Click to expand...

All right....now I'm getting confused with what you wrote. There is NO math,
period. No adding, dividing....okay.

This whole posses is best done with a degree wheel,..the crank pulley is too
small in diameter! For a PROPER job you need to go BOTH CW and CCW for
the dial indicator. Now,..nothing against "a bass" at using .050" down the
hole, okay! I like .100" down the hole,..it gives a better reading.

Offer still stands on a verbal tutorial..
 
That appears to be an 'A' timing cover, but, a more frontal picture would help determine where the hole is located in the raised boss.
So, if it is an A cover then the next thing is to check the gear alignment marks.
 
Check to see that the timing gears are aligned correctly.
 

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"the need for this method of splitting the difference for TDC is because of the offset crank."

The concept of a piston stop can be applied to any motor....there is a lot of dwell adjacent to TDC.
 
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