Inliners International Forums General Discussion Engines Top dead center

I originally took my tdc on my 261 from the flywheel, marked my balancer and made a pointer to indicate tdc. I've always been a little wary of the accuracy of this but it seems to work reasonably well within a degree or two. I am now changing my balancer to one from a SBC and will have all the timing marks. In pulling the old balancer at what I considered tdc, I noticed the keyway was at 12 o'clock. Is it true that at tdc, the keyway is at a true 12 o'clock? You couldn't really see this on the old balancer. Any hints would be greatly appreciated. I now run a Tom Langdon adapter to run a TH350 so no more factory timing marks.

Thanks
Warren

Hi Warren . . .

That's always been my understanding - the crank key-way and number-one/number-six crank throws are all aligned at 12 o'clock essentially establishing TDC. But it's the corresponding installed alignment of the crank-gear and cam-gear that ultimately orient valve-timing/spark events to TDC on the number-one cylinder.

regards,
stock49

I would suggest getting a sparkplug piston stop tool and find the true TDC and mark it on the new balancer. Also make a nice indicator to bolt to front of engine.

I want to thank both of you for great replies. I currently have the head off and can see the piston movement. Since the arc of the wrist pin is oval, there are a few degrees when the piston is at top that there appears to be no movement in the piston vertically. I can move the ring gear several teeth in both directions and the piston does not seem to move. I made a nice pointer and it works with the new balancer, I just wanted to make sure I am starting at 0 degrees. By the way Tom, that snout tap tool is one of the best tools I have ever used considering the investment.

Thanks
Warren

The BEST technique for finding TDC is to stop the piston short of full up stroke with a reference point on the balancer or with a degree wheel and pointer, using some kind of stop that is rigid and stays put, and mark that spot. Rotate the reciprocating assembly in the opposite direction until it reaches the same point and mark that spot on your wheel or balancer. Find the point that is exactly halfway between the two and that will be the Top Dead Center point of your rotating assembly.

There are 2 factors that favor using a piston position as far down from TDC as your tool can reach:
1. piston motion is much greater per degree of crank rotation as you approach 75° ATDC = better accuracy.
2. the stock stovebolt piston pin is offset 5/64" (.078") and will have asymmetric motion before vs. after TDC during the first period of rotation. This error vanishes as the piston retreats down the cylinder.

Blackwater, Panic; I followed both you hints, took the piston down 1.5" which equalled about 75 degrees in both directions and marked on the ring gear, about 35 teeth. Found the middle tooth and tada, TDC. My marked TDC on the old pully was off about 3 degrees. I am installing an HEI distributor with vacuum advance and I want to take advantage of every degree.

Thanks again
Warren

Excellent. In that range, each degree is about 1/32", at TDC it's less than 1 thousandth per degree, which is why it appears motionless.

Great!! That'll get you as close as you'll ever need!!