Thread: 350 SBC harmonic balancer swap

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Finishing a 350 using short water pump. Vibration damper (8 inch) has marks for timing straight up at 12 o'clock position. New timing tab is for earlier SBC with balancer marks on drivers side at about 2 o'clock. Anyone know the measured distance between the two marks so I can remark the dampener?

the dampers from the 62-68 engines have the mark 2 degrees from the keyway centerline

the dampers from the 69-up engines have the mark 10 degrees from the keyway centerline

given that the circumference of a damper is pi times the diameter, you can measure your diameter, multiply by pi and then take 8/360 of that result.......gives you a distance.

Originally posted by robot
the dampers from the 62-68 engines have the mark 2 degrees from the keyway centerline

the dampers from the 69-up engines have the mark 10 degrees from the keyway centerline

given that the circumference of a damper is pi times the diameter, you can measure your diameter, multiply by pi and then take 8/360 of that result.......gives you a distance.

Now all I need is the exact diameter of both harmonic balancers. Mine is approximately 8 inches. Will measure it with calipers. The early balancers I remember were 6 7/8 and / or 7 inch. Any help?
Thanks! Jim

This way is easier and requires no math. Take a bolt, brass if you can get one, and thread it in through the spark plug hole. Try not to thread it all of the way against the cylinder wall on the other side of the cylinder. Slowly rotate the engine until you feel the piston hit the bolt. Back the bolt off a little and turn the engine until it gently hits the bolt. Repeat until the engine doesn't hit the bolt again. You have just passed TDC when this happens. Mark the balancer where it lines up with the 0 and you should have yourself a TDC. There is some error in this because the piston remains at TDC for some number of degrees of crank rotation so you can move the crank back and forth a few degrees and the piston will remain at TDC against the bolt. But, it has to be more accurate than trying to relocate the mark 10 degrees.

you can also go to the speed shop and buy timing tape for an 8 inch balancer. Use the tape to find your 8 degree offset.

By the way, 6 3/4 dampers are available from GM for the new style engines. Also, a fluidamper is 7 1/4 dia.

accurately measuring the balancer diameter doesnt make a bunch of difference......

for instance, if the balancer is exactly 8 inches diameter, the offset for 8 degrees is 0.5585 inches

If you have a balancer that is 8 1/4 diameter, the offset for 8 degrees is 0.5759.....a difference of 0.017......less than your scribe line width.

Ok. Now I feel dumn! Haven't used high school algebra since.... well since high school! On the way home the light came on (dimly) and I got it. Did the math, measured and scribed a new line. Fired the engine and checked and the mark is perfect. Will pick up a timing tape at the local speed shop and verify just how many degrees I moved the mark.
Thanks a bunch for making an old man think!
Jim

Hmmm......it's been a long while, but seems like to find TDC, you'd thread that brass bolt into the spark plug hole (I used to braze a piece of brass into a broken spark plug) and turn the motor over gently till the piston contacts the bolt. GENTLY ! ! ! Mark the damper at the TDC arrow on the block. Turn the engine backwards till the piston contacts the stop again. Mark the damper again. !/2 way between the 2 marks is exactly TDC. Did I remember it right ??

Lar.

Originally posted by biglar
Hmmm......it's been a long while, but seems like to find TDC, you'd thread that brass bolt into the spark plug hole (I used to braze a piece of brass into a broken spark plug) and turn the motor over gently till the piston contacts the bolt. GENTLY ! ! ! Mark the damper at the TDC arrow on the block. Turn the engine backwards till the piston contacts the stop again. Mark the damper again. !/2 way between the 2 marks is exactly TDC. Did I remember it right ??

Lar.

Yep, cant get any closer than that with the heads on. Especially if you have a digital 6 inch caliper.

Originally posted by biglar
Hmmm......it's been a long while, but seems like to find TDC, you'd thread that brass bolt into the spark plug hole (I used to braze a piece of brass into a broken spark plug) and turn the motor over gently till the piston contacts the bolt. GENTLY ! ! ! Mark the damper at the TDC arrow on the block. Turn the engine backwards till the piston contacts the stop again. Mark the damper again. !/2 way between the 2 marks is exactly TDC. Did I remember it right ??

Lar.

I finally use the system u described I threaded a bolt into a spark plug and had my boy turn the flexplate at the trans and found the damper pulley had rotated and 2 miles test later it came off completely and fell againt the pulley. that was good thing and is now replace and accurate. Ed ke6bnl