Timing Tab Position

[Mike B.]

I am looking for an answer to a question that has nagged at me.

The timing marks on my 1961, 283 seem to be a few degrees off. I have checked and double checked TDC against piston and valve position, and have also swapped out the hamonic balancer in the event that one may have slipped, but both are right on the same mark. I am able to time it using a new paint mark at TDC, but of course it would be better if I had the original mark.

The timing tab is spot welded onto the timing cover which looks to be period correct. Could it be that there are different covers through the years, with the tab in a different position?

Thanks, Mike

[Clem Z.]

Re: Timing Tab Position

I am looking for an answer to a question that has nagged at me.

The timing marks on my 1961, 283 seem to be a few degrees off. I have checked and double checked TDC against piston and valve position, and have also swapped out the hamonic balancer in the event that one may have slipped, but both are right on the same mark. I am able to time it using a new paint mark at TDC, but of course it would be better if I had the original mark.

The timing tab is spot welded onto the timing cover which looks to be period correct. Could it be that there are different covers through the years, with the tab in a different position?

Thanks, Mike

use a piston stop to find TDC. rotate the engine ccw till it stops,mark the damper in line with the "0" mark on the timing tab. rotate the engine cw till it stops and mark the damper in line with the "0" mark on the tab. if all is correct the TDC mark on the damper should be exactly half way between the 2 marks you put on the damper

[John H.]

Re: Timing Tab Position

The timing tab is spot welded onto the timing cover which looks to be period correct. Could it be that there are different covers through the years, with the tab in a different position?

Thanks, Mike

Mike -

The "0" mark on the SBC timing cover tab was in the same place on Corvette engines until sometime in the mid- to late 70's, although there were many different configurations of tabs. It changed on passenger cars in 1969 (moved 10* CCW) when those engines went to the new long water pump accessory drive system, but Corvettes didn't use that system and stayed with the short water pump.

It's easy to determine if the inertia ring has "slipped" on a harmonic damper; the timing index line on the inertia ring should be exactly in line with the keyway slot on the hub and a radial line from there through a pulley bolt hole. The GM drawings require that the inertia ring not move relative to the hub at any less than 1,000 foot-pounds applied to the ring with the hub held stationary, so "slipping" is a lot less common than some folks think.

[Joe L.]

Re: Timing Tab Position

Mike -

The "0" mark on the SBC timing cover tab was in the same place on Corvette engines until sometime in the mid- to late 70's, although there were many different configurations of tabs. It changed on passenger cars in 1969 (moved 10* CCW) when those engines went to the new long water pump accessory drive system, but Corvettes didn't use that system and stayed with the short water pump.

It's easy to determine if the inertia ring has "slipped" on a harmonic damper; the timing index line on the inertia ring should be exactly in line with the keyway slot on the hub and a radial line from there through a pulley bolt hole. The GM drawings require that the inertia ring not move relative to the hub at any less than 1,000 foot-pounds applied to the ring with the hub held stationary, so "slipping" is a lot less common than some folks think.

John----

Although Corvette kept the short waterpump right through 1991 (1955-70 "standard short"; 1971-91 "Corvette short"), the timing mark did change for the 1969 model year. For 1969, all Chevrolet small blocks, including Corvette, changed. So, I suppose it's possible that the engine in question has either a 1969 and later timing cover or balancer installed.

[Clem Z.]

Re: Timing Tab Position

GM moved the timing mark because of the location of the power steering pump made it hard to see the timing mark so GM moved the mark

[Mike B.]

Re: Timing Tab Position

Thanks for all of the suggestions. I am going to take another look this weekend.

From my recollection the balancer timing mark lines up with the keyway so it is doubtful that it slipped.

I recently found a removable tab in my parts bits - the type that is held in place by two timing cover bolts. My first sanity check will be to superimpose that on the welded-in-place tab to see if they align. I don't know the vintage of that removable tab but I think it was pretty early SBC.

Failing that I will go back and confirm the TDC position using Clem's method.

The last resort will be to remove the timing cover which won't be anytime soon because it is nice and tight with no leaks. I'll just scribe a few new marks and stop looking at it after that!

[Joe L.]

Re: Timing Tab Position

Thanks for all of the suggestions. I am going to take another look this weekend.

From my recollection the balancer timing mark lines up with the keyway so it is doubtful that it slipped.

I recently found a removable tab in my parts bits - the type that is held in place by two timing cover bolts. My first sanity check will be to superimpose that on the welded-in-place tab to see if they align. I don't know the vintage of that removable tab but I think it was pretty early SBC.

Failing that I will go back and confirm the TDC position using Clem's method.

The last resort will be to remove the timing cover which won't be anytime soon because it is nice and tight with no leaks. I'll just scribe a few new marks and stop looking at it after that!

Mike-----


All of the bolt-on timing tabs I'm aware of are for the 69+ timing marks. At least, GM never offered any for pre-69; there may have been aftermarket pieces, though.

[Mike B.]

Re: Timing Tab Position

Joe,

I believe I found the problem...there is a later model timing cover installed on the 1961 engine. The removable timing tab lines up with that of the welded-on tab. If as you say all removable tabs were post 1969 (when the tab moved down for the long water pump), that explains everything!

At this point my best recourse will be to accurately find TDC and mark that on the balancer at the 0 degree tab indicator and then set the timing from there. I am not about to yank it all apart. There is nary a leak anywhere. And it runs damn good right now so it is mostly academic anyway.

Thanks to everyone for the help.

[Clem Z.]

Re: Timing Tab Position

use a piston stop to find TDC. rotate the engine ccw till it stops,mark the damper in line with the "0" mark on the timing tab. rotate the engine cw till it stops and mark the damper in line with the "0" mark on the tab. if all is correct the TDC mark on the damper should be exactly half way between the 2 marks you put on the damper

a very good piston stop is available thru your chevy dealer part # 12364087