?'s concerning the 305 engine in my sons 76 GMC sprint.

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?'s concerning the 305 engine in my sons 76 GMC sprint.

Post by Bruisr on Mon May 27, 2013 11:34 pm

I learned today that my sons 76 GMC sprint has a factory 305 4BBL engine. What concerns me is: the timing tab on the timing cover doesn't line up with timing mark on the balancer. The tab is welded to the cover at the 12 o'clock position, however with the heads removed I noticed that TDC on the intake stroke puts the mark on the balancer at the 2-2:30 clock position. Well past tab welded to the timing cover. I'm afraid the balancer might have slipped but want to verify. I don't think over heating the engine caused the engine to jump timing. Also, does anyone know if the cam gear has the nylon coated teeth? If so, then I'm thinking the nylon coating deteriorated and caused the cam to jump a few teeth. Can anyone provide some insight? Another weird thing I found: The head on the passenger side has pressed in rocker studs, the drivers head has screw in rocker studs! Am I right to assume the head was rebuilt, or is this a factory blunder?
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Re: ?'s concerning the 305 engine in my sons 76 GMC sprint.

Post by 77mali on Tue May 28, 2013 5:52 am

Bill posted this on my cars thread
pila wrote:At some point you need to verify where top-dead-center is on #1, and what the timing mark on the balancer indicates at TDC.
A piston stop is is the easy/simple way to locate TDC. Easier to do it now than later....
So, with #1 on compression stroke (thumb in the spark plug hole)......back it off about 45 degrees before top center. Remove all plugs. screw piston stop in #1 plug hole. Rotate engine clockwise by hand until piston stops. Mark balancer. Then. rotate in opposite direction until piston hits the stop. Mark balancer.

Halfway between your marks on the balancer is top-dead center, which you can find my measuring, and that is where the timing mark should be, if it isn't. Older balancer rings sometimes slip on the rubber mounting. Without knowing where TDC is, timing will be a guessing game to some degree...


With the cylinder head off- at least #1 TDC will be easy to locate.

Also- I don't think that a factory 76 motor would have had the screw in type rockers. I'm not familiar with the GMC Sprints but would doubt that this would have been the case.
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Re: ?'s concerning the 305 engine in my sons 76 GMC sprint.

Post by bracketchev1221 on Tue May 28, 2013 8:23 am

I don't seem to think they went to the 12 oclock timing position to much later. I will take a look at my 76 305 tonight to be sure. I know my 86 Caprice had the 12, but I seem to remember my 78 and 79 having the standard 2? oclock position for the pointer.
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Re: ?'s concerning the 305 engine in my sons 76 GMC sprint.

Post by pila on Tue May 28, 2013 11:41 am

I had a '76 El Camino, and the timing tab was at the usual ( for those years) 2 o'clock position.
And that engine did have the plastic on the cam sprocket teeth, by the way....
Sounds like the front of your engine needs to be opened up for inspection of the timing chain & sprockets. If the timing jumps very far, a few bent valves might be the result when turning it over...

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Re: ?'s concerning the 305 engine in my sons 76 GMC sprint.

Post by Bruisr on Tue May 28, 2013 12:39 pm

I'm able to verify TDC, the heads are off, I can see the intake rocker on #1 starting to lift. That puts the mark on the balancer at 2 o'clock position. An aftermarket bolt on timing tab lines up as it should. I have to assume, whoever had the screw in studs installed on the drivers head may have also installed a newer model timing cover. I think all is good, just waiting Mali to send the heads. This is why I love this site, lots of help and experience to those in need!
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Re: ?'s concerning the 305 engine in my sons 76 GMC sprint.

Post by driveit on Tue May 28, 2013 1:45 pm

I second that, everyone here is very helpful and knowledgeable!
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Re: ?'s concerning the 305 engine in my sons 76 GMC sprint.

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