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Went to replace my timing chain on a '77 GMC Motorhome with a 455.
Turned over the crank, by hand, to find TDC on the old timing chain I noticed the marks on the old chain were not together. Both marks are at the top of the gears. The lower has a "O" on top, and the upper has a " ^ " on top. The guide pin for the top gear is at the bottom half. I checked that the exhaust valve on #1 piston is fully closed. The distributer is facing #1 on the cap. When I turn over the were they match it is not on compression stroke and the valves are open.
The engine has been running strong with no issues before tearing into it. It appears to be the original timing set.
Any thoughts on why this may be or should I just slap the new set on (which both marks are on the top also) and don't worry about it?
Thanks everybody.
Don't mind those chisel marks on the lower gear. I didn't harm the crank
When the cam gear is at 6 oclock and the crank gear at 12 #6 is firing. When the cam gear and crank gear are 12 and 12, #1 is firing. The normal installation procedure is to line up the dots with the crank at 12 and the cam gear at 6, then turn the crank 1 full turn (crank at 12 and cam at 12) and drop in the distributor with the rotor pointing to #1.
Why do people lose so much sleep over this? All that matters is that the timing marks line up and the distributor rotor points to the no. 1 plug when that cylinder is at TDC on the compression stroke. From the factory, Olds motors have the timing marks at 12:00/12:00 when number one is at TDC on compression. If you didn't move the distributor, you can reinstall the timing set at either orientation. The distributor needs to be synced to the cam, period. If you didn't disturb that relationship, then it will still be correct.
Please replace the timing set. This past October the cam gear in my 330 came apart, what mess. The nylon is known to come apart, if it does like mine did you can do a lot of damage to the engine. You are already at the point where it will be easy to do.
