The Stovebolt Site Forums The Parking Lot Big Bolts 503 engine timing mark mystery-your ideas?

In regard to engine timing with a timing light, the Maintenance Manual says: "Rotate distributor to line up steel ball (6 degrees BUDC) in flywheel with indicator, with engine idling at 400 to 450 rpm." I can read and follow instructions. The problem is that there is no "steel ball" in or on the flywheel. There are a couple of barely discernible lines and lettering on the flywheel which are very difficult to read mainly because you can't get close enough to the opening to decipher it. I'd be willing to worm my upper body down in there, but there is way too much in the way. I wish there was a "steel ball".

Jay - Without being able to see what you see, it's hard for those of use not up on the 503 engines to offer much help. Can we assume (I dislike this word, but don't know how else to ask) that you've rotated the engine through the full circle and still not seen anything other than the "...barely discernible lines and lettering..."???

Can you get a picture of the lines and letters and post it for a look see? It might help someone (perhaps one of our GMC experienced folks) to offer some good advice or suggestions.

Is it running or are you trying to do an initial timing? The timing specs really aren't very relevant. Gasoline had a lot lower octane in the 1950's and with today's octane the engine will be a lot happier with a little more advance. The 503 spanned the entire 1950's and the newer ones had more hp than the older ones because octane rose during the 1950's. If it is running advance the timing a little until it starts pinging and then retard it a little. If you are trying to get an initial timing, remove the no. 1 plug and rotate the engine until gets to a compression stroke on the no. 1. Then use a flash light to see when the piston is all the way up. That will be pretty close to TDC and it should start. You can keep advancing it until it pings and then retard it a little. That's how I did it over 60 years ago and it will still work. I was a 14 year old kid when I built my first GMC engine in 1957.

Thanks for the responses! The truck is running but the timing hasn't been adjusted in ages. When hot, it cranks with difficulty and is hard to start. I'm not against timing it as suggested, but wanted to try the timing light method. I may be able to reach far enough to get a little sandpaper on the flywheel, then rub a little light paint into the area, then reach a phone down there to take a picture.

Some pix attached, one from the manual and one of real life. I got the area inside the peep hole cleaned up and legible, using a small drill operated wire brush. The difference from the picture in the manual and what I have is that there is no pointer and there is no ball or bump on the flywheel, rather there are lines stamped in the housing and the flywheel. I marked the 6 degree and housing lines with oil-base white paint and will get to the timing later this week. As suggested I will not go just by a timing light, but I still believe it has a useful role in engine tuning.

Got it timed, using the light and instinct. The timing light is interesting. The aim direction is tough, as the target is way down and way back. Do you hold the light high or do you hold it low? You can't get a good direct right angle on it. Did the best I could, splitting the difference, and using the ear.