gw204

I'm elbows deep in changing the intake and intake gaskets on my son's '86 Trans Am and have a question about setting the motor to TDC.

Before we stripped everything apart, I rolled the engine over via the crank bolt so that the rotor was pointing directly to the no. 1 cylinder on the compression stroke (put my thumb over the hole to verify). However, it wouldn't stop building pressure until the rotor went past no.1. When I leave the rotor pointing at no.1, the piston is still about 1/4" from the top of the cylinder...so not TDC. I'll admit that I'm no expect and this is my first time working on a small block, but that really doesn't seem right. However, I've seen reference to a similar thing in this thread about harmonic balancers (see post #7 by J.C. Denton):

https://www.thirdgen.org/forums/tech...ic-damper.html




New balancer is going on this thing as well since the outer ring has separated from the inner, walked back and is kissing the timing cover.



I don't know much about this history of this car, so I can't say if the distributor has ever been out or not. Is it possible/likely that it was installed a tooth off (or more) or is this normal?

Tuned Performance

iTrader: (92)

Chain stretch . Or the slipped balancer

Vader

Chain stretch/cam timing is one potential issue, but when the timing mark on the balancer and timing cover tab indicate "TDC" and the piston is obviously not at TDC, the problem is not the chain, but the balancer/tab. Find TRUE top dead center and re-mark the balancer, or mount an appropriate timing tab.

That's a strange-looking '86. I'm pretty sure that I see dog bones and a spider in the valley, which would make it at least a 1987 engine. If the balancer was purchased to fit an '86 and the engine isn't, that could explain the position marking discrepancy.

Tuned Performance

iTrader: (92)

Good point, the balancer is not correct to match the cover. Depends on the accessory drive you might not be able to see a different pointer.

sofakingdom

iTrader: (1)

Another thing you have to remember is, there are TWO instances of TDC for each cylinder during a complete engine cycle.

One of them is of course "firing", at the end of the compression stroke and beginning of the power stroke.

The other is the end of the exhaust stroke and beginning of the intake stroke.

As far as the crank, pistons, timing mark, etc., they are identical in every way. The ONLY thing that makes them different is the relationship of the positions of the cam lobes to the position of the crank.

So, using your thumb to find compression, is absolutely correct, and SHOULD leave the motor at #1 firing. You can also watch the lifters: the cyl 4 cyls away in the firing order is exactly 1 full turn of the crank away from a cyl (360° of crank rotation and 180° cam), and its TDC behavior is therefore exactly opposite the one you're working on. In the case of #1, the opposite is #6; so, you can watch the #6 exhaust lifter (5th one back on that side), and as it's passing through full lift and is nearing getting back to its rest position, #1 is coming up to firing on the comp stroke. The same thing applies to any other pair of cyls with that firing order relationship, which can be used for example to adjust valves PERFECTLY without the engine running. That way, you can even find which instance of TDC you're on without relying on compression, meaning you can do it without the heads on it if you need that for some reason.

But as Vader pointed out, as I also do in just about EVERY post where somebody asks about "setting the timing" and then going on endlessly about "mark" and "light" and "tab" instead of "does the engine run the best it possibly can for that particular adjustment", those are RARELY accurate. Not least because of mismatched parts, which most often consists of a 60s tab associated with a "krome" timing cover although many times it's a plain steel one like yours, coupled with an 80s damper, as Vader describes and you have in front of you; BUT ALSO, because the "mark" is NOT on the damper itself, but rather is on the inertia ring around it, with the only connection being a thin layer of spooge. The inertia ring is ALMOST ALWAYS displaced from its correct orientation by the time it gets past about 10 yrs or 100k miles or so. The rubber disintegrates and allows the inertia ring to wander wherever it wants, taking the "mark" along with it.

All of which means your "mark" is not to be believed. And furthermore, even if you make a new mark that's 100% guaranteed accurate, it probably won't stay accurate for very long.

As another point of reference, the crank key is EXACTLY in line with the #1 rod throw. Therefore when it's pointed at EXACTLY 45° to the driver's side, #1 is at TDC.

So, once you are certain that you're working with the comp/power stroke instance of TDC and not the exh/int stroke one, use the actual piston location to set up the distributor, which means, set the engine to where it's around 12° of crank rotation before #1 reaches TDC. Install the dist so that the rotor points at the #1 plug wire tower, which is supposed to be at the very front and about 10° to the driver's side from straight ahead; and set the dist body to the point where the little starwheel "teeth" on the shaft piece are exactly lined up with the stationary "teeth" of the pickup coil. The motor should start RIGHT UP INSTANTLY if you pour an ounce or 2 of gas into the carb before hitting the key. Personally I always FULLY ENTIRELY COMPLETE the assembly of the engine BEFORE trying to start it; coolant in, air cleaner on, etc. etc. etc.; none of this "I just want to see if it runs" or "I want to hear it run" or ANY of that. In fact when a customer is present, I often used to CLOSE THE HOOD before starting it up the first time, and reach in through the window to turn the key. Then of course open the hood back up to top off fluids, check for leeeeeks, etc. before the test drive.

gw204

Thanks for the replies all!

I think my post is causing some confusion though. The first image is not of my engine. It is a '92 305 and is the engine being asked about in the post I linked to. I simply used the pic because it shows the same position of the no. 1 piston that I achieved when I set mine so the rotor was pointing straight at no. 1.

The second pic is of the balancer on mine. I'm not concerned with how the marks on it didn't line up with the tab because I know it's shot....and I'm almost certain it's not the correct one for this engine anyway. In fact, now that I reread the post I think that's where the confusion is coming in. I should have just left out all mention of the balancer.

Much appreciated sofakingdom! That explanation makes sense given my observation. With my rotor pointing to no. 1 there was still a bit or rotation left before the piston reached TDC. While I suspect it was more than 12° more for the piston to come all the way up, that could probably be explained by the timing chain stretch that Tuned Performance mentioned. After all it is nearly 40 years old with 140K+ on it...

You guys are awesome!

Tuned Performance

iTrader: (92)

Nevermind about chain stretch I was way off. Thought there was 2 issues the slipped or wrong balancer and the distributor facing issue.

gw204

If the distributor is driven off the cam, why wouldn't chain stretch come into play?

Either way, timing set is getting replaced, I pulled the front crank seal last night so I could throw a new one in to go with the new balancer. From the little bit I could see of the crank sprocket, it didn't appear that the chain was making good contact. Got out my little endoscope to look inside the cover to try and gauge stretch and found broken nylon teeth on the cam sprocket. Yanked the water pump and timing cover and found more slack in the chain than I ever would have expected.

Tuned Performance

iTrader: (92)

Clean your oil pump pickup well

Vader

If the cam sprocket has nylon teeth, it's time to replace it. Aside from that, bear in mind these facts:

• Both the ignition and valve timing events are generally expressed in degrees of crankshaft rotation, not camshaft rotation;
• Common configuration for an aftermarket double-row roller is 22T at the crank and 44T at the cam sprocket, and factory or HD sets can be as low as 18T/36T for the sprockets;
• A distributor (rotor) would be about 8 - 10° of rotation "off" the ideal position if the timing chain had actually skipped one tooth, and far less than that if the chain had only stretched;
• The general suggestion that the distributor rotor should be oriented toward the #1 cylinder when that cylinder is at TDC on its compression/firing stroke is merely a very relative guide to indicate how the factory assembly would be done, mainly to prevent confusion for future service. (An SBC can run in ANY distributor orientation since the ignition secondary wires can be moved to any of 8 positions on the distributor cap, and the distributor can be rotated to infinitely adjust ignition timing.) If you can visually see less than a 9° degree difference at the distributor it would still be pointing relatively at the #1 cylinder.
• The rear cam drive and distributor driven gears usually have 13T, which accounts for around 28° distributor rotation (56° crank rotation) per tooth. If the distributor is installed one tooth off, it is generally fairly apparent.

Thus, the "rule of thumb" for distributor orientation is defined, and should be considered with all those variables and caveats. It is not always a good indication of chain slack.

gw204

I'm gonna take a wild guess and say that the pickup probably faces away from pan drain. If is that correct, I have no chance of getting the pieces out without pulling the pan, right?

gw204

That makes sense. Thanks!

sofakingdom

iTrader: (1)

Several additional points for consideration:

While the engine can be made to "run" at practically any dist orientation, the plug wires are at their neatest, most orderly, and ... made-to-fit sets FIT ... - when the dist is at the factory orientation, as described. Practically anywhere else, they turn into a bird's nest.

The cam sprocket might have nylon teeth, or they might be phenolic. The factory thought materials like that would run quieter than straight-up steel. Whether that's true or not, the organic materials ALWAYS fail. If you see that in your motor, GET IT OUT. Doesn't matter if it's "OK"; doesn't matter if there's teeth missing; doesn't matter if you're "on a budget"; DOESN'T MATTER DOESN'T MATTER DOESN'T MATTER DOESN'T MATTER. It WILL fail, and it will do so at the worst possible time (think, member of your desired attractive sex sitting in the pass seat, bad part of town, not dressed for walking, etc.). REPLACE IT if you're already looking at it.

Virtually all replacement timing sets will advance the cam 4° from where a 71-up factory set would have put it, even when brand-new. Your engine will not run the same after changing out the timing set: it will almost certainly have more low-end torque, less tendency to rev higher, and won't sound the same.

The rotor/cap relationship changes as vacuum and centrifugal advance mechanisms do their thing. The spark will ALWAYS occur before where it looks like it's going to under static (sitting still) conditions, when the engine is running. Since the rotor turns CW, that means that sitting still, the rotor needs to line up to the very CCW-most relation to the cap contacts.

The oil pump pickup is largely impossible to get at with the pan on. Probably not something to concern yourself about since there's mostly nothing you can do about it if there's foreign matter in it. The most harmful obstructions I've ever seen in them, is gasket material and sealer; like, sheets of RTV that squirted out and fell in, and other such as that.