Working on stock L48 on 75 C3 with 108k. PO had replaced harmonic balancer about 3k ago, but no record of checking timing chain and looking at bolts on timing cover does not look that cover has been disturbed in a long time.
Recently replaced plugs and wires, distributor cap and rotor trying to eliminate some causes of misfiring / rough idle. considering condition of rotor, cap and wires I am surprised that car even started. After replacement engine was running fine, I also checked manifold vacuum and constant at 19in and compression on all cylinders range from low of 136 and high of 141.
Based on some comments I have read it would be a good idea to change the timing change / gears if unable to verify if it had been done recently. so thought I would tackle the job today but having some issue confirming TDC.
Mark on balancer indicates at TDC but when I check the rotor position it was closer to firing on No.8. Started engine again and lined up at TDC but then rotor was around No. 6, tried it with taking off vacuum advance line to see if that made a difference but rotor just ended up in another position. Didn't want to take apart timing cover until I got some advice as it seems to be critical to have the timing at TDC for No.1 before removing the old timing chain.
Is there something I am missing? I figured if it was the timing chain it would not seem to idle so smoothly.
Appreciate your help

 

Don't worry about exact TDC before you take the cover off. Just "bump" the engine over until the harmonic balancer is somewhere close to the "0" mark on the timing tab, and verify that the distributor rotor is pointing roughly towards #1 and not roughly towards #6. If it's pointing towards #6, rotate the crankshaft 1 turn to get it pointed towards #1. One turn of the crankshaft turns the distributor 1/2 turn.

Once the timing chain cover is removed, you can then accurately align the timing marks by turning the crankshaft with a large crescent wrench against the crankshaft key to get both the gear dots pointing straight up. This is TDC on the #1 cylinder on the compression stroke. In this position, the distributor rotor should be pointing roughly towards the #1 plug wire tower position.

If you have reason to believe, or any fear of, the balancer being slipped, do a TDC check before tearing anything down so you can get a new balancer procured before starting the job. You can correctly install the timing gears with a slipped balancer, but you cannot correctly set the timing with a slipped balancer.

For detailed instructions, e-mail me for a copy of my following papers:

"How to Check TDC"
"How to Replace your Timing Chain"
"How to Install Your Distributor"

This will give you all detailed info needed.

Lars
V8Fastcars@msn.com

 

Before you dig in, you might want to do a little testing on your current chain:
With no spark plugs in the engine (to make it easier to turn by hand), as viewed from the front rotate the engine clockwise until the balancer line is aligned with the -0- mark on the timing tab. If you overshoot, don't back up, go around again. Next, while watching the distributor rotor, turn the engine "backwards" and stop immediately when you see the first movement of the rotor. Look at where the balancer line is on the tab -- the difference from -0- is slop in the timing chain. As far as I know, there are no set guidelines for how much is acceptable, as all chains will have some degree of slop, but if it were my engine something like 1°–5° is fine, 6°–10° of movement is approaching borderline, anything more than 10° would be a definite chain replacement.

I hate to publicly disagree with Lars, but... before removing the old chain, I think it's much easier to rotate the engine so both dots are opposite each other so at TDC #6 (cam @ 6:00, crank @ 12:00). This makes visually aligning the new timing set much easier when you're bent over the fenders. In either case, don't remove or loosen the distributor from before the swap, it'll still be timed and start fine.

One more tip: heat the crank gear in an oven for 30-45 mins at 200°F. This will expand the gear so that it's a quick slip-fit by hand onto the crank nose and keyway – it will quickly contract and lock onto the snout.

 

Thanks for the responses - Tried the check on the timing chain by turning the engine backwards and it was 3 deg max before the rotor moved.
I have just tried the timing light to see where mark was showing on No.1 and not even visible = then went through the other 7 wires and only time it showed up was on No 5 at about 20deg ATDC - however went through the wires again and mark not visible on any of the wires, getting really confused now.

In addition to the timing chain I am also planning to install a new Edelbrock 2101 intake manifold but will hold off until confirm about the timing as that will require the distributor to be removed.

Lars - will send you a separate e-mail for your papers

 

The chain test sounds good and I don't think there's any need to change it out.

If it starts and runs fine right now then, then yes, don't change the distributor setting at all until you get the timing flash figured out!

I don't want to insult your knowledge, so forgive me if this is too basic: the #1 spark plug wire is the front driver's side closest to the radiator. When you hook up the timing light, are you sure the light is set at zero if it's a dial-back or digital light? If the mark isn't near the tab for #1, leave the light hooked up and find where the line is when #1 is firing. If you have access to another light, it would be good to confirm what your first light is showing.

Can you post photos of your harmonic balancer and the top of the engine, from distributor, with plug wires attached, all the way to the water pump?

 

another problem is there's a bunch of different balancer / timing tab combinations on small block chevies over the years . First thing to do is verify your tab and balancer are the right ones . Then try set the timing .

 

I’m having a similar issue with the ‘76 I bought a month and a half ago. I’m only getting the marks to line up with around 60 degrees of initial timing! Obviously this isn’t actually what the engine is running on, so I suspect the balancer is slipped. Before I replace it though I’m gonna find true TDC with a piston stop and mark the balancer. You can buy a piston stop for fairly cheap, or make one yourself.

 

By all means, email @lars and follow his TDC instructions. They work.

My harmonic balancer was dead-on, until it suddenly wasn't.

 

I have no issue with that, as long as the person doing the work is aware of the fact that the "dots together" position is the exhaust stroke on #1, so the distributor must be installed with the rotor pointing to #6 - not to #1.. Most people believe that the "dots together" is the firing position for #1, and end up have nothing but trouble when installing the distributor in this position and then trying to get the engine to fire... Having both dots "straight up" through the process makes sure that you assemble everything correctly. It's actually easier to see a gear "one-tooth misalignment" with the dots further apart... The Chilton's and Hayes Manuals will tell you to assemble it with the dots pointing together, but fail to tell you that this is not TDC on the compression stroke.

That's only 1-1/2 degrees of crankshaft movement retard on the cam timing, which is only 3/4 degree on the cam. That's nothing. I wouldn't bother with a chain replacement at this time. Nothing to gain by that. Do a good TDC verification check to verify your balancer timing marks and then set your timing.

Lars

 

Hi, I always learn a lot from threads like these and appreciate the way experienced members share their knowledge - y'all have helped me several times.

Question - I believe a sbc of this era was still using a nylon timing gear. If so, as long as the OP is 'in there', wouldn't it be a good idea to get a quality metal set or double roller set for a little preventive maintenance? The engine is 50 years old and has over 100k miles. I learned the hard way several years ago. Just a thought. Best regards, Paul

 

Completely agree if the distributor is out of the engine or its position moved for some reason during the install. For just a timing chain R&R there is no need to touch the distributor at all – just leave it wherever it is and things will still be timed close enough to start the engine and fine-tune the timing with the new chain.

Not just small blocks, big blocks got them too – intended to quiet the chain noise but the nylon didn't have the longest life. The nylon cracks and breaks off in chunks, leading to significant chain slop. Since the OP has such little movement between the crank and cam, I highly suspect the chain has been changed before. Another check on the nylon chain would be to lower the oil pan and inspect it for chunks of nylon.

Getting to the chain is not a difficult job as far as required skills, but the coolant must be drained and a lot has to be removed to get to it: fan/clutch, all belts (so need to loosen P/S, alternator, A/C, smog pump to get belt slack), water pump, balancer, oil pan, and by extension, the steering drag link, oil filter, and starter in order to lower the pan. Only after all of this will the timing cover be free for removal.

 

Ok
You are misfiring and you want to check TDC.
Sounds like the chain has been tested and is OK for now.

Lars gave you the basics in post #2. When it points to the #1 distributor cap location you are at #1 firing.
Since it runs, it must be close.
At that point the balancer should be within say 10* +/- of TDC. Any more and you have either balancer or timing tab issues.

If you need to get closer, buy a cheap TDC tool, or make one out of an old spark plug and a bolt.

Move the crank slowly with a wrench, and softly "bump" the piston stop from each direction, and mark the balancer, tape and sharpie. Half way between is exact piston TDC. Hopefully the balancer/tab line up, or you can correct.

On your misfiring issue, a very common reason is spark plug wires shorting to ground. Check them all for ohms for resistance. Check the engine running in the dark for sparking to ground. Very carefully check the routing of #5 & #7 wires, they should be separated in the looms as much as possible, and definately not touch or even next to each other, or they will cross-fire.

 

Thanks for the responses - didn't have time to check the engine again until this weekend. Used the paper from Lars and others on locating TDC and found following. The position on No.1 piston on both strokes is shown on the attached picture (top edge of red/white spot is correct (hand slipped while marking) Looking at that the mid point is around 5 deg. Second picture shows the engraved mark on the balancer - had to rotate the engine about 40 deg to see this mark. The red mark is very close to where the rotor is aligned with no 1 plug wire. Next question is does this mean this balancer has slipped? In my first post said that balancer was replaced around 3k ago - meant to say 3 years ago but car has only done around 400 miles since then.
Appreciate your comments.

 

Sounds like your new balancer is off. 40*.
It isn't old enough to have gone bad.
It was probably the incorrect part in the first place.
There were several timing mark locations on balancers over the years.
You just have a mismatched balancer and timing tab.
Sounds like you have an 86 balancer with either a 69 or 75 timing tab/cover.

You know now where true TDC is.
You need to fix one or the other, tab or balancer.


There is a tab and balancer combo that will give you exactly 40* off.

Many ways to fix this:

• new correct balancer
• custom timing tab
• timing tape on balancer
• or hacksaw marks
• or paint stripe

It is important enough I would suggest something a little more permanent that just paint.