Piston Slap: Piston Stops and Finding Top Dead Center


Tom writes:

Hello Sajeev, I am the original owner of a 1973 Chevy Camaro with all-original drive train. The engine is a 350 ci, and the car underwent a frame off restoration. During the engine rebuild, a points eliminator kit was installed. Shortly after getting car back, the eliminator kit’s module failed. I put points and condenser back in. (I did not remove distributor).

I set the dwell at 30 degrees, and checked timing (with the vacuum line plugged). Timing mark was about 3/4″ past the timing tab. I set timing to 12 degrees. After doing so the engine backfires thru carb. I checked plug wire routing and all of it was right. Then I set #1 piston at TDC on compression stroke. The rotor was pointing at #1 wire on cap. (The #1 wire was about at the 6:30 position on the cap.) I put timing back to 3/4″ from tab.

The backfire stopped, but it’s not where the timing belongs. I am worried about damaging a newly rebuilt engine. Any ideas?

Sajeev answers:

Hi Tom, thanks for this comprehensive question, that helps me give a better answer to you. Putting the timing back was the right move, and I doubt you damaged to the motor if you kept the backfiring to a minimum.

There’s something wrong with the harmonic balancer (or perhaps the timing mark?), but I am woefully under qualified to narrow down this issue. Instead, I shared your question with Hagerty’s own Brandan Gillogly, and his answer cleared everything up in a hurry:

“It sounds like his new balancer doesn’t agree with the timing tab. I think he should use a piston stop and find out where top dead center (TDC) is and make sure things line up.”

Bingo. Let’s step back and learn how to use a piston stop tool (which you can make yourself) with this video of an early Chrysler Hemi from our pals at Holley.

I am hoping the marks you put on the balancer line up with the timing tab on your engine. Or maybe I do not, because that suggests a defective balancer.

I suspect the timing mark on the balancer is not in the right place (like the video above) due to a manufacturing error, likely from the mating of the two metal parts and the elastomeric band between them. Aftermarket balancers can be of dubious quality, and I am hoping that’s your problem. Considering the quality of the points eliminator kit that came with your motor, there’s a good chance I am on the right track.

Bad parts wind up in our collective engine bays far too often these days. This usually isn’t the repair/machine shop’s fault, as most places have the best intentions. The quality of vendor, on the other hand, is a bit harder to confirm, and it’s often situations like this that bring issues to light.

So this is where I pass the torch to the Hagerty Community. Does Tom’s Camaro have a defective balancer or is something else wrong with the ignition system? Your comments are always appreciated.

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.comgive us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.


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    Is it a new harmonic balancer or a used one. They do sometimes slip as they age. Because “everyone knows all SBCs are the same” you can run into problems because they are in fact not all the same. Some parts will physically interchange, “work”, but not be right and you end up with things like this. https://www.enginelabs.com/news/what-i-learned-today-with-jeff-smith-dont-trust-timing-marks/

    So yes the best plan of action is to get/make a piston stop and see what it tells you.

    Who installed the distributor? It sounds like they did the “just stab it and wire it from there” method. While this certainly works it isn’t the greatest idea when you have to do things later. It can also ocassionally cause problems with the spark plug wire length and routing.

    In the case of some other brands it really messes with you since the factory thought about the mechanics that would be working on it later, so to make things easier a cap built to the proper specs actually has a 1 next to the proper #1 terminal.

    At least on the older SBC the way they left the factory was with the rotor pointing ~at the #1 cylinder when the engine is at TDC #1. That would put the rotor at 4:30-5:00 when looking at the engine from the front of the car. Definitely not your problem and no need to rush out and change it. but a good to know thing if you have to pull it out again.

    Defective balancers are rare.

    I expect the distributor is off one tooth and may have been since the rebuild. They can run a tooth off but setting timing is impossible.

    The other issue could be they got the cam off a tooth too.

    Find top dead accurately and them check where the distributor is. Missing by one tooth is easy and a common issue.

    My initial thought is one of the things that hyperv6 mentioned…if it was a complete rebuild, the cam was at least out and likely even replaced. Without pulling the timing cover, Tom can’t know if the rebuilder installed and timed the cam properly. That is the basis upon which EVERYTHING else in ignition timing is built. Of course, it’s also the issue to check out that’ll take the most work (disassembly and reassembly, even if it’s 100% correct), but if it’s wrong, all of those other things will just be wrong too!

    Before suspecting the cam, I would definitely do some basics first. Look at the balancer. How does the rubber look? can you spin the outside of the balancer? I’ve found on bad ones you can usually pull the outer ring right off the balancer. Does ‘all original drivetrain’ mean stock? Does it have an aftermarket cam? how/where is it idling? disconnecting the hose disables the vacuum advance, but mechanical advance is still working, and will throw things off if your idle is too high. If you are running a wild cam you can throw the stock timing spec right out the window. Is there a broken mechanical advance weight spring? I have seen this more than once.

    I would tend to lean to the marks not being lined up right either due to wear or wrong parts. Despite what others say, you can drop that distributor in any position and still be able to time the engine (barring any interference like cable-driven tach). Cam being off a tooth is a different story, but I suspect you would have other symptoms if it is off enough to drive the timing mark out of range

    Last thing… make sure your timing light coil is actually on the #1 plug. Look for the simple stuff first. Do the piston stop TDC test second… and absolutely last, when all other options have been ruled out look at the cam timing

    Well it is a new balancer so that is not likely.

    I would move the distributor one tooth first. To get TDC you can get a Whistle valve to put in #1 to get true tdc. Or pull a valve cover.

    As for the cam even changing just the chain people mess up. No easy way to check so save that for last.

    Do you know what NEW stands for in today’s replacement parts world? Never Ever Worked. So the best assumption is that if it is new than there is a very good chance it isn’t right.

    I work in the performance aftermarket and deal with quality control.

    I am well aware what new means and unless you are dealing with a no brand off name chinese part a bad balancer is very rare.

    I do also see human error as a very common thing and often that is the problem. The timing tab being wrong can happen. Generally they do not fit but if it is reversed it could be wrong.

    To be honest in 30 years I have seen very few bad balancers.

    Chevrolet also moved the timing tab around on the front cover. Since “All SBC Are The Same”, it might have gotten a different front cover by accident.

    Whoever assembled the engine should have made sure the timing mark was at 0 deg at TDC, and also that the cam was installed according to the cam card. That said both aren’t difficult to accomplish and there’s a million videos online showing the process. Many cams today are advanced 4 degrees, so I’d double check that, too. Once you know the timing mark is right and cam dialed in, I’d ignore idle timing. Instead set timing at full advance ( usually somewhere over 4000 rpm to 36 degrees. Hopefully you’ll end up with around 17-20 degrees at idle with the vacuum disconnected. Connect the vacuum advance and take it for a spin. If it misfires and sputters at 45 mph light throttle, but runs fine otherwise, you’ll need to limit vacuum advance by buying a different vacuum cam (Napa has a fine selection) or buying a kit to limit the advance. You just want to limit it enough to eliminate the spitting/sputtering. Get a handle on timing and carb tuning and you can make that bone stocker run much better than it did new.

    Key words here being “should have”. Yep, the assembler should have, but that doesn’t necessarily equate with “did”. Unless Tom is the rebuilder or knows the assembler well, he has little to no knowledge of how this motor was put together. IMO.

    There’s so many variables at play, most of which have been mentioned. I think Sajeev’s suggestion is the best way to go about correcting the issue. So, find actual TDC, and use a paint pen or some other device to make a mark on the balancer where the pointer (wherever it may be located) is at that point. Then use that mark to set the timing via the usual and customary method.

    As I wrote earlier: I’d vote for the incorrect mark, on the balancer. Chevrolet had several different balancer sizes and also different tabs and balancer marks to correspond with the different applications. Most of the SBC engines before the mid 70’s would have had a timing tab, just to the right of the engine center line ( maybe 30-40 degrees off the centerline) and the timing mark can be checked with a timing light just above the power steering pump pulley.
    In the 70’s or early 80’s they moved the timing mark to straight up, so you had to time the engine by shining the timing light right behind the eater pump. The timing mark on the balancer are in different places. Plenty of videos and pics online to help with that. Additionally if you can see the balancer keyway, on early balancers the timing mark is nearly in line with the keyway. In ’69 or so, the line moved, roughly 10 degrees in the counter clockwise direction. Then in later years, when the timing is checked looking down behind the water pump the line was moved again, much further in the counter clockwise direction. Do a google search for these changes. In the end if the problem is not obvious, then you will need a piston stop, a proper degree wheel and a bit of time to set it up correctly.

    I agree. I just did an engine swap in my ‘65 El Camino. The 327 that was in the car (truck?) when I got it had that balancer and timing cover from the late model, along with the long water pump. Checked the timing by shooting the light between the water pump and the timing cover.

    Is the timing light really on cylinder #1? Is the timing light one of the advance type, with a dial back advance setting. is it at 12ºBTDC, or at 0º? The timing light indicates the set advance when the timing mark used is the “0º? mark on the damper. I have seen many engines improperly set when the user didn’t know how to use the advance timing light properly. Clue: GM #1 cylinder is the front one on the driver’s side. (Opposite of Ford…).
    Make sure that your dial back advance timing light is set to the desired advance, and that you time using the “0” mark on the damper and the pointer.

    If the engine was running well with the points removed, then it should run properly with points installed, but the timing will change as the location of the points mechanism will be in a different position than the magnetic trigger. However, this is only a few degrees, and should not be causing the problems described.

    Also, I would get rid of the points and put a Pertronix Ignitor in the distributor. Simple and more reliable than points. Properly set the timing and you won’t have to mess with it again.

    Everyone else has already suggested everything I would’ve thought of so I can only add this: unless you’ve gotten into pre-ignition (knocking, pinging) there’s been no damage to the motor.

    But I just thought of this: I wouldn’t use a spring loaded TDC device on an SBC, the plug hole angle is sharp enough versus the bore that the piston could bend it as it comes up.

    Good thought. I fabbed an indicator eons ago by drilling out an old spark plug and inserting wire down through it. But don’t use mild steel wire! I bent my first attempt and this went with stiffer stuff – but I also curled the tip into a loop so there wasn’t a sharp end to possibly score the piston top. I’ve since replaced that with a dial indicator extension rod with a roller on the tip.

    Allow me to offer a small tidbit of Chevrolet initial ignition timing set up. Chevrolet distributor rotors do NOT point at #1 cylinder. Referring to the many factory GM shop manuals that I have, the rotor and the vacuum advance can vacuum hose nipple are aimed at a point near the #8 intake valve (rocker cover removed.) Viewed from above, the “clock” position would be at about 6:30 or 7:00. I have had to correct this snafu on countless Chevy V8’s over the last 50+ years of working on these things. A definite indicator of where TDC on #1 is to remove the left rocker cover and turn the engine over by hand. Watch the exhaust valve open and close, watch the intake valve open and close and then continue to rotate the engine until the damper TDC line aligns with the timing tab. As stated by others, make sure that the damper and timing tab is correct for the engine. It should go without saying that quality, name-brand parts should be used on any engine that is being attended to. If the box those parts come in says “Made in …” anywhere else but USA, I would recommend that another source should be considered. Not an endorsement, but we have had excellent results with Fluidamper products.

    Well the Made In USA is not always a sure sign anymore.

    We do see failed American parts too due to poor quality. But also just because it is made overseas is not a sure sign of failure either.

    The key is how the plant is supervised in China. We have some things made there but we have Americans on the ground doing QA and to make sure the MFG sticks to the engineering drawings and quality of parts.

    This is how Apple and other electronic firms do this as they catch them cheating now and then. Apple dropped one MFG because they were using an unapproved screen.

    It used to be Made in China or made in American was a sure sign but no more. Stick to reliable brand names and with good warranties.

    There are a number of parts today you just can’t find made in America anymore I am ashamed to say. Just no production on them here.

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