Finding TDC on a SBC...But Think Out of the Box!

Hello LYN: I didn't see the picture of the fancy tool. I certainly can see that it over work great on a HEMI, have you tried it on a wedge engine. The angle of the tool on a wedge shape combustion chamber seems like it might be problematic.

Never tried it on a small block. Always had access to the front. And, if just pulling the distributor to repair or replace, there's ne need to do anything, except hit the right cog when re-installing.

I'd think that by creeping up on it, you could get there with the dial indicator without damage.

I think the OP has it well in hand.

This usually works....

https://www.youtube.com/watch?v=RENhHI9n65I

http://www.summitracing.com/search/part-type/piston-stops

I don't have a damper, can't even access the crank because the blower is in the way. The only way to rotate the engine is with the starter or by using a special hand tool on the flywheel. I have that tool. This looks interesting:

http://www.summitracing.com/parts/ipa-7880/overview/

It's not a stop, but an indicator of piston position. Anyone used one of these? Wonder how accurate it is. Same with the TDC dial indicator; how accurate will it be? I imagine they would be good on a hemi, but wonder re SBC, where the plus is at an angle to the piston.

I got in the ballpark (long ago) with a cut off piece of coat hanger poked into the #1 spark plug hole. But I had someone who could turn the crank from the front of the engine.

With a ratio of 2:1, the distributor will be pretty close.

I sure wouldn't use any kind of hard stop in the spark plug opening if you are going to use the starter to turn the engine over with. And even turning the engine over by hand with anything solid in the spark plug opening can be detrimental to a piston.

Maybe this one...

http://www.summitracing.com/parts/cca-4927/overview/

I think that is a cam measuring device. I'm trying to find a dial indicator that screws into a plug hole, but no luck yet.

So Welderman's idea might actually be best as it can be set up to both points well before TDC....less guessing?

----and cheaper!!

Those darn piston indicators seem to have warnings about only using them on hemis, and angled plug holes could break the indicator. I think the SBC plug is at 45º to the piston top. I might try one anyway.

If I pull the blower this winter, I can see the crank. And I THINK the keyway in the crank matches the #1 throw. So if I set the keyway at 45º (the cylinders being at 45º), the piston should be at TDC. What you think?

That is a good point. I would not try a dial indicator if the spark plug to cylinder angle was greater than 15 deg. Otherwise the indicator stem would bend to the side giving inconsistent readings and probably destroying the indicator.

Given the restrictions of spark plug angle that have been mentioned, perhaps a dial test indicator with a probe long enough to reach through the spark plug port and contact the piston near TDC could work. The indicator would be held by a magnetic indicator holder attached to the side of the block

Thats a very good point that several have made here that actually dissuades me from even thinking of doing it in this way. Not forgetting the price of the gauge!! (My family come from Scotland!)

Also, looking for TDC, even when such a gauge is available and it works correctly, is still fraught with problems, as already pointed out, TDC is "seen" over quite a wide range of angle, maybe even more than 5° for example, so the accuracy of the instrument is completely destroyed by the way a piston behaves at that point.

Basically the piston comes to a stop and reverses direction.....bad for exact measurement.

I liked welderman's well thought out simple and cheap method using an old spark plug with an extra piece of steel firmly welded into it. Hopefully long enough to meet the piston at a roughly halfway point between its top and bottom of travel, where the angular difference will be very obvious, reducing the possible error to probably well under one degree.

Marking the two points on the flywheel, making the middle point an exact TDC.

A light hand when turning the motor back and forth as well as exact and permanent marking...I would spray paint the rough TDC point white (if normally unseen) on the flywheel and mark it with punch marks once it is certain as being correct...

The expensive but nice tool shown by some, could be used, as long as it would not be damaged or bent in use (as you yourself pointed out correctly), provided it too was used at a point, roughly in the middle of piston travel, between top & bottom, meeting it from both directions, as with welderman's idea.

The person doing this must simply be careful as to which "middle point" he eventually finds as it could simply be BDC instead if he is not careful!!

I personally have learnt a great deal from all posts on this blog, though in my experience I have always been able to see the original timing marks, but in the case of such a problem, I now have a good fix!!

Many thanks.

GA

GA...and pull ALL of the plugs so you can rotate by hand!

Install a piston stop in cylinder number one or in cylinder number six; they rise and fall in unison, so it doesn't matter which one you use. Turn the engine slowly BY HAND until the piston hits the stop; this point will be obvious because the engine won't turn any more. Make some sort of rigid pointer which aligns with one of the teeth on the flywheel, and mark the tooth which is in alignment.

Turn the engine in the opposite direction slowly BY HAND until the piston hits the stop again. Mark the flywheel tooth/notch which aligns with the pointer. Remove the piston stop and turn the engine BY HAND in the "second" direction until the point halfway between the two marked teeth (the halfway point which could not be reached with the stop in place) is aligned with the pointer. The engine will be at TDC for #1 and #6; well, really close.

With 168 teeth on the flywheel, each tooth is roughly two degrees from the ones on each side. If you are really careful about your pointer and marks, you can get within one degree, assuming that the piston is reasonably tight in the bore.

Welderman posted while I was typing.

If you know the cam you have, pull the valve covers and watch the rise and fall of the valve. Place a degree wheel on the crank shaft and from the known dell timing off cam you can get it down to a few thousands of an inch. But it takes a bit of math. Once located mark the harmonic balancer. Which should have that info on it anyway.

From the original post:

"A Potvin blower attaches to the front of the motor and therefore there is no damper or timing marks"

Contact Moon Equipment, they use to produce and sell Potvin materials. According to what I heard they are still around and may assist you.

Yes, Moon bought Potvin in 1963 and kept the molds, but didn't market the blower drive. I called them in 1993, and they made a front cover for my blower from the original mold. They still sell the drive kit (manifold, air tubes, adapter, etc), special order for $3k, less the 6-71 blower itself.

If its a 2 stroke (I have no idea) all the methods should work, but if its a 4 stroke, then it may not be the TDC required!!

But observation of valves (both closed) or where the distributor (if it has one, I don't even now if it has more than one cylinder or not!) is may shed more light on "which" TDC it is!!

Some twin cylinder engines for example fire both the plugs on each TDC for both cylinders, even though one is only exhausting....no distributor needed....

I liked Welderman's idea too....

Plus, it would be great to to have more detailed description and maybe fotos of the engine....

Here's the motor:

It's a 350 Gen I Chevy crate motor. As you can see, the blower eliminates the damper and any access to the crank. I changed distributors and lost TDC. I have it running now, but just adjusted the timing for max vacuum. I don't have the actual numbers for initial or speed or vacuum advance.

Short of a magic solution, I'll probably pull a head this winter and use a dial indicator to get TDC. And I'll mark it on the flywheel for future reference. Then I can use a dial-back timing light to check the advances.

NICE Ride! Any more pictures?

I thought you'd never ask. Here's a bunch of pics:

https://plus.google.com/photos/104971212334307682869/albums/5552767776876190417

Thanks - very nice project. I like the cooling system!

The craftsmanship is impeccable, the plumbing is awesome and the wiring is bare minimal. Ignitions are my specialty but I think they have about covered it.

Well SSC Welderman and Ed Watts have the right idea about the stops and finding center at the fly wheel. If you are using an automatic you can drill an observation hole so you may at a later date use a timing light through the lower cover and see your marks once you have found TDC, mark the next few teeth on the fly wheel with different colors to set your advance or retard thus making it easier next time you need to do this. The same applies to stick, it will just be a little bit harder to drill the blow-proof bell-housing to get your marks. Unless you can remove the lower part of the bell-housing and view from the front then approximate the location of the marks and drill an observation hole like the auto.

Also how is your fuel injection determining when to pulse without accurate timing. You must be pretty close or it would not fire on the right sequence and cause a misfire.

Hey, LockDuke. You know, I have never had a flywheel cover on the car! Tranny is TH400. I use the flywheel for timing and it's just convenient to not bother with a cover.

My FAST EZ-EFI gets a tach signal from the dist and it is a batch-fire, all the injectors fire simultaneously without regard to crank position. Sequential-fire, where each injector is timed to fire when the intake valve opens, requires another signal, one that indicates crank/cam position. I've thought about it, but what I've read sounds like it's only a minor improvement unless you're racing.

On reading your initial post I was going to suggest marking the flywheel. You already do this so I guess you want to find TDC as an initial start point. Can I suggest an old spark plug minus guts. Drill right through from one side a 1.6mm hole. Flatten a piece of 150mm long fence wire about 50mm in from one end. Drill a hole 1.6mm Dia in this flat. Next drill a 1.6mm hole straight through the plugs hex section and insert a piece of welding rod through the plug, flattened wire and then out the other side of the plug. You now have a lever arm inside a plug. You can figure the rest from here.

Jim

Replying to my own post,duhhh. Should have proof read it. I mentioned drilling a hole through the plug twice! This just confused the issue. Delete the first mention, "Drill right through from one side a 1.6mm hole.".

Jim

I'll probably mark the flywheel, maybe a small hole, at TDC.

And...I could fab a timing wheel on the front of the blower, since it is 1:1 coupled to the crank:

Yes, I should have been an artist...maybe abstract.

Another thought.

One problem I see with using a hard stop is developing a feel for when the piston just touches the stop. Remember that the piston will be near TDC when it hits the stop and you will have a tremendous mechanical advantage with whatever you are using to rotate the crank.

My idea is to drill our punch out the electrode of an old spark plug, leaving the insulation. Now insert a drill bit with a tight fit that extends out enough to contact the piston. Connect the other end of the bit to a 12V test light. Now you can see when the piston makes first contact.

One advantage over a dial indicator is that you can see the test light from where you are when you are rotating the crank.

A light is a good idea, but I would not use a test light to complete an electrical circuit. The piston will be coated with crud which will make circuit completion iffy.I suggest using a pair of normally open relay contacts that close when they hit the top of the piston. If you use this arrangement in the same manner as the hard stop, you should get an accurate reading. Also, rotating the crank a few degrees B&ATDC will eliminate the geometric ambiguity.

SSCpal, If the ignition you have is like that used on Chevrolets Sequential Fuel Injection systems then you can forget most everything you know about setting the timing. Even if you had the damper you cannot use a timing light. On these systems you have to use a computer scanner to find the correct timing or another option is kind of trial and error until you hit the right spot. I will provide a link but you may want to search out some others until you completely understand what's involved. I will cut and past below. You will be looking for the section on (VARIATIONCRANKSHAFT RELEARN) (CASE Learn) I have heard that GM charges one hour of labor to do this (about $70.00) and it takes about 15 minutes.

http://www.fullsizechevy.com/forum/general-discussion/performance/port-injection-96-obd-ii/164702-gm-computer-relearn-procedure-including-crankshaft-relearn.html

Default
VARIATIONCRANKSHAFT RELEARN (CASE Learn)
"CASE" means: crankshaft angle sensor error. A crankshaft variation relearn must be performed if:
A diagnostic trouble code of P1336 is present.
The computer has been replaced or re-programmed.
The crankshaft position sensor has been replaced.
The engine, harmonic balancer, clutch, or flywheel have been: disassembled, removed, or replaced.
The crankshaft position variation learning feature enables the computer to compensate for part manufacturing tolerances. This allows the computer to accurately detect an engine misfire throughout the engine RPM range. The learning process is stored in the computer's memory and does not have to be repeated unless one or more of the above conditions are present.
To perform the relearn, proceed as follows:
Connect a scan tool to make sure there are no trouble codes stored in the computer's memory. If there is any power train trouble code other than P1336 (Crankshaft Position Variation not learned), the computer will disable the relearn function until the problem that aused the code has been eliminated. Also, make sure that the engine coolant (check it when the engine is cold) and oil levels are at an acceptable level.
1. Set the parking brake and block the drive wheels. Make sure that the hood is closed.
2. Start the engine and make sure that the engine coolant temperature is at least 158 degrees F. (70 degrees C.)
3. Turn the engine off for at least 10 seconds.
4. Select the crankshaft position variation learn procedure (CASE Learn)on your scan tool.
5. Make sure that the transmission is in Park. Start the engine.
6. Apply the brakes and hold the pedal firmly.
7. Follow the scan tool instructions.
Remember: That you are going to increase the engine speed to approximately 3000 RPM, 4000 RPM, or 5150 RPM. That's the variation learn fuel cutoff RPM (depending upon the engine), and that it's important to release the throttle when the engine RPM starts to decrease as a result of the fuel cutoff going into effect. Failure to do such will result in over revving of the engine, causing possible engine damage.
8. Once the engine has returned to idle, check the status of Diagnostic trouble code P1336. If the scan tool indicates that the CASE has been learned, the relearn procedure is now complete. If CASE has not been learned, check for the presence of other power train codes. If any exist, correct the problem, then repeat this procedure.
NOTE: If the Crankshaft Variation Relearn is not accessible through your scan tool (some truck applications 1998 & later), perform the relearn in the following manner:
1. Turn off all of the accessories. With the Air temperature sensor and Coolant temperature within 5 degrees (Centigrade) of each other, start the engine and let it idle in Park or Neutral for two minutes.
2. Accelerate the vehicle to 55 mph at part throttle. Cruise at 55 mph for 8- 10 minutes until the engine reaches operating temperature.
3. Cruise at 55 mph for another 5-6 minutes.
4. Decelerate to 45 mph without using the brakes, and maintain 45 mph for 1 minute.
5. Perform 4 deceleration cycles, without using the brakes, of 25 seconds each where no specific speed is necessary. Returning to 45 mph for 15 seconds in between deceleration cycles.
6. Accelerate to 55 mph and cruise for 2 minutes.
7. Stop the vehicle and idle for 2 minutes with the brake applied and the transmission in Drive (automatic trans.) or Neutral (manual trans.) with the clutch depressed. Refer to diagram A on page 8.
BASE IDLE RELEARN
The Computer will reset the idle air control valve whenever the ignition switch is turned to the "On" position, then to the "Off" position. However, before you return the car to the customer, it is best to bring the engine up to operating temperature in Park with the air conditioning off. Then, turn the air conditioning on. Next, turn the air conditioning off. Next, place the transmission selector in drive. Finally, with the car in Drive, turn the air conditioning on. That's it!

OP here. Great discussion, guys. I have some new ideas now to work with. I really like the light that comes on with the piston stop, since I am under the car turning the flywheel. You've all saved me from having to pull a head.

This has been one of the more informative threads in a while and "IF" I could give a GA to all the replies, I would! There's been some real thinking out side the box Engineering .

GA for everyone

I didn't read through every post, so this might already be up. But if you can easily pull the valve covers, the second part of step #1 looks promising.

The other method requires the valve cover or intake to be off. You can look at the lifters or rockers of #1 and #6 cylinders. When #1 is at TDC, #6 will have both valves open slightly. If you move the balancer timing mark to each side of the timing pointer, you should see both lifters or rockers of #6 moving up and down and #1 should remain stationary. If it is vice versa then you are on TDC #6 and need to move the crankshaft 360 degrees or one full turn.

http://www.73-87.com/7387garage/drivetrain/newengine.htm

How's this:

I figger about 1" to 2" reach into the chamber should do it. Little springs will keep from damaging the piston as a hard stop might. When the contact wire touches the piston and then touches the loop the light goes on. Very sensitive and repeatable each side of TDC. I'll put the bulb and battery in a little box with a magnet to attach near where I am under the car marking the flywheel. Your comments are most welcome.

I LIKE THAT! Very simple and effective evolution of my initial idea, and solves the "crud on the piston" problem.

This thread has been one of the best examples of "coffee room" or "bar room" engineering that I have seen here in a long time. Give us a problem, and between us, with the help of a few beers, and good fellowship, we can find a solution.

In order to determine TDC, the piston must be on a compression stroke and not on an exhaust stroke.

You can use the flywheel to determine TDC by pulling the starter or dust pan cover.

Remove all sparkplugs from the engine.

Place a compression gauge in, or your thumb over, the # 1 cylinder spark plug hole and rotate the engine until pressure is realized.

Install a soft metal or plastic bolt into the # 1 cylinder sparkplug hole.

Slowly continue to rotate the engine in the same direction until the # 1 piston engages the bolt and stops. (Hand rotation only)

If using the starter motor mounting hole, mark the flywheel/flex plate at the extreme top of the hole.

If you have access to the flywheel by removing the dust cover, mark the flywheel at the top on the right side (passenger).

Slowly rotate the engine in the opposite direction until the # 1 piston engages the bolt and stops.

Mark the flywheel accordingly in the same method at the same location.

Measure the distance between the two marks and TDC will be at the half-way point.

Mark the half-way point as o deg TDC. (A metal stamp or scribing tool works well for permanent marking.)

Divide the distances before and after TDC in 1/8" increments and scribe/stamp as BTDC or ATDC.

This will allow easier future tuning of the engine.

If there is not access to the flywheel, and you are using a toothed drive belt, it is possible to mark one of the pulleys using this same procedure and utilizing a reference point.

Here's my first try at a TDC detector. It uses the block as the ground, so only one wire connects the plug to the box. The box has a battery and an LED, and a magnet on the back to hold it where I can see it. The magnet is also the ground connection.

I'm now working on a different sensor, which uses a flexible strip (small robots use them to detect obstacles). The strip changes resistance with amount of bend and a simple circuit will light up at an adjustable threshold, so I can choose the point BTDC/ATDC. Stay tuned.

Time flies. I built the new improved TDC finder with a flex resistor strip, an adjustable threshold and a beeper instead of a light:

Screw it into #1 after making sure it's on compression stroke, then get under the car and move the flywheel until I have TDC, midway between the beep points. Cut a mark on the flywheel and made a pointer to always find TDC later. And I can use a dialback timing light to check timing (which is fun with my head next to the exhaust pipe).

Yes I can, read the thread sorry about that but all the answers are there. In your particular situation I like the positive stop method. You can buy a tool for this or just use an extra long spark plug, I believe the procedure was described in post number four.

I'm not sure what you're going to use to set your timing marks on, one post described marking the flywheel great because it's so large it gives a very precise mark.

One other thing you might try there are several electronic ignitions High dollar timing lights and such that will calculate a timing mark once you have top dead center located never used one for that but they should be adaptable.

BTY had a friend which ran into the same problem on a big block Chevy just a funny tip. If the headers glow red the timings retarded.