3.8 OMC Ignition Timing

Just finishing the V6 engine rebuild but now find the water pump double groove pulley is so big it swamps vision of the timing scale on the crank pulley.
I can just about see it from the side - but the other unfinished engine has an additional timing scale bolted (to the timing chain cover) on the right of the original scale (which is spot welded to the timing cover)
Which should I use? They both show 0 to 12 degrees before and after TDC - except the bolted on one is about an inch further around the crank pulley!

Depends what style indicator you prefer.
The typical set up will have the crank pulley cut in one place with it lining up to the 'stock' welded on timing indicator '0' mark.
If you prefer to use the bolt on all you do is set the engine at TDC cylinder #1 and cut a small groove in the original crank pulley that lines up with the '0' of your new indicator. To set it at TDC so you can mark it properly you can
a) use the original timing indicator and line up the crank pulley cut bolt on your new indicator and mark it
or
b) pull #1 spark plug crank engine over by hand and set at TDC by either using a prob in cylinder to feel piston at TDC, or use light to look in

Clear ??? Hope this helps.

Later

Ah, but finding actual TDC will be very important for you. Without actual TDC, your ignition timing may be incorrect.
If you know how critical this is for a Marine gasser, then you'll know how important this will be to get correct.
(some of this is cut & paste from other postings that I have left for people)

Since you are still in the engine assembly process, why not do a PPS (positive piston stop) procedure, and get this dead on?
No offense, Gimlit......, but probing, lights, feeling for compression, touching the piston top to see when it stops moving, etc..... will NOT get you true TDC.
You'll find that a crankshaft (at/near the top of the stroke) can rotate a degree or two, without much piston travel.
So this is NOT a good means of determining actual TDC.
Close... Yes! True.... NO!

The only true means, is the PPS method, or a piston travel dial indicator reading.... and doing either from two equal directions of crankshaft rotation measured in degrees, not piston travel.
Somewhere in the neighborhood of 30* BTDC and 30* ATDC, will give you a more accurate reading, than if this were to be only 10*, for example.

NOTE: Be very careful regarding the articles that you can find and read on this.
We DO NOT achieve actual TDC until AFTER the degree wheel (or pointer) has been equalized, and the piston stop has been removed.
Some do not get this right, and the verbiage is entirely misleading and can be difficult to understand.

Do the above, and your timing marks can be adjusted to give you a more true TDC.
Long as you're there.... give yourself an additional 25* and 30* BTDC mark.
This can later be used for checking/setting your ignition TA.

Good luck on it!


.

No offense taken I was basing my advise on the fact he has 'assembled' an engine therefore he has

1. already has motor assembled and heads are installed therefore can't dial a piston for TDC as well a camshaft has been installed and timed which IMHO is far more critical than ignition timing
2. has original timing indicator on the timing cover as well as a crank pulley with original mark on it, all he wants to do is mount another/different indicator than what it had previously
3. has enough knowledge to get to this point but isn't sure with the relocated timing indicator

As far as timing being critical for a 'marine gasser'
Not sure why this engine with 99.9999% chance of points for a ignition trigger and a carb for fuel delivery would need timing set any more accurately than any other.

Anyway...... you are 1000% a dial indicator with head off would absolutely be the most accurate method of determining TDC.

Agreed...... all engine assembly procedures are very important....., but we cannot remove the importance of Ignition timing once the engine is being tuned and ready to be put into service. To gain a correct ignition spark advance, we must know precisely where the piston is (in terms of crankshaft angle) in relationship to when the ignition spark occurs. 2 degrees can be the difference between Ignition caused Detonation and NO Detonation, given the right load and RPM. Therefore we must have an accurate timing mark regarding TDC, BASE advance, and most importantly, Ignition TA......, of which was my point from the beginning.
The OP apparenty had enough concern to have prompted initiating this thread here. If he is trying a tab component that may not be correct, then he may be creating a recipe for ignition timing issues. Hence my "suggestion" of the PPS.

gimlit said:

1. No offense taken I was basing my advise on the fact he has 'assembled' an engine therefore he has
2. 1. already has motor assembled and heads are installed therefore can't dial a piston for TDC
3. as well a camshaft has been installed and timed which IMHO is far more critical than ignition timing
4. 2. has original timing indicator on the timing cover as well as a crank pulley with original mark on it, all he wants to do is mount another/different indicator than what it had previously
5. 3. has enough knowledge to get to this point but isn't sure with the relocated timing indicator
6. As far as timing being critical for a 'marine gasser' Not sure why this engine with 99.9999% chance of points for a ignition trigger and a carb for fuel delivery would need timing set any more accurately than any other.
7. Anyway...... you are 1000% a dial indicator with head off would absolutely be the most accurate method of determining TDC.

Click to expand...

1. Thank you..... I don't want to offend you or anyone here.
2. Correct, but a PPS (positive piston stop) procedure can still be done via the spark plug bore on many engines.
3. Very important.... I agree. However, do not discount the importance of ignition advance, particularly for a Marine Gasser.
4. Case in Point!
5. Again.... Case in Point!
6. Correct.... whether mechanical contacts or electronic triggering, we have ignition. However, the importance of ignition advance does not change any!
7. A dial indicator with head off is great, but no more accurate than if the PPS were to be used. Both require that two equal degree readings be taken coming from both directions towards theoretical TDC....., and both require splitting this reading in order to locate true TDC! This is not speculation, this is a tried and proven procedure.

Thanks again for taking no offense. Hopefully this would be no different than if we were sitting have a coffee or beer together.

I won't get into Detonation in this thread, but if unfamiliar with what may cause it, and/or the damage that it can cause, you may want to read this recent thread whereby the OP's engine was damaged, and very likely due to Ignition caused Detonation. No Marine Gasser is immune to this. I could make a small mis-adjustment to any of our Marine Gasser ignitions, and create detonation damage under certain load/rpm conditions.

IMO, it isn't worth rolling the dice on this.

.

A beer or a coffee.... ya I would likely be saying the same thing. Bonus would be that we were enjoying one or the other preferably it would be past noon (somewhere it is always past noon) and it would be a beer.

Cheers