Finding TDC for distributor installation

heut · Post by **heut** » Wed Dec 21, 2011 8:36 pm

I am trying to find TDC on my M38 to install the distributor. Is there a sure way to determine TDC being I cannot verify piston travel with a pencil or the like through the plug hole. I now know the timing makes are on the flywheel but from what I have read it is also possible that the flywheel could be 180 off. Not sure because I have not verified that yet. First time working a flat head so it's somewhat new to me.

Thanks in advance

Merry Christmas

Post by **wesk** » Thu Dec 22, 2011 1:04 am

Remove all four spark plugs.

Make sure your 5 degree BTDC mark and reference point are colored brightly so they are easy to see. (we really do not need to deal with the TDC mark)

Note the position of the #1 spark plug wire in the distributor cap. Make a crayon or felt tip mark on the lower housing edge just below the cap even with the #1 wire tower.

Place your thumb over the #1 spark plug hole.

Have a helper turn the engine by hand. You can do this with a socket on the crank pulley nut or by placing your left hand on the generator belts between the generator and the water pump and turning the fan blade in it's normal direction of rotation.

When you feel strong air pressure trying to push your thumb off the hole stop turning the engine. Look at your timing marks. They should be close to lining up. Very slowly continue to turn the engine until they line up.

Now look at your distributor. Is the rotor pointing towards the crayon/felt tip mark you made on the lower housing? If it is then you are now at 5 degrees BTDC and where you should be to set the distributor so the points are just starting to open.

Bretto · Post by **Bretto** » Thu Dec 22, 2011 7:18 am

I had to do what Wes suggested a while back by getting a piece of wire bent just right to get back into the cylinder in order to find the TDC.

http://www.willysmjeeps.com/v2/modules. ... hlight=tdc

Post by **wesk** » Thu Dec 22, 2011 9:17 am

The extra accuracy of the wigler wire is not really needed unless you have no timing marks and wish to make some.

The phrase so many use; "Bring #1 TDC on compression" is really not the best way to say what needs to be done. It would be better described as; "Bring #1 halfway up on compression then stop turning when the firing mark (5 Deg BTDC) lines up."

heut · Post by **heut** » Thu Dec 22, 2011 8:34 pm

Thanks for the info. I may have to go the wire route as I do not have any marks and the distributor is out. Once I get it set I will make some marks. I read something about marks on the flywheel, however I did not look for any when I reinstalled it.

Appreciate the help

Cheers

Post by **wesk** » Thu Dec 22, 2011 11:32 pm

What is the serial number of your engine?

Which starter do you have installed? Top or bottom of the photo below?

heut · Post by **heut** » Fri Dec 23, 2011 9:33 am

Bottom photo is what I have. The engine is a remanufactured. Not sure on the serial number. Still learning these jeeps and the uniqueness of them. I really appreciate all the response and info. I am planning on messing around with it this weekend so I will update as I can. I won't be firing it up for awhile as I am still in need of misc items. Just thought I would get the distributor set. Working the carb linkage and lines now.

Thank you again.

skyjeep50 · Post by **skyjeep50** » Fri Dec 23, 2011 10:31 am

One other thing to consider is making sure the oil pump is installed in the correct position so that when the distributor is installed, it will be pointed in the right direction. The oil pump and distributor are connected via a offset key so they only go together one way. If incorrect, you can take off the oil pump and turn the shaft and reinstall it so everything will line up. If your engine is a reman, it may be worth your while to start at the beginning. When I was setting up my M38's engine prior to installation, I first verified TDC with the head off so I could directly observe #1 piston travel with a dial gauge. You may not have the opportunity to do it this way but the thumb or feeler methods may help you find TDC. I have a "late" 129 tooth flywheel without timing marks but the front pully had the hole drilled in the back of the hub which did line up with an embossed rib on the crank drive gear cover at TDC. Then I verified distributor orientation. Then measured and cut TDC and 5 degree before TDC marks on the edge of the fan belt pulley. Then installed the distributor and rough timed the engine so that the breaker points were just separating at the 5 BTDC mark. After a lot of other work on the jeep I finally got to start it for the first time, it ran perfectly - checked the timing with a timing light and it was right on the money. And again, thanks to Wesk for providing all the directions as I've related above to get this job done right! Good luck with your project this weekned.

Post by **wesk** » Fri Dec 23, 2011 10:42 pm

If you have the bottom starter/flywheel/bell then you have no access to your flywheel to view the timing marks. The top parts are the early set up and have that small round window next to the starter mounting hole to view the flywheel marks.

Now let's take a look at the front of your engine. Are there any timing pointers, raised marks on the timing cover or notches in the rear edge of the crank pulley?

Don't get too concerned with the oil pump orientation right now. If the distributor will time correctly and have enough room to rotate so you can adjust timing then the oil pump position is very unimportant.

One very painful part of setting up a new set of timing marks is trying to find the true TDC point on the crank. The crank will actually rotate as much as 5 degrees either way while the piston remains perfectly still at TDC. This motion must be taken into consideration when determining your marks.

Let us know first what configurations you have on the front of your engine.

Here's a few photos of different timing mark setups.
http://www.willysmjeeps.com/v2/modules. ... _album.php

heut · Post by **heut** » Sun Dec 25, 2011 9:33 pm

Thanks again guys for all the info. I used the bent wire method and to the best of my knowledge it seems to have gone in correctly. Used the org maintenance manual to get the rotor oriented towards # 1. Seems to have gone it correctly without much trouble. From the way it looks the distributor drives the oil pump with a slotted key way. Seems it can only engage either correct or 180 degrees off. Does that make sense? After getting the distributor seated I turned the motor over buy hand with my thumb over the # 1 hole and after I felt pressure the rotor was oriented where the manual indicated # 1 should be. Looks like there is enough room to move the distributor to set the timing. It will be a while before I crank it but if here is something I missed please let me know.

Appreciate ALL the responses.

Thank you very much

skyjeep50 · Post by **skyjeep50** » Mon Dec 26, 2011 11:03 am

heut - That's correct on the distributor and oil pump. Since your engine is a reman, the mechanic that put it together may have already put the oil pump in with the correct orientation so when mated with the distributor everything is lined up. But since the oil pump gears can mesh with the drive gears in several possible ways, the key that drives the distributor can be off. Your guy apparently knew what he was doing - a good thing. Did you find your timing marks?

Post by **wesk** » Mon Dec 26, 2011 11:11 am

From the way it looks the distributor drives the oil pump with a slotted key way

Heut you are incorrect in your understanding. The cam drives the oil pump and the oil pump drives the distributor. It is for this reason that the oil pump should be properly indexed when it is installed to insure the distributor aligns correctly when it is installed later.

heut · Post by **heut** » Thu Dec 29, 2011 5:10 pm

Thanks Wes. I guess I ment to say it that way. I understand the cam driving the oil pump then the distributor being driven by the pump via the cam. Being the distributor is slotted I would guess that if the pump is in correctly the distributor can only be installed either correct or 180 off. Does that make sense or am I not understanding it. Anyway I am going to pull the pump and double check that and then check the distributor and reinstall if needed.

I appreciate all the info

Happy New Year

Heut

skyjeep50 · Post by **skyjeep50** » Thu Dec 29, 2011 9:54 pm

I got your drift even if it didn't quite come out right in your post. But, you don't have to pull the pump to determine if it was inserted in the right position, just mount the distributor and check to see if the orientation is correct. The oil pump shaft and distributor are connected by a slot and offset key - that is the key is not on the center line of the distibutor shaft, its offset to one side. So the shafts will fit together only one way. Shine a flashlight down the tube where the distributor mounts and see where the slot is on the oil pump shaft and slide the distributor in until it bottoms lining up the key with the slot. Make sure the motor is at 5 degrees BTDC and see where the rotor is pointed - toward #1 spark plug wire with the distributor in the correct position? The military elongated distributor housing needs to clear a bunch of obstacles on its side of the engne - generator, starter, cables, crosstube so orientation is important. Wes may (will) have a neat pic to explain all.

Post by **wesk** » Fri Dec 30, 2011 1:29 am

Being the distributor is slotted I would guess that if the pump is in correctly the distributor can only be installed either correct or 180 off.

Not correct. The distributor will mate to the oil pump in only one position.

I prefer to call the end of the distributor shaft that mates to the pump the Offset Tang and the top end of the oil pump shaft as the Offset slot.