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4.0 Timing Question

scca28

NAXJA Forum User
Location
N.E.Philadelphia
Half way through rebuilding a '90 4.0. Crank and cam are installed as well as a new timing set. Now, I have the two timing marks on the sprockets pointing towards each other in a straight line through the center of both shafts. Number one piston is at TDC. So far, so good. Only problem is in this position, the two cam lobes are facing upwards, which means the exhaust would have just finished closing and the intake would just be starting to open as that piston is going down. Now, if I rotate the crank one full revolution, the cam lobes are pointing downwards which would indicate both valves closed. That would be the "firing" stroke. So that is where I should install my distributor, pointing towards the number one plug terminal, right? But, in this position, the cam sprocket timing mark is now 180 degrees away from the crank sprocket. It don't make sense. Why wouldn't they design everything so that the dist goes in with the two marks lined up? I double checked this with all the old parts that came out, and it's not a mistake.
I guess they figured that:
1. If you are just replacing the timing set, you won't be moving the two shafts so timing should stay the same, or:
2. If you are R/R the dist, you would just crank the engine around until you feel the air come out of the plug hole to find the compression stroke.

But the directions I have seen so far don't take into account a total rebuild.
It's just a good thing I noticed this, because nothing I have read so far makes mention of this. I would have set that dist in towards piston one with the timing marks lined up and...and then I would be throwing wrenches when the engine didn't start.
Anyway, am I making too much out of this, or is this something anyone rebuilding should be aware of???
 
I just went through a nightmare on my 89 4.0 while installing a new timing set.
My piston was atdc and the cam timing mark was out 180* turned the motor so that the marks lined up but now the piston was not on the compression stroke.

Got to poking around and noticed that for some reason the cam has two holes in it that the cam sprocket "key" can fit into.The rebuilder installed the pin in the wrong hole!!
Oh yea I feel ya about being pissed if I would have buttoned everything up just to find that the cam isn't timed right!!!

It was all good after I figured that out....the only problem was I had to make a new timing mark 180* from the real mark on the cam sprocket to get the timing right.

Sorry for the longwinded post but its something worth checking out....is the cam key in the right hole?
BTW I would wait to install the timing set untill after you get the head on and valve train hooked up so you can be sure that the valves are closed atdc...
 
Last edited:
outlander said:
Got to poking around and noticed that for some reason the cam has two holes in it that the cam sprocket "key" can fit into.The rebuilder installed the pin in the wrong hole!!
Sorry for the longwinded post but its something worth checking out....is the cam key in the right hole?

Yeh, I remember you posted this problem two weeks ago when you found you were missing the thrust pin. The thing is, when I hold up the old cam and sprocket next to the motor, everything is lined up the same as the new parts. So, I don't think the cam is pinned wrong. Must be "A Jeep Thing"! I guess I will scribe another mark like you did, and mark it "C" for compression, for future reference.
I did some research on other types of engines, and on Autozone.com they show how to replace the timing set on a small block chevy engine. The picture shows the timing marks pointing towards each other, and here is the caption under it, " Fig. 4: Position the timing marks (arrows) as shown, which is TDC on No. 6 cylinder's compression stroke, not on No. 1 cylinder-small block V8 engines ".
By the way, if you are still looking for the thrust pin, I just found this last night:

http://www.amcyclopedia.org/node/425

Camshaft thrust pin package includes cam bolt, spring and pin (no
washer): Chrysler "PIN PKG C", part number 83502890, special order
part, cost was $5.28 on 4-10-2007 from Brennan Dodge, 318-251-9874, in
Ruston, LA.
 
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