TOH Oil pump question

cowdog

Member
TOH, I have a question about an 8N oil pump rebuild please.
1. The clearance between the pump gears and the housing is .002. Someplaces it is .003 but it is tight fit. Will new gears take up some of that slack?
2. the bushing is out of round. .570 one way & .566 90 degree from that. The shaft is consistently .562 + or - a half a freckle. I assume with that amount of clearance it would be a wonder it worked at all when hot? 9/16th gears.

The 3/4 gear change out is not an option at this time.
 
(quoted from post at 16:39:12 02/14/15) TOH, I have a question about an 8N oil pump rebuild please.
1. The clearance between the pump gears and the housing is .002. Someplaces it is .003 but it is tight fit. Will new gears take up some of that slack?
2. the bushing is out of round. .570 one way & .566 90 degree from that. The shaft is consistently .562 + or - a half a freckle. I assume with that amount of clearance it would be a wonder it worked at all when hot? 9/16th gears.

The 3/4 gear change out is not an option at this time.

The shaft is not worn. Trying to measure the pump gear lash or tip clearance with a .004/.008 wobble in the gear shaft is not meaningful. Replace the bushing and measure with a properly located gear. A good new number for tip clearance would be .001/.002. The wear limit is .005. Don't forget to check end clearance as well....

TOH
 
what about this? I am not sure when this broke off> But will it loose prime etc because of this?

When you say end clearance, Do you mean between end of gears and the suction tube plate? There was a crescent shaped groove in it that I milled out.
a182909.jpg
 
(quoted from post at 19:38:04 02/14/15) what about this? I am not sure when this broke off> But will it loose prime etc because of this?

When you say end clearance, Do you mean between end of gears and the suction tube plate? There was a crescent shaped groove in it that I milled out.

Yes - end clearance is between the end of the gears and the plate. Also between the bottom of the housing and the other end of the gears. That chipped area is not good and will be a potential source of internal bypass. It isn't going to help matters but how much it adversely affects the pump is anybody's guess. Looks like it may have been caused by the driver gear wobbling around....

TOH
 
Thank You, you have been very helpful. This tractor was loosing it oil prime. Just a cursory inspection of the pickup tube revealed nothing. I will dunk it in a tank of water tomorrow. If it passes it must have been the pump. What else could it have been besides pump clearances, pickup tube or gasket? I think that about covers it.
 
(quoted from post at 22:27:47 02/14/15) Thank You, you have been very helpful. This tractor was loosing it oil prime. Just a cursory inspection of the pickup tube revealed nothing. I will dunk it in a tank of water tomorrow. If it passes it must have been the pump. What else could it have been besides pump clearances, pickup tube or gasket? I think that about covers it.

These pumps and the lost prime phenomenon are still a mystery to me. The "obvious" explanation is that the gears wear. The driver gear bushing also wears and that gear wobbles so much the clearances open up even more. The result is it can't produce the oil film needed to pull the initial vacuum at startup. I think that is mainly true.

Another concern is a worn oil pump housing. Once that driver gear starts to wobble around it begins to rub on the housing and wear it out. When that happens replacing the gears won't fully restore the clearances and you may still have low flow and/or loss of prime issues

Leaks in the pickup tube or pump sealing surface are also potential problems as are the gear end clearances.

Some people have also theorized that a worn shaft bushing allows the residual oil inside the pump to drain back into the sump once the engine is shut down. I am more skeptical of this one as there is enough space below the level of the bushing to hold plenty of residual oil.


That is why I built this Rube Goldberg bench test rig for the L-head oil pump It lets me test a pumps before it goes back in the tractor. And the reason I say loss of prime is still a mystery to me is that I recently rebuilt a pump that primed and bench tested fine but if I allowed it to sit on the test rig for 3-4 hours it would not prime itself again. All of the clearances I could measure were fine and there was no evidence of any external leakage while it sat. But that sucker simply would not self prime for some reason.....

TOH

IMG_1851.jpg
 
When I rebuilt my 8N pump I was concerned with the housing to end of tooth clearance as some teeth on the new gears from the kit dragged (some did and some did not) so I ended up lapping those that did, but was unsure how much clearance to maintain. In reading I tripped across an old Ford flathead V8 rebuild book that indicated that tooth to housing clearance to be 0.002".

That makes sense, the 0.0015" wear limit recommendation on the shaft bushing would allow full wear on the bushing and shaft without damage to the housing.
 
Actually the specification for shaft clearance new is 0.001", but also on inspection of used pump the bushing should not exceed 0.566" dia and the shaft not less than 0.560" dia suggesting up to 0.005" clearance is acceptable, but that would allow the gears to do a number on the inside of the housing.
 
(quoted from post at 17:10:41 02/15/15) Actually the specification for shaft clearance new is 0.001", but also on inspection of used pump the bushing should not exceed 0.566" dia and the shaft not less than 0.560" dia suggesting up to 0.005" clearance is acceptable, but that would allow the gears to do a number on the inside of the housing.

Here is how Ford designed their fits fo rthe 120 engine. The information is from Chapter IV - Fits and Tolerances of the OEM [u:cbb7e00e31]Ford "120" 4 Cylinder Industrial Engine Manual[/u:cbb7e00e31] which IMO is far better than the I&T manual:

[i:cbb7e00e31][b:cbb7e00e31]Generally speaking all bores are made to a standard size (so that standard reamers, plug gauges, etc. may be used) with a [u:cbb7e00e31]plus tolerance[/u:cbb7e00e31]. The maximum size for male parts is usually a standard size less the [u:cbb7e00e31]minimum clearance[/u:cbb7e00e31] required for the type of fit desired. The minimum size for male parts is the maximum size minus [u:cbb7e00e31]the tolerance[/u:cbb7e00e31][/b:cbb7e00e31][/i:cbb7e00e31]

It is important to distinguish the difference between a tolerance and a clearance which is why I underlined those references. So let's consider the oil pump bushing as an example:

The "standard" size for the bore/shaft is is 9/16" (.5625). Assuming a standard 9/16" reamer with a .0005 plus tolerance the design specification for the bore would be .5625/.5630.

The design oil clearance for the the shaft is specified as .0015/.0029. So the shaft manufacturing specification works out to be:

Maximum = .5625-.0015 = .5610
Minimum = .5630 - .0029 = .5601

That is what the factory would have been trying to produce. The shop manual says that for a used engine the wear limit on the shaft [u:cbb7e00e31]clearance[/u:cbb7e00e31] is .005 or .0021 more than the maximum manufacturing tolerance for that clearance. That isn't a lot of allowable wear.

I have never found any specifications for the gears or the housing. How the factory might have tried to control tolerance stackup between the shaft clearance, OD of the gear, and ID on the housing is anybody's guess. About the only way we would ever know is if someone with access to the original prints cared to share those callouts ;-)

TOH
 
I get what you are saying. I took my oil pump down to a machine shop today. They do a lot of work with pumps. Bigger no doubt. He theorized that someone had rebuilt the pump and reamed that bushing w/o a guide . He showed me how the gear was in fact cocked in the housing a freckle. I did not have the gear face with me, but I told him of the crescent wear marks in it. He guessed correctly that one was larger than the other. That's when he showed me the cocked gear and were it had started to wear on the pump housing . He said he probably has a bushing for it, or he could make one. All in all he said it should pump and unless the gear face was making it loose its prime he could see no reason why it wouldn't work. Now that I tuned the gear face up. He said take it back put it toghethor and try it in a bucket of oil..,. Sure enough it took right off. I am now in the process of trying to duplicate the "Un priming". Much like your bench test. We will know more in the morning. I know a lot of people will claim heresy over this next statement. I am not convinced that upstream of the pump doesn't play a part in pump priming. Much like you hold your finger over a straw or hand over an irrigation tube . Some will say the oil path is open all the way around but just a chance that the engineers created a gooseneck to prevent drawback. Perhaps a slight Negative pressure effect to enhance suction? Just a theory. What I am saying is perhaps not just the suction side of the pump or pump integrity is at fault. Lastly along that line I dig out the pieces in the picture out of my Bypass . I wish I could say it came from someone else. But it was my own fault of TOO much permatex on the timing cover squeezed into the bypass channel. Maybe it was enough to hang it open? Who knows. Some of you will claim pure hogwash. That's ok I respect the current thought. But how is it explained that as perfect as can be Oil pumps loose prime? Not a common incident, but it does occur. In fact I talked to a Very Respected tractor rebuilder this morning who was struggling with this same issue.
 
Just cant make the picture thing work tonight. Anyway It was an o Ring created when I applied too much permatex and it was in the presuure relief cavity surrounding thee spring/ plunger.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top