1955 oil pump gears

[buick5563]

Hi guys,

I just cleaned up my oil pump today, and the replacement gears I bought from Terrill are straight cut as opposed the spiral cut gears I pulled out. From what I have learned from looking at my 1956 manual, 56's had straight gears. The parts book lists two styles for 55 (early and late).

My question is:

Is there a difference in the actual oil pump housing?

I did install the straight gears, and there was the factory play (.004) so I imagine they would work. Just want to know if somebody knows differently.

Thanks.

[old-tank]

Hi guys,

I just cleaned up my oil pump today, and the replacement gears I bought from Terrill are straight cut as opposed the spiral cut gears I pulled out. From what I have learned from looking at my 1956 manual, 56's had straight gears. The parts book lists two styles for 55 (early and late).

My question is:

Is there a difference in the actual oil pump housing?

I did install the straight gears, and there was the factory play (.004) so I imagine they would work. Just want to know if somebody knows differently.

Thanks.

Mike

I don't know the answer to your question on the housing, but you might call Terrill and see if they have gears like your original---that way you will have no doubt.

Willie

[special55]

Mike

I don't know the answer to your question on the housing, but you might call Terrill and see if they have gears like your original---that way you will have no doubt.

Willie

I agree with Willie Mike. It would make me a little nervous making that change without confirming with the parts sipplier.

Oil pump is not something you want to question on your fresh rebuild.

Just to be double sure you can pump up your engine with a drill and a pressure guage to make sure you're where you want to be before firing it up. A practice I use every time regardless if I have questions or not. Just makes me feel a bit better about things.

Rich

[buick5563]

I'm gonna call em tomorrow.

[buick5563]

Just talked with the guy at Terrill. He said that the pumps they rebuild all use the straight cut as opposed to HELICAL cut. He said originally Buick used Helical cut gears because they were supposed to be quieter. He also told me that they measured the same, and all gears from 53-56 V-8's use the same height gear.

Well, there you have it.

[NTX5467]

Thanks for the update, Mike! I believe that all of the oil pump gears I've seen on more later-model engines were "straight cut"--on all makes.

There might have been some flow theory to justify the helical cut oil pump gears, other than just noise control. I also suspect that they were more costly with little real benefit for the ownership experience.

A local transmission expert (in this case, the word really DOES apply) noted that when GM makes a parts change, it can depend upon who is making the decisions that day, more than anything else. From one "good design" to another "good design", usually. MANY variables!

I have noticed that just as soon as problems start to happen "in the field" on a new model GM vehicle, a TSB (back when they were sent out on paper!) would magically appear with the new and upgraded part listed to be installed to fix the problem. Kind of like they KNEW what was going to happen and had already planned to use the "second good design" to replace the "first good design" which failed. Or perhaps the "second good design" was supposed to have been the "first good design", but wasn't for one reason or another? This seemed to really happen in the 1980s and 1990s.

Sometimes, these new part numbers were not yet available. GM Service/Engineering sent out the TSBs but had not made arrangementw with GMService Parts Operations to get them to the dealers. In some cases, there were HUGE disconnects between those two parts of GM! in some cases, it was quite amusing.

Just some thoughts and observations . . .

NTX5467

[Bill Stoneberg]

Just to be double sure you can pump up your engine with a drill and a pressure guage to make sure you're where you want to be before firing it up. A practice I use every time regardless if I have questions or not. Just makes me feel a bit better about things.

Doing this also ensures that you have not forgotten a plug behind the bell housing that will dump the oil on the floor when you turn the drill on. Dont ask me how I know this but I wished I had found it when the engine was out of the car.

[buick5563]

Further update...

I was looking for something in the 55 Product Service Bulletins, and on page 7 it says:

The new Spur type gears are made from powdered metal which is compressed under extremely high pressures, then subjected to extremely high temperatures causing the powdered metal to fuse together and form the gear, thus eliminating extensive machining originally required. This type of gear requires a tighter press fit on the drive shaft than the helical type, consequently, the bore size of the new spur gear is slightly smaller than the bore in the helical gear.

...therefore, if you were to find NOS "Spur" (straight cut) gears, you could not use them in the original helical (spiral cut) pump body, since the mounting hole is smaller...

FYI

[NTX5467]

Are they mentioning that the pump body's internal diameter is smaller with the spur gears than with the helical-cut gears OR is the pump body's ID the same, but the ID of the gears, where they are pressed onto their respective shafts where the smaller ID is?

It kind of sound like 5563 is talking about "pump body" ID whereas the PSB is talking about the ID of the gear's hole where the driveshaft (for want of a better word) interfaces with the gear? I highly suspect that if the PSB had mentioned the new gears being different (and in what respect), IF other related parts would have been needed to ensure their correct operation in the engine (i.e., pump body ID), then the necessary parts would have been mentioned in the PSB. Hopefully, there would have also been some clearance specs in the PSB, too, for the gears themselves and also in the pump body.

If, per chance, the clearance between the gears and the pump body might be a little larger (as 5563 appears to suspect), then the PSB would have been remiss for not mentioning this and a possible slight loss in oil pressure at lower rpms would be expected. BUT in our modern world, having oil pressure a little lower than original 1950s specs can also release more horsepower to the flywheel (taking less gasoline in the process, too).

Therefore, if the spur gears were not intended to be a "drop-in" replacement for the original helical-cut gears, there would have been comments regarding other parts needed to make them work at least as well as the helical-cut gears. That's my gut suspicion . . .

Enjoy!

NTX5467

[buick5563]

I'll reread it and try to figure it out. I unfortunately don't have a scanner, so I had to type all of the above bold type from the manual. There IS more...

[old-tank]

Mike

That's the rules in 55...how do the currently available replacements fit the rules? My understanding from reading this is that the spur type idler gear would not fit on the pin of the pump originally equipped with helical gears. Apparently it fits, so you should be ok. I would not be afraid of using the original gears if they look good and measure out ok.

Willie

[buick5563]

The replacements from Terrill fit like a glove, and specced out exactly like the service manual stated.

[Guest Silverghost]

The big thing that effects oil pump pressure & output is how tightly both gears mesh and the clearence from the outside of the gears to the pump housing.

Usually you place a feeler gauge between the outside of the gears and the inner pump housing. Also on top to the pump housing cover.

There should be tech, specifications for this in an engine rebuild manual.

I would go with the new gears you bought.

I suspect the new straight gear set is slightly oversize in outside diameter to account for normal pump cavity wear.

Use a new gasket and put locktite on the pump cover fasteners and you now should have a like new oil pump in performance !

The spiral Helical gears are great...

BUT much more costly to machine.

A real gear shop job with costly special machine equipment.

I suspect because of this he had straight tooth gears reproduced !

The powder metal fused gear process is much cheaper than cutting gear teeth by machine.

I guess they don't want to machine cut gear teeth in China or Mexico ?

Another reason for the staight gear teeth !

Sounds like these pump gears were made to sell at a low affrodable price ?

He possibly had the shaft hole Inside diameter made slightly smaller for a tight shaft fit on a variety of old & warn pump shafts.

We can't always get everything we want with reproduction parts.

These new spray powered meal straight gears will never last as long as that old set....

BUT~

Just be glad he reproduced them for your application !

We are in luck anyone is making these old parts in the first place...

Be Happy

&

Good Luck !

Edited October 25, 2010 by Silverghost (see edit history)

[NTX5467]

Thanks for those updates and additional information!

NTX5467

[Guest Silverghost]

If the supplier were in fact selling Helical Spiral machine cut gear sets they might have to charge you $1000. per set !

Take the old gears to a local machine shop and ask how much they would have to charge you for one machined custom one-off set ?

The production & set-up would only be for very low production volume.

[buick5563]

Thanks for your input Silverghost. The main reason I am posting a lot of this is for future reference for people using the search feature on the forum. I should reiterate that I am not concerned about the quality of the reproduction parts I purchased. All of the tolerances were exact per the specs in my shop manual. There is absolutely no slop to speak of. I am much more happy that these are gears made in Texas than machined elsewhere, not in a jingoistic way, but I have not been happy with other reproduction parts with the made in China label on them. The quality control can be atrocious on some of these parts, like the motor mounts I got that were 1/8" off... Unacceptable!