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1G replacing oil pump straight gears with helical

dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
I have an early 6bolt with original straight cut oil pump gears. Since those gears are discontinued, can I swap directly to the helical gears or do I have to replace the front case along with the gears?
 

1990TSIAWDTALON

Moderator
9,146
4,975
Nov 14, 2013
Independence, Kansas
I would keep the gears with the case. If the case is messed up, the gears may have issues also. Just what I would consider the right thing to do. I would be afraid of clearances that are so important in an oil pump.
 

dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
I thought it would be prudent to replace the oil pump after 270k. Nothing is "wrong" with my old setup, though. I read maybe there was a design change on the front case between the early and later 6 bolts.

However I do want to check clearances to see if the old straight gears are still in spec. I hear they are better flow and longevity than helical.
 

steve

DSM Wiseman
14,473
1,402
Feb 3, 2002
St. Charles, Illinois
I wish I had saved all the TSBs or could find them all online. Customers were complaining about the gear whine on the 90 and 91's so they issued a TSB and switched over new builds. I looked for a receipt when the dealership swapped mine but it doesn't look like I got copies of work done via a TSB.

CAPS says the gears match the front case so if you're replacing the 90/91 MD129347 case with a MD175762 the gears need to change as well.
 

pauleyman

DSM Wiseman
7,999
2,605
Nov 19, 2011
oklahoma city, Oklahoma
I wish I had saved all the TSBs or could find them all online. Customers were complaining about the gear whine on the 90 and 91's so they issued a TSB and switched over new builds. I looked for a receipt when the dealership swapped mine but it doesn't look like I got copies of work done via a TSB.

CAPS says the gears match the front case so if you're replacing the 90/91 MD129347 case with a MD175762 the gears need to change as well.
As I was one of those customers too. It was a distinct sound. If I recall they did not do anything for me. I've read the tab and if I recall the gears match the case as you said. I have an nos pump with straight gears for my next build. I kept the original block and crank from that same 90. Never bored or machined.
 

dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
I can't say I've even noticed the whine. Actually, now that I think about it, that probably wasn't my alternator.

I kept the original block and crank from that same 90. Never bored or machined.
That's what I'm working with, but an early 91. I feel bad I'm going to have to machine it.

So it seems I either need to source straight gears or buy a full front case if I want to replace the pump.
 

dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
I should have done this before posting. Here's a little data to back up the front case vs. oil pump gear changes. Definitely the '90 and early '91 case/gears were different than later 6 bolts. Although the front case changed between 6 and 7 bolt, the helical gears did not.

Front Case:
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Large oil pump gear:
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Small oil pump gear:
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1990TSIAWDTALON

Moderator
9,146
4,975
Nov 14, 2013
Independence, Kansas
Being an engine builder, I have used ACL front pumps, with helical gears and not had an issue. Pressures are 17 hot idle, 85+ cold start.
Use what you feel comfortable with. I won't build any engine without a new oil pump, however you do it. Only my opinion.
 

DSMPT

DSM Wiseman
2,002
1,553
Jun 12, 2014
Japan / Mexico, Arizona
You can swap with the helical gears but if I were you, I would buy a new OEM case and gears as the first plan, if the budget would allow me. The second, I would keep using the same OEM case and gears if the clearance is still fine. The third, a new aftermarket pump.
Just a little tip. Don't over tighten the gear cover bolts. And avoid to use a gear cover that is taken from another case (Do not mix it when you keep several used cases. The case and the cover are originally paired from factory).
 

iugrad92turbo

Supporting Member
12,884
813
May 22, 2007
Kalamazoo, Michigan
Agreed keep things together with in the same specs of the gears, oem pump will be the choice, other that that many use the acl pump, just use the oem gears. I believe i have a oem pump ive had so many 6 bolts if forgot but im sure im running a oem pump, with straight cut gears.
 

dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
I was inspecting my front case last night and realized it's the piece that takes most the wear, so replacing only the gears doesn't accomplish much. Those gears are so hard I can't see how an aluminum case is going to cause much, if any, wear on them. Maybe only a small bit of wear on the teeth where the two gears mesh. It'll be a new OEM case and helical gears for me! Thanks everyone for your responses, it really helped point me in the right direction.

As a side note, I noticed some oil inside my oil pump sprocket and further discovered the sprocket requires a bit of RTV where it seats against the drive gear. It's got an oil seal for the outer lip, but nothing where the drive gear inserts into the sprocket. I didn't see this mentioned in the FSM anywhere.
 

1990TSIAWDTALON

Moderator
9,146
4,975
Nov 14, 2013
Independence, Kansas
HERE is some example oil pumps in a thread I did a while back, just for reading and the pictures and specs.
I've never used RTV on the oil pump, are you sure the castle nut wasn't leaking?
 

dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
HERE is some example oil pumps in a thread I did a while back, just for reading and the pictures and specs.
I've never used RTV on the oil pump, are you sure the castle nut wasn't leaking?
Castle nut was fine and not leaking. I did find a bit of old RTV in the sprocket and on the drive gear shaft when I disassembled it, possibly from the factory because I didn't do that last time I had it apart for BS delete. It really confused me when I started looking at that seal, because it seals around the outer lip of the sprocket, but the sprocket has a hole through it to fit the drive gear shaft. I'll try to snap some pics of what I'm referring to.

Here's some pics showing that there is no oil seal between the sprocket and drive gear shaft. Only between the sprocket and case.
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DSMPT

DSM Wiseman
2,002
1,553
Jun 12, 2014
Japan / Mexico, Arizona
I noticed some oil inside my oil pump sprocket and further discovered the sprocket requires a bit of RTV where it seats against the drive gear. It's got an oil seal for the outer lip, but nothing where the drive gear inserts into the sprocket. I didn't see this mentioned in the FSM anywhere.
Maybe remaining oil on the shaft entered to inside the sprocket after you removed the nut.
The sprocket should be sealed at the green line after torquing the sprocket nut. So usually not much oil get in there if everything is in its spec including the nut torque. If you see oil leak through there, your drive gear shaft or sprocket may be deformed or cracked or the nut isn't torqued enough or the sprocket isn't installed properly. The deformation/crack is sometimes found after bending valves badly.
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dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
The amount of oil was negligible, but grimy like it had been leaking slightly. I wouldn't expect a metal-on-metal mating surface to be oil tight. Usually those spots call for RTV.
 

Mech Addict

Supporting Member
982
414
Jun 9, 2019
Jackson, Wyoming
So your oil pressure has been ok? It the best lubricated moving part in the engine, right? After that many miles, my own not very experienced opinion is if the clearances are not close to the limit, why mess with a new case & gears? You got a good one, it seems.
 

DSMPT

DSM Wiseman
2,002
1,553
Jun 12, 2014
Japan / Mexico, Arizona
Usually those spots call for RTV.
No. But you can do that if you don't care using RTV in oil pump, close to a tiny drain hole. If you would need to remove the sprocket later, you may drop leftover RTV residue and may make the hole clogged though.

I tested this before. Exactly the same reason as you wondered.
You can test it when the gears are out of case, set the sprocket on the drive gear shaft and torque the nut. Make it stand up and pour liquid inside, above the contacting point of sprocket and shaft. Try various conditions, all new/used, clean/unclean surface, torqued/not torqued etc, and then see how each condition would affect the time to start to ooze out. If all the parts are in good condition or new, it would take long even with the pure water.
If all the parts are in good shape and the surface including the back side of nut flange is flat and clean, and the nut is torque properly, usually no much oil get inside from there. Yes, sometimes you find a very small amount of oil oozing out from the sprocket nut, in that case I recommend to re-torque the nut first, or/and you can slightly apply RTV (Or maybe loctite) to the nut flange/thread part, instead of applying inside the sprocket/oil pump side. This would stop it.
 

dwb

Proven Member
168
64
Sep 9, 2021
Broomfield, Colorado
So your oil pressure has been ok? It the best lubricated moving part in the engine, right? After that many miles, my own not very experienced opinion is if the clearances are not close to the limit, why mess with a new case & gears? You got a good one, it seems.
Never had any oil pressure issues and I know this pump had at least 200k on it. Probably was more likely the factory pump, so 270k. I will not be reusing it. Not worth the risk.

No. But you can do that if you don't care using RTV in oil pump, close to a tiny drain hole. If you would need to remove the sprocket later, you may drop leftover RTV residue and may make the hole clogged though.
Makes sense that RTV would not be ideal inside the pump. I get it. Hence why I'm having this discussion now and not after I do it :)

I just want clarify that my initial diagnosis of the oil leak was incorrect. What I saw was oil inside the sprocket, thus it had to be the case seal. I do not see any oil on the outside/nut side (pic below was right after pulling t-belt cover). I think I'm just being overly paranoid about oil leaks. I'm sick of 30 years of oil all over this thing and I'm finally getting it cleaned up.

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motomattx

Proven Member
3,697
1,436
Dec 9, 2010
wampum, Pennsylvania
Oil pumps are one part that I will use over and over again unless I notice that they have too much wear or are out of spec, there are cars out there with a million miles or more on the original oil pump, a couple hundred thousand is nothing with today's oils as long as there is no wear or another reason to replace it, as far as I'm concerned they are "seasoned" at 100k-200k and prime parts.
 

DSSA

Supporting Vendor
598
398
Jul 26, 2002
Hatfield, Pennsylvania
A customer sent me this thread with a question about using the straight cut gears in the later style cases.

As noted above Mitsubishi put out a TSB in 1992 and discontinued the MD129347 front covers, which superseded by the MD175762 unit.

The straight cut gear noise was causing owners to complain, so they redesigned it with helical gears.

The TSB also stated to *NOT* interchange earlier gears into later cases, or the opposite.

Straight cut gears are louder, but produce less axial load. So, it was an upgrade (no/little noise) and a downgrade (more axial load). This is similar to when they went to the later 1G 3/4 gear sets (1993+ Model Year) with a larger diameter--easier shifting for those who said the transmissions felt "notchy", but of course they were much weaker as a result.

All of the above said, there's one MAJOR point of data to take into account before accepting the above as gospel....

Knowing that the early style front cover was discontinued in 1992, there have been at many hundreds, if not *thousands* of builds since that time which used the straight cut gears, especially on 4G63s that were going to see high-rpm punishment (remember the "axial loading" thing?). If the older case wasn't available, guess which case they were loading them into and running them with success? The straight cut gears were what sold at (guesstimate from watching the market, and selling them over the years) about a 5-1 ratio for over 15 years, until they too were discontinued about 2 years ago.
 
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