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Cat 931 engine mystery....

TnTrouble

Member
Joined
Apr 5, 2023
Messages
16
Location
Tennessee
Starting a new thread as advised. Purchased a Cat 931 without doing due diligence and now I have a bucket of questions that need help sorting out. I have been unable to find a serial number on the machine bevel gear housing will be my last try in the morning. The machine had a jasper engine installed and threw a rod after 20 hours of ownership. Been a long time I was snoogered, but I got taken on this one. Motor medic and saw dust will stop a knock until it burns off it seems. I will post pictures of the Jasper engine but it is a 3204 of unknown arrangement. I have procured a 3204 long block with a serial number of 45v22532 arrangement 7N5139. Replacement block is at the machine shop and I have the out of frame internals for it with 3 ring pistons.
 

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  • jasper head.jpg
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TnTrouble

Member
Joined
Apr 5, 2023
Messages
16
Location
Tennessee
Starting a new thread as advised. Purchased a Cat 931 without doing due diligence and now I have a bucket of questions that need help sorting out. I have been unable to find a serial number on the machine bevel gear housing will be my last try in the morning. The machine had a jasper engine installed and threw a rod after 20 hours of ownership. Been a long time I was snoogered, but I got taken on this one. Motor medic and saw dust will stop a knock until it burns off it seems. I will post pictures of the Jasper engine but it is a 3204 of unknown arrangement. I have procured a 3204 long block with a serial number of 45v22532 arrangement 7N5139. Replacement block is at the machine shop and I have the out of frame internals for it with 3 ring pistons.
 

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  • hole.jpeg
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  • Jasper piston.jpg
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  • jasper head.jpg
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OzDozer

Senior Member
Joined
Jan 18, 2007
Messages
2,207
Location
Perth, Western Australia.
Occupation
Semi-Retired ..
What made the conrod let go, was it broken big end bolts, or did the little end open up? The opening seems to indicate broken big end bolts.

They really need to be replaced at every rebuild, IMO, I've seen too many 3200 engines with ventilated blocks.

Those pistons don't look correct for the DI engine, I don't believe they should have the heat plugs - but if they're aftermarket, the aftermarket manufacturer may have been producing an older Cat design piston.

That block is repairable, a good cast iron repairer should be able to recover it, as long as there's no major internal damage to the casting.
The crankshaft will need to be checked out for alignment and whether it can still be reground.

There's a great deal of interchangeability all through the 3204 engine range, the major differences is when the engine went from PC to DI.
The Arrangement number is your best guide, my 931B has the original engine with a S/N of 45V29120, and the Arrangement number is the same on my tractor - 7N5139.
 

Nige

Senior Member
Joined
Jun 22, 2011
Messages
28,979
Location
G..G..G..Granville.........!! Fetch your cloth.
What's the tractor Serial Number.? Edit: I just saw that you said you couldn't find it.
Not to worry, take a look at the table in the attachment and pick out the options (e.g. type of transmission and does it have standard or LGP undercarriage) that your machine has. At least we ought to be able to narrow it down.

It's looking very likely that the 7N-5139 long block you've bought could be very close to or even identical to the original engine arrangement that was installed in the tractor when it was built.
 

Attachments

  • SEHS7458.pdf
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OzDozer

Senior Member
Joined
Jan 18, 2007
Messages
2,207
Location
Perth, Western Australia.
Occupation
Semi-Retired ..
56Wrench is correct, that's a PC cylinder head, I wasn't looking closely enough at the head photos, I was concentrating on the block damage photo. That's why the pistons have the heat plugs.
Not sure what the engine S/No break was, for the change from PC to DI, but it was around 1978 or 1979 IIRC, my 931B tractor is 1980 production, and it's DI.
 

Welder Dave

Senior Member
Joined
Oct 11, 2014
Messages
12,257
Location
Canada
If it's a straight 931 I'm pretty sure it would be a PC engine. A pic. of the machine would verify it's a straight 931. Very noticeable difference around the operators station. I think sometime in early 931B production is when they went to the DI engine. I have an 86 931B but also a Cat bulletin on improvements to the 931B. The 931B may have been when they went to the DI engine. The bulletin lists the 931 at 62 HP and the 931B and improved 931B at 65HP. A brochure from 11 82 lists a DI engine.
There is a thread from earlier this year where someone rebuilt a 3204 and made some modifications but I think it was for a D4H. Can't remember exactly though.
 

TnTrouble

Member
Joined
Apr 5, 2023
Messages
16
Location
Tennessee
See below pictures. It is a straight 931. No LGP. I believe the arrangement will work. If you notice in the photos the previous owned bypassed the fuel pump in the accessory drive and ran an inline electric pump straight to the filter. My question regarding conversion is, Will the fuel flow be enough for a DI engine. I do not see where any different internals are listed for a PC injection pump Vs DI. Also will the timing require advance or retarded? Another issue is the glow plug set up. Without the PC cups that the glow ran to, how would i go about the prestart warming sequence? I am sure more issue will present themselves as work progresses.

The engine i am building as a replacement requires a ,020 cylinder bore and a .1 rod bearing turn. I have already gotten the new internals on hand.

Is there anything I should go over regarding the transmission while i have it out? How about the injection pump, should i rebuild it? Oil pump gears and bearing appear to be in good shape, should i refit new gears anyway? Torque converter need any attention? Shifted fine when he engine was running.

Anything else you all may suggest would be greatly appreciated. I have turned plenty of wrenches in my time but I am not familiar with the CAT 3204 engine and do not want to miss something and have to pull the drive train again...... Once was more than enough.


Thanks in advance.
 

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TnTrouble

Member
Joined
Apr 5, 2023
Messages
16
Location
Tennessee
What made the conrod let go, was it broken big end bolts, or did the little end open up? The opening seems to indicate broken big end bolts.

They really need to be replaced at every rebuild, IMO, I've seen too many 3200 engines with ventilated blocks.

Those pistons don't look correct for the DI engine, I don't believe they should have the heat plugs - but if they're aftermarket, the aftermarket manufacturer may have been producing an older Cat design piston.

That block is repairable, a good cast iron repairer should be able to recover it, as long as there's no major internal damage to the casting.
The crankshaft will need to be checked out for alignment and whether it can still be reground.

There's a great deal of interchangeability all through the 3204 engine range, the major differences is when the engine went from PC to DI.
The Arrangement number is your best guide, my 931B has the original engine with a S/N of 45V29120, and the Arrangement number is the same on my tractor - 7N5139.
Conrod let go at the crank bolts. Have yet to pull the crank and pistons to get a better look.
 

TnTrouble

Member
Joined
Apr 5, 2023
Messages
16
Location
Tennessee
What's the tractor Serial Number.? Edit: I just saw that you said you couldn't find it.
Not to worry, take a look at the table in the attachment and pick out the options (e.g. type of transmission and does it have standard or LGP undercarriage) that your machine has. At least we ought to be able to narrow it down.

It's looking very likely that the 7N-5139 long block you've bought could be very close to or even identical to the original engine arrangement that was installed in the tractor when it was built.
Nige,

Thanks a million sir. Thats exactly what i needed. I should have read it completely before my last post.

You are a great resource sir!
 

Welder Dave

Senior Member
Joined
Oct 11, 2014
Messages
12,257
Location
Canada
The DI engines had an ether injection system for cold weather but you could just give a tiny spray in the air intake. I took my ether injector off because it needed a new temperature sensor that goes in by the thermostat housing. It wouldn't allow the system to work when the coolant was warm. There was a loose wire that arced one time and I think shot a tiny bit of ether. I don't use my machine much in the winter so just took it off. My machine will start without ether down to about -10C maybe even colder but I usually give it a tiny shot to save the starter. Plugging in would be best but I don't have power at my property.
 

Nige

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Jun 22, 2011
Messages
28,979
Location
G..G..G..Granville.........!! Fetch your cloth.
Thanks a million sir. Thats exactly what i needed. I should have read it completely before my last post.
So based on it being a straight 931 rather than a B there are only 2 choices of S/N prefix because both the 8N & 10N prefixes are LGP machines.

78U & 81U were both built in Japan. Production of 81U started in 1973 and of 78U in 1976.
If your machine is/was a lower S/N than 81U930 or 78U3913 the original engine would have been a 3204 PC of 4N0169 Arrangement Number.
If it was 81U931 or 78U3914 or higher, it would originally have been built with a 3204 DI engine of Arrangement Number 7N3800.
The arrangements above are broadly similar to what would have been installed in a straight (non-B) D3 tractor of the same era.

7N-5139 appears to be a DI engine arrangement only used in D3B tractors with a 3F-1R type transmission as opposed to the 3F-3R. I'll do a bit of digging and see just how different it is to a 7N-6938 that would have been installed in a machine with a 3F-3R transmission.

EDIT: And............ I'm back.

The only difference between those two engines above is the flywheel.
7N-5139 Arrgt has a flywheel Part # 4N-0184.
7N-6938 Arrgt has a flywheel Part # 7N-6937.
 

Welder Dave

Senior Member
Joined
Oct 11, 2014
Messages
12,257
Location
Canada
Nige, does it list different HP ratings between the PC and DI engines in the straight 931 machines? I'm sure I read somewhere that there was an improved DI engine in later 931B/D3B machines. Also curious what is different that a 931C got a jump from 65 HP up to 75 HP?
 

OzDozer

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Messages
2,207
Location
Perth, Western Australia.
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Semi-Retired ..
Nige, I believe all the 931's were built in Japan.

TnTrouble, I see no reason to bypass the standard 931 fuel pump, the system works just fine if the lines and tank are in good condition.
But the tank and lines can end up with internal corrosion (and external corrosion will cause pinholes and create more trouble) - and both these conditions will create fuel flow problems.

Also, tanks often have a buildup of debris inside them, such as fluff from rags, foreign materials accidentally dropped in the tank - and dirt and water (from condensation).
So a fuel tank cleanout and new fuel lines are always a good start when rebuilding - particularly if the lines display any form of corrosion.

You don't have to buy Cat fuel lines as a part, you simply manufacture new ones with some new steel tubing, and a small cheap tube bender.

Re the transmission, seals and o-rings go hard and cause leaks and shifting problems, so replacing as many as you can get at without a complete transmission disassembly would be a good move.
Check the transmission oil pump for wear levels (clearances), and the pump screen for debris and trash and look for metal. The oil pump gears should not need replacement if they fall within wear specifications. As a general rule, .003" to .005" clearance is good, .006" to .008" clearance is getting into the region where you probably should replace the components that are required to produce and hold oil pressure.

There is a bushing that supports the transmission valves control shaft where it goes into the transmission.
Replace that bushing and the seal on it, as wear here allows dirt entry. Check the shaft condition, to ensure the seal seating area, and bushing support area, are not severely worn.

The injection pump should not need attention, these old style injection pumps have a very long life as long as the filters have been working properly.

I don't have the injection timing degrees readily to hand for the PC engine, but the DI engine injection timing runs at 21° BTC.
There will most likely be a difference in timing between the two engines, the service manual advises the injection timing under "Engine Specifications".
 

Nige

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28,979
Location
G..G..G..Granville.........!! Fetch your cloth.
Nige, does it list different HP ratings between the PC and DI engines in the straight 931 machines? I'm sure I read somewhere that there was an improved DI engine in later 931B/D3B machines. Also curious what is different that a 931C got a jump from 65 HP up to 75 HP?
I think confusion reigns between sources that quote Gross Power and those that quote FWHP.........
From the earliest model to to C there is only 5 BHP of difference.

1685409177871.png
 

OzDozer

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Joined
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Messages
2,207
Location
Perth, Western Australia.
Occupation
Semi-Retired ..
The main ID for the 931 are the two aluminium tags - one for the tractor, which is normally located on the upper outer vertical face of the RHS loader tower support (just above the "CAT" decal on TnTroubles tractor) - and the other on the upper left rear of the engine block, just below the head gasket line.

Both these tags appear to be gone from TnTroubles tractor. Some Cats have the numbers stamped in the metal under the tag - but the tags on my 931B are still in place, so I can't check that angle on my tractor.
I have seen some Cats with the engine S/No stamped into the metal of the lower front left of the engine block, just above the oil pan join line - but I haven't checked my block for that, and my block is currently still at the engine reconditioners.

The other angle TnTrouble could try, is to find a casting on the tractor with the NUMERALKOD in it, and acquire a date from that casting. The 2-post ROPS indicates an early 931. The 931 was first manufactured in 1973.

"The Caterpillar numerical code contains the date of manufacture. This date is translated to a format of
day, month, and the last two digits of the year, and then coded into a six-digit sequence with no spaces
in between digits.
When the day or month is less than ten, a zero is inserted before the number to maintain the six-digit sequence.
Use the following legend to decode the date:

N U M E R A L K O D
0 1 2 3 4 5 6 7 8 9

Example:
December 18, 1985, is translated to 181285 which is coded UOUMOA
September 7, 1985, is translated to 070985 which is coded NKNDOA"
 
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