WARNING!!! Rebuilding GPW/MB Engines

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.
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17thAirborne
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by 17thAirborne » Sun Feb 11, 2018 8:14 am

harve wrote:
Sun Feb 11, 2018 7:48 am
Same exact thing happened to me on my last 3A engine i had rebuilt , carbon copy of events
Did you determine a cause? Was it the wrong rod installation or something else?
Oz

Feb 43 GPW 98532 USA 20206257
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by Chuck Lutz » Sun Feb 11, 2018 8:55 am

If the bearing was basically "welded" to the crank, then the crank will need to be turned probably...

However, the missing dowel pins that are often missing allow main bearings to spin....another problem....but not the one your friend had occur.

But...I'm kinda thinking that artificer's suggestion that the incorrect installation of the rod bearings denied the flow of oil to that bearing and the heat caused the problem.

No offence to any professional mechanic, but these jeep engines are 75 year old technology. Added to that there are manuals with incorrect information in them.
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by Joe Gopan » Sun Feb 11, 2018 9:11 am

Trained or experienced mechanics are not dumb when it comes to Jeep engines.
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by Joe Gopan » Sun Feb 11, 2018 9:16 am

Chuck Lutz wrote:
Sun Feb 11, 2018 8:55 am
If the bearing was basically "welded" to the crank, then the crank will need to be turned probably...

However, the missing dowel pins that are often missing allow main bearings to spin....another problem....but not the one your friend had occur.

But...I'm kinda thinking that artificer's suggestion that the incorrect installation of the rod bearings denied the flow of oil to that bearing and the heat caused the problem.

No offence to any professional mechanic, but these jeep engines are 75 year old technology. Added to that there are manuals with incorrect information in them.

Incorrect installation of "rod" bearings? Did you forget to say "Main" bearings?
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HONOR GRAD-WHEELED VEHICLE MECHANIC SCHOOL 1960 - US ARMY ORDNANCE SCHOOL(MACHINIST) ABERDEEN PG 1962 - O-1 BIRD DOG CREWCHIEF - 300,000+TROUBLE FREE M-38A1 MILES
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by harve » Sun Feb 11, 2018 9:30 am

Mine was a main bearing improperly installed. Come to find out the dowl pin was missing and bearing spun just enuff to block oil flow from the oil gallery in the block. Thank God i didn’t keep running it

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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by 17thAirborne » Sun Feb 11, 2018 11:56 am

I will see if he took some pics.
Oz

Feb 43 GPW 98532 USA 20206257
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by harve » Sun Feb 11, 2018 12:02 pm

Some pics , I’m curious also

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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by Joe Gopan » Sun Feb 11, 2018 3:59 pm

it is a good idea to check the length of the used connecting rod journal for excess wear against the length of a NOS Rod. too much wear allows fore and aft motion as mentioned by Luc, It does make a difference. I do not have the measurement handy.
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by Michael Browne » Sun Feb 11, 2018 7:48 pm

Without echoing previous comments too much I would be inclined to think the bearing clearances on the #4 rod were too tight and coupled with a wrongly oriented rod and perhaps a bent rod the early failure has occurred. A bearing that is stressed by the offset positioned wrongly and perhaps a bent rod will quickly overheat and close up any clearance it had and lock up, or try to lock up but the engine will overcome this and either throw the rod out or weld the shell to the crank pin.

Maybe the main bearing shell has only partially turned and blocked off most of the oil hole to the bearing and there was sufficient oil supplied to the main bearing to prevent seizure but not enough to supply the rod bearing and it has overheated and locked up and the babbit/white metal has deposited itself onto the crank. or a blocked oil drilling to the rod journal ....Pure speculation here as no photos to verify.

Wonder why he chose to continue driving it after he heard the noise. Do that on a bigger engine and you risk catastrophic failure and big big dollar repairs :?:

When I assembled my first jeep engine a couple or three years ago I followed the manual but on close inspection the rods didn't look right at the piston end... and the caps didn't look like the ones shown in the photos in the manual. It was being assembled as it had come apart. I quickly changed them out to correct this and also changed the manual to read correctly. Over the years since, I have seen many many jeep engines with the rods in wrong.

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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by twinflyer17 » Mon Feb 12, 2018 3:41 am

Oz, thanks for calling this out and posting the corrections to the incorrect section. I have my original GPW block disassembled and ready to be dropped off at the machinist - just waiting until I somehow find more time - then will hopefully continue with the rebuild this spring. I've see others on the 'G' call out this error before, but posting here will allow me to easily reference the corrected version when the time comes.
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by 17thAirborne » Mon Feb 12, 2018 6:34 am

And from John Barton's Guide on building engines. I always get confused on the "OFFSET" Here is the correct picture of what 2 and 3 should look like.
Image27.jpg
Image27.jpg (76.99 KiB) Viewed 577 times
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by Chuck Lutz » Mon Feb 12, 2018 8:46 am

We got one engine back from a "professional engine rebuilder" who had transposed the sizes of the new rod and main bearings. While he had done dozens of L-134s, he didn't get that one right. It took awhile for us to discover the problem since we aren't "pros" by any means and who were we to blatantly question one who was. Good news was we figured it out prior to trying to start it so no damage done. with the correct rod/main bearings things went as they should have gone.
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by rjbeamer » Mon Feb 12, 2018 12:18 pm

Good day Chuck. So what you are saying is the Bearing shells were too small for one type of journal ie: Rods or Mains. And too large for the other. Was this an assembled engine or just a machined engine and loose parts. Ether way i am at a loss as to why it would take a while to find the problem.

Roger

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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by 17thAirborne » Mon Feb 26, 2018 6:14 am

bantamj wrote:
Mon Feb 12, 2018 8:17 am
I have posted this manual page before but here goes again as it is important.
Be aware that the oil squirt hole must be orientated AWAY from the piston slot and TO the piston thrust side (for if one has piston's without slot).
The squirt holes are facing AWAY from the camshaft side while the small end from the off-set is TOWARDS the nearest main bearing.
So there are 2 things to be cautious for : conrod off-set & oil squirt orientation!!
And to be able to do that one needs 2 different conrod's...2 for cyl 1&3 and 2 for cyl 2&4.

Best Regards,
Luc
That is a great pic and good to see again. Thanks
Oz

Feb 43 GPW 98532 USA 20206257
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Re: WARNING!!! Rebuilding GPW/MB Engines

Post by KiwiMB » Mon Feb 26, 2018 12:33 pm

I keep seeing the words "Professional Engine Rebuilder" used in threads such as these. The reality here is that many such folks have never seen a Jeep engine, let alone a side valve type, and as such are ill equipped from an experience perspective, to effectively overhaul these engines. This is by no means a criticism nor intended as a broad statement but simply to point out that our engines are now over 70 years old!

Offset con rods are not uncommon, but it seems to me that these professionals lack the necessary observation skills to see how an engine came apart, nor the discipline to take photo's during the dismantling process. Incorrectly installed con rods will often be evidenced by excessive thrust on the piston skirts and thus an engine that is (very) tight to turn over during the assembly process. Yes, the manual is incorrect but "professionals" do not look upon the manual as gospel and question when things seem contradictory or their instincts tell them something does not add up.

In addition, Professionals actually measure the crankshaft journals and then measure the inside diameter of the bearing that is installed into the main bearing or con rod cavity and compare those measurements to ensure that the correct clearances have been achieved. I personally have no time for plastigauge, I use a micrometer and I record ALL the clearances when rebuilding Jeep or dodge engines. The professionals will then check the bearing "crush" in the conrod by releasing one conrod bolt and measuring the gap between the conrod cap and the conrod, beside the loosened bolt. It's this "crush" that ensures that the bearing shells are held firmly in place in the con rod.
Professionals also insist on checking conrod alignment (straightness), again this ensures that the piston will be concentric within the bore and avoid piston skirt damage.
This has been a great thread and shows that the Gee is indeed a great resource.

Cheers
Ian
Ian J
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