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AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Mon Aug 17, 2015 10:01 pm
by muleman7
I will attempt to list and show with pics the differences between the AO42 Mule and 2AO42 Power Plant engines.
First a short introduction. The major internal Long Block engine components appear to be the same. They both are 42 cubic inch displacements and use identical Cylinders, Pistons, Crankshafts, Camshafts, Hydraulic Lifters, Push Rods, Rockers and Heads. The Valve Train differences are the Valves and the use of a 0.050 Copper Preformed Head Spacer/Gasket along with a different set of Piston Rings for the AO42 Mule Engine. One of the engineers that helped develop the Mule engine said they dropped the Compression Ratio to produce more low RPM Torque since the engine would run under varying loads and RPMs where the Power Plant engine ran at a constant RPM and varying loads. I have no engineering data to explain the TMs reference to the Power Plant 10 hp rating and the Mule rating of 14 hp. Remember the muscle car adage increase compression increase hp. The compression on the Mule is around 125 #s where the Power Plant is around 145 #s.
The following chart is a pic of an Excel spreadsheet where I have listed the Valve Train parts including the FSN and ORD #s for both engines. Some of the parts did not have FSN #s so those cells are blank. I also listed the TMs / Month-Year published and referenced the fig #s and Item #s to make it easier to check.
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Spreadsheet pic listing Valve Train components and Piston RingsImage
Camshaft from Emory Mule Supply
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Left Head is a 2AO42 and the Right Head is an AO42 notice the height of the Valve Stems
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Closeup of the 2AO42 Head the top of the valve is nearly flush with the valve spring keeper/rotator
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Closeup of the AO42 Head the valve is 0.050 longer from the keeper grove to the top of the valve it sticks above the valve spring keeper/rotator
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These are valves from each engine the top one is a 2AO42 valve and the bottom is an AO42 valve
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This is a Piston Ring set for the AO42 engine the #s match with the spreadsheet pic
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This is a piston with rings for the AO42 engine
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This is a piston with rings for the 2AO42 engine
I will post a pic of the Copper Preformed Head Spacer/Gasket later
I hope this clearly documents the differences as I see it. From my experience using the 2AO42 Long Block for the Mule I find it more responsive with better power. But I have not tested it over the full RPM rage fully loaded. I will save that for another day and edit this post

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Copper Head Spacer measuring 0.065 thick

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Copper Head Spacer in comparison to micrometer

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Copper Head Spacer installed in the head

I used a digital caliper to measure the top of the groove to the top of the valve and found:
The 2AO42 valve measured 0.200
The AO42 valve measured 0.250
The Copper Head Spacer measured 0.065 and the 0.015 difference must be for crush allowances when the head is torqued

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Tue Aug 18, 2015 8:30 am
by Auto Shop teacher
Great information, thanks for putting all of it together so we can see the differences side by side. Considering the size of the engine and the fact that both are de-rated industrial engines I would have a hard time believing that the differences would be noticed when you are bouncing along in the dirt. As long as the engine is in tune and the governor is in adjustment, either one should perform well.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Wed Aug 19, 2015 10:13 am
by HoustonMule
Question: Are the heads I see on Ebay 2A042 style? What needs to be done to them if you are installing them on an A042 engine? Are there any issues using the 2A042 cylinder jugs and pistons on A042 engine?

Thanks

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Wed Aug 19, 2015 3:03 pm
by muleman7
All of the heads listed lately on eBay are 2AO42 versions. They can be identified as 2AO42 if the valve stems are flush with the top of the Keeper/Rotator.
These heads may be used on the Mule engine but do not use the copper head spacer. The copper head spacer is only used when the longer stem AO42 valves are installed.
The cylinders and pistons are the same for either engine only the rings are different. The Mule engine uses 2 chrome compression rings and a 4 piece oil control ring. The 2AO42 rings use a chrome top compression and a cast 2nd compression with a 2 piece oil control ring. Either set of rings will work.
My preference is using both compression rings chrome.
I built modified mules for the Chessie System that were used in their Classification Yard in Cincinnati, Ohio 24 hours a day and 7 days a week. The first engines I built used Primeline rings with both compression rings cast iron. When I received them back for rebuilding the rings were worn severely and sometimes were in pieces when the piston was removed. The cylinders were worn and unable to be used without boring. I switched to chrome rings only and used the 4 piece oil control ring. The engines sent back allowed me to flex hone the cylinders and reuse them.
Use the eBay heads and if the valve stem is flush with the keeper/rotator then do not install the copper head spacer they will give you great service.
Use the cylinders and pistons and remember to stagger the ring end gaps.
One more thing; the pistons and cylinders are stamped with either A, B, C or D. the piston is stamped on the top and the cylinder is stamped on the top fin. You should match A piston with A cylinder. You do not have to match side for side. This is going to be extremely difficult because the sellers do not send matched sets. Try to use a lower letter piston with a higher letter cylinder. If I remember correctly they step up in 0.0005 increments. The D piston used with an A cylinder may cause scuffing and seizure because the skirt clearance may be less than required [Mfg new 0.0018-0.0033 maximum permissible 0.0053] per DMWR 5-2805-259 Jun 1968. Check your piston skirt clearance with a feeler gauge.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Thu Aug 20, 2015 9:01 am
by Auto Shop teacher
Hi Muleman, where do you get your Chrome rings for your engines? Is there a civilian engine that uses the same size rings? Any break in problems with the chrome rings?

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Mon Aug 24, 2015 2:06 pm
by muleman7
I bought Perfect Circle rings in bulk and I will go through my notes to check on the commercial application because these were shelf items.

I had no break-in issues using chrome rings after flex honing the cylinders that ended up within tolerance for piston skirt clearance. I washed the cylinders in hot soapy water and immediately oiled them to prevent flash rusting.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Mon Aug 24, 2015 5:19 pm
by Gunslinger1$
This great info!!!! thanks
Is there any where you can buy the brass head spacers?

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Tue Aug 25, 2015 1:02 pm
by HoustonMule
So what is used between head and jug for sealing when not using the copper spacer?

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Tue Aug 25, 2015 2:18 pm
by muleman7
I am checking my suppliers, Emery Mule Supply has a few takeouts. I have not found any NOS copper Head Spacers but will post when I do.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Tue Aug 25, 2015 2:24 pm
by muleman7
The 2AO42 allows the cylinder ridge to be in direct contact with the head and this provides an effective seal. The Copper Head Spacers main function for the AO42 was to reduce the compression ratio not used primarily as a seal.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Fri Sep 11, 2015 7:06 am
by 13bravo
The difference in the valve overall length would be so you didn't have to use different length pushrods then. If you were to install an A042 head on an engine without the copper head ring gasket you would most likely hold the valves open.

Another possible difference between the HP ratings could also be due to the different restrictions in the exhaust and intake systems used. They use different carbs and the airflow might actually be different.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Fri Sep 11, 2015 7:32 pm
by Chuck W.
The difference in the valve overall length would be so you didn't have to use different length pushrods then
Keep in mind that the engine uses hydraulic lifters, so it would not make a difference on the pushrod length.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Fri Sep 18, 2015 4:12 pm
by 13bravo
Chuck W. wrote:
The difference in the valve overall length would be so you didn't have to use different length pushrods then
Keep in mind that the engine uses hydraulic lifters, so it would not make a difference on the pushrod length.
Depends on the lifter though. If too long a rod is used without being able adjust the rockers you can bottom the lifter out and hold the valve open. The .065 thickness can sometimes be too much for the lifter to compensate.
I don't have any experience with these engines but have lots of experience with automotive engines in general.
By the way I believe the lifters on these are the same as one of GM's engines.

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Mon Sep 21, 2015 9:58 am
by HoustonMule
13Bravo is correct on hydraulic lifter preload especially with non-adjustable rockers. One more question regarding matching Letters on pistons/cylinders:
I contacted several suppliers about matching the sets .... Replies to request "NO".
Is there no indication on the NOS P/N stickers of package which indicates letter size? If not how did Military match the two together?

Thanks

Re: AO42 Mule vs 2AO42 Power Plant Engine Differences

Posted: Fri Sep 25, 2015 9:48 pm
by muleman7
HoustonMule wrote:Is there no indication on the NOS P/N stickers of package which indicates letter size? If not how did Military match the two together?
As I recall there is no indication on the NOS P/N sticker indicating Letter designation and I do not know how the depot maintenance people would match without unpacking. They could check the skirt clearance and if within limits a D piston will work in an A cylinder. I will check my stock and find a D Piston and check it against an A cylinder then post the results.