Dexta Main bearing block bores

This forum is about the Fordson Dexta, Super Dexta and Petrol Dexta.
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1962 model
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Dexta Main bearing block bores

Post by 1962 model »

Hi,
I haven't been on the forum for a while, I am about to rebuild my 1958 Dexta engine, and I need the block main bearing bore specification, as it is hard to find, can anyone please help? IT is not the crankshaft journal size I am looking for.

I need to know what the main bearing bore size is in the engine block ( 1958 model ) as the old bearings have evidence of polishing on the mating surfaces in the main caps. Extensive experience is that almost all engines of all brands stretch main bearing caps.

This is a problem that most engine manufacturers have, it does not come about from wear and tear. This occurs for a couple of reasons;
1. The cast iron is not aged on original engine manufacture and;
2. the crush on the bearing shell together to repeated engine warm- ups over a couple of years is enough to cause the caps to stretch, by a minimum of .001.
Working the engine just adds to the problem, it does not cause it.

Now this .001 stretch does not sound like much, and one would be forgiven for thinking that an extra .001 on bearing clearance would still be within specifications, well it is not.

The .001 stretch in the main bearing caps because of 3 times diametrical effects results in about .003 extra bearing clearance, and when added to the working clearance, is either on the limit for a worn engine, or outside most normal engine bearing clearances.

The Dexta Main bearing clearance should be between .0025 and .0045 so if an engine crankshaft and bearings are in perfect condition and intended to have .0025 main bearing clearance ( which is what an unworn engine should have ), and we then add .003 for the main bearing cap stretch not putting the correct crush on the bearing shell, then we get .0025 plus .003 this equals a main bearing clearance of 0055, which is a bad clearance for a rebuilt engine that should be expected to last another 60 years at least, if it had the correct bearing clearance to start with on the rebuild.

No engine that I have measured in 50 years has had the correct main bearing bore sizes, all had stretched!

Can anyone please tell me what the main bearing bores are meant to be when they are the correct size?

Mike.

Billy26F5
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Re: Dexta Main bearing block bores

Post by Billy26F5 »

The manual unfortunately doesn't cover that, but if we add the thickness of the bearing and the clearance it shotld come to roughly to 2.6465" using the thickest parts and minimum clearance in the specs. I've never heard it for Dexta's but Major's could have the main bearings to outside +0.015 O/S, but the whole block had to be line bored to the new size. This is not now an option as there are no new bearings like that available. In order to do that you would need to make your bearings to the new specs.
Sandy
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1962 model
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Posts: 170
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Re: Dexta Main bearing block bores

Post by 1962 model »

Thank you,
I will give it a measure.

I also have a Power Major with oversize main bearings, which I need to rebuild, fortunately the crank and bearings are in reasonable condition, and the previous owner gave me the rest of the 6cyl bearing set that he had not used on the rebuild in the 1980's.

Billy26F5
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Re: Dexta Main bearing block bores

Post by Billy26F5 »

That's very lucky. I don't know if there might be some manufacturer making O/S bearings for 6 cylinder engines perhaps aimed at Thames Trader's.
Sandy
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1962 model
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Posts: 170
Joined: Wed Feb 02, 2011 12:01 pm
Location: West Australia

Re: Dexta Main bearing block bores

Post by 1962 model »

The previous owner is still furious ( he is in his 80's now ) that he had to fit O.S. O.D bearings and had to get the crank reground when the crank grind was not necessary. He was forced to do all this as the caps were stretched ( as all Majors are ) and he could not get standard size O.S.O.D bearings.

I have been having main bearing tunnels bored back to the correct size for years, because as a Machinist by trade I understand what the clearances are supposed to be.

Agriline or another supplier should be supplying Over Size Outside Bearings by rights because almost all 6cyl engines should have their main bearing bores machined out. This is supply and demand based of course and most people will not even realize the main bearings on their 6 cyl engines are completely shot!

Engine Machine shops which have the right gear can actually line bore the caps/block without making the bearings O.S. O.D. they simply grind the caps then bore them back to round with a mandrel bar aligned with the front and rear bore. ( fixes block bend and caps stretch in one go )

The 6 cylinder engines are a disaster in regards to main bearings because the problems of the 4cyl are accentuated considerably because of the greater block length.
This comes about because ( like all engine blocks ) they have more cast iron on one side so must shrink more on one side when cooling from the block casting Moulds, I have not made a study of it but I would say the blocks likely all bend in the one direction.
The bend that results from usage of a block which is still settling during service must be about .015 or .020 as the last 6cyl block I had the mains bored on still had witness marks on one side in the middle main, and on the opposite sides on either end mains. I can see why they made O.S.O.D. mains for the 6cyls.
The 6cyl cranks wear terribly because of three things ( one more than the Major tractors )
1. water at times leaks into the sump. 2. diesel at times leaks into the sump, 3. the block bend puts side pressure on the crankshaft and can even bend it.
There are up to three reasons diesel leaks into the sump and that is assuming someone had done the job right to Ford specifications.
The Ford Specifications for the leak off pipe are definitely wrong other manufactures fitting sims injectors have a better system for the leak off return pipes that do not damage the engine.
The banjo fittings fitted to the tops of the injectors in other models have single use copper washers on both sides of the banjos, this prevents diesel leakage, other manufacturers have these on the outside of the rocker cover so leakage is easily seen, Ford just has this leakage leak into the sump thin the oil and wear the bearings.
Things don't end there either as the lead solder joints often crack on the Fordson and drop Diesel into the sump.
In turn this lead cracking can come from tightening and loosening the injector pipe nuts, I now try to loosen on the injection pump when I have to bleed an injector line.

As you can see the cranks suffer terribly so never buy a 6cyl engine assuming the bottom end will be usable without machining, and don't think you can simply slip in a reground crank and new bearings, even with stretched caps there is not enough clearance for the crank, I know I tried it, the crank just locked up!

Mike.

Billy26F5
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Re: Dexta Main bearing block bores

Post by Billy26F5 »

That's some very good points you've made Mike, and yes I was intending to inspect a 6 cylinder engine if we do end up getting one. I would add another reason for problems with 6 cylinder cranks: they've been run at much higher speed (2000+rpm instead of 1600), thus making all the already mentioned problems bigger and more serious in less time. Super Billy's con rods were given such treatment before fitting, so they should be fine, we'll see what the rest does when we get him running again.
Sandy
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1962 model
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Posts: 170
Joined: Wed Feb 02, 2011 12:01 pm
Location: West Australia

Re: Dexta Main bearing block bores

Post by 1962 model »

Hi Billy26F5,
I have eventually measured the main bearings, and they are out of round, and outside specifications as I suspected, and most likely not aligned with each other either.
I phoned the engineers who usually do my main bearing bores and the machining tolerance is 2.9165 to 2.9175 my number one main bore measures 2.915 to 2.917 which is unusual to be undersized I wasn't expecting that.
The rest of the mains measured up between 2.914 and 2.9178.
This means I will be getting the main bores machined out for sure!

Billy26F5
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Re: Dexta Main bearing block bores

Post by Billy26F5 »

Hope you manage to get a good finish, I'm keen to see how it all ends up as if we ever have engine trouble again I'm keen to have a go at it too.
Sandy
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1962 model
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Posts: 170
Joined: Wed Feb 02, 2011 12:01 pm
Location: West Australia

Re: Dexta Main bearing block bores

Post by 1962 model »

I did get my engine finished in my 1958 Dexta, I had it at a local show yesterday and was complemented on it.
I had it fitted with my Ford back blade and leveled the ground between each tractor pull participant, I was complemented on a good job there too.
I am considering doing some firebreaks with a TPL offset discs to help settle in the engine more, as blade work doesn't work it very hard.

The front gearbox shaft spigot is worn, as normal from incorrect pilot bearing fitted, which means I will have to rebuild the gearbox some time.
The gearbox is a 1959 casting and has a dual stage clutch, so I need to be careful which ratio it is.
I have another 1958 gearbox housing and hope to rebuild it and swap over in future, unfortunately that housing had water in it and was retrofitted with later gears.
I would like to know when they started to fit dual stage clutches, I assume it could be from the beginning of 1958 production as the housings are the same and the Major fitted them from mid 1957.

I don't know how to post pictures as there seems to be no browser box to add it, and I don't have a photo bucket account, tractors I understand, computers are just a pain that take me away from tractor work.

1962 model

Billy26F5
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Posts: 1912
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Re: Dexta Main bearing block bores

Post by Billy26F5 »

For pics try this viewtopic.php?f=1&t=5024&p=53955#p53955 I'm not great on computer stuff either and it works fine for me. Glad your engine is doing nicely.
Live clutches were there from the very beginning, so there are very early tractors with them. I'm almost certain you'll have the early ratio, the new ratio appeared on 957E-63953 in 1960. There is a service letter covering rebuilding an early gearbox with later gears, but then the tachometer information will be out for certain gears depending on what you need to fix. The bell housing changed in 59 with a longer starter apperture I think for the petrol engine, unfortunately I can't find where the exact detail was at the moment (might be elsewhere on the forum).
The Major live clutch appeared on 1417988 in February 57.
Sandy
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