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OT - How was this engine's main cap machined?

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
Another oddity...the upper main cap from a 1961 Plymouth aluminum block Slant Six.

Most engines back then had a lower cap only, but these sat above the crank and let you avoid having to try to directly seat the bearing shell in an aluminum block.

In any case, a guy wants me to consider making him some out of steel (these are also steel as best I can tell).

I can figure it all out easily except for the top OD 'curve'. It's a machined finish made to a tight tolerance, just like the ID. The ID I can cut in a lathe. But the top's shoulder prevent me from making it in a lathe...so how did they make this in 1961? I've looked for tooling marks and there are very few. The few I do see suggest they used a shaper but I'm not sure on that. Could a shaper cut this profile? I'd guess they'd need to have some sort of holding fixture that allowed the cap to be clocked around a little bit per pass?



Slant Six Main caps.jpg
 
Could they have done it with one form tool on a big shaper and slowly lower it down to cut more and more as it goes. I mean stroke parallel to the crank axis.
Bill D
 
Why?

They may have been roll formed to general shape, other stuff done as a long length then cut in slices and finished.

They would likely need to be line bored in the engine to be correct.

No junk engines available?

Slant 6 was a good motor in its day but...

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Given the sheer number of those slant 6's made I would guess some one and done process. I have seen them use giant broaches on things like that.

FZ2_354.jpg

Also a large custom form cutter on a horizontal mill would also be a guess.

IMG_20150517_161959_115.jpg
IMG_20150517_162013_178.jpg
 
Given the sheer number of those slant 6's made I would guess some one and done process.

They made less than 50,000 aluminum versions. They had a lot of issues with them and today very few survived. This piece is specific to the aluminum version.
 
Regardless of how the OEM cap was made- probably with a surface broach, what aligns the cap in the crankcase?
Looks like possibly the dowel sleeves and the flat surfaces at both ends? Profile the shape and then clean out the radius by tipping them upright. Don't even try to match the bore size- you will have to leave them undersize and line bore them after assembling and torqueing them in place in the crankcase.
 
They can be built, the machining of the cap is doable, but it would be a little interesting as the block would needed to be line bored after the replacement caps are installed. You still would have to maintain correct cap wall thickness/profile so as to hold correct crank shaft center line. The two opposed top and bottom bearing inert tang groves would not be too hard, but time consuming. This project can get costley fast.
How did the factory build them, easy. I have a 1940 Ford machine shop manual displaying the custom Milling machines built for just one production line part.
 
Another oddity...the upper main cap from a 1961 Plymouth aluminum block Slant Six.

Most engines back then had a lower cap only, but these sat above the crank and let you avoid having to try to directly seat the bearing shell in an aluminum block.

In any case, a guy wants me to consider making him some out of steel (these are also steel as best I can tell).

I can figure it all out easily except for the top OD 'curve'. It's a machined finish made to a tight tolerance, just like the ID. The ID I can cut in a lathe. But the top's shoulder prevent me from making it in a lathe...so how did they make this in 1961? I've looked for tooling marks and there are very few. The few I do see suggest they used a shaper but I'm not sure on that. Could a shaper cut this profile? I'd guess they'd need to have some sort of holding fixture that allowed the cap to be clocked around a little bit per pass?



View attachment 286227

Old friend, that engine. Iron head, regardless of the block. Four mains and serious ugly but very strong crank, and yet.. it turned out to be ruggeder by easily double the run-and-forget mileage than a similar concept BMW slant six with SEVEN mains and easily twice the machine-tool time pumped into it.

One of ours was run (by me late Mum, pissed-off at Dad during the divorce before they remarried each other) in SERIOUS anger... bone-dry ...until the paint burnt clear off it, finally quit when the insulation melted off the underhood wires and they shorted-out.

A cousin bought the remains from the inch-hoorance cumpnee, put an ignition and wiring harness on it, a head gasket (it was JUST shy of needing resurfaced), and drove it for many more years.

The BMW six? Aluminium head casting. Or so they THOUGHT it was!

I replaced three cracked heads on my 520 carburreted and had shed the earlier 3.0 "Bavaria" full of Barr's leaks. I'll be able to trail-ski across the Atlantic on the ice before I buy another Jerry-built POS.

AFAIK, MOPAR had that item yer lookin' at investment cast to damned-near finished size. Light cut on a specialize machine did the rest - and FAST. MOPAR and their contract suppliers did s**t like that all the time to save half a penny multiplied by a large count of units.

There would not be a lot of tool marks.
 
Yea...they built these only for a couple years so today they are scarce. Even in the early 1980's, through all of my salvage/junkyard days, I only saw one (1) ever go through the scrap pile. And by the time I saw it, the forklift had ensured it was dead for good.

Yes, the Slant Six was reliable but I have talked to a number of people who tell me if you ran one without oil for more than a couple months, you could start to have problems. lol. Actually, my friend worked as a mechanic for a Cincinnati cab company in the early 1970's and he said they would run the Sixes to 450,000 miles, re-ring them, then run another 450,000 miles before selling them off. Never went into the rest of the engine.

Did investment casting exist in 1959? That's when these were birthed.

I agree, line boring will be required in the ID but on the OD, the fitment gets trickier.

The caps fit the block in the side-to-side direction by means of machined pads on either side, just like most other engines. It's a tight fit.

He's going to bring me the block, too, so I'll be able to look at it and maybe learn more.
 
You can see the corners have that sort of 'forging flash' look to them so they were not machined from a solid 'billet'.

As for locating, these also used a thin sleeve as shown on one side. The other side is left unsleeved to allow oil to flow down along the bolt then crosses through the drilled hole into the crank journal bearing.

Slant cap 4.jpg
 
I will agree with the others that say surface broach. Until you have seen one, you can't believe the speed. Full contact face for a cylinder head from raw casting is about 20-30 seconds machining, about 50 second total cycle time. Shapes? No problem. Ground into the teeth, inset into the broach.
 
Did investment casting exist in 1959? That's when these were birthed.

1959? Surely you jest?

:)

Oldest use of investment casting predates most written languages.

Wax or vegetable fiber - shit can work - mud or plaster, molten metal.

All we added, modern times, was the rubber mold instead of hand-carved wax, and there's most of the joolry industry and a significant part of firearms.

Use for modern STEELS of complex shape predates the First World War.

See Springfield rifle receivers, model of 1903A3 in the USA,
Older yet, Basque country France and Spain.

Basque - and Anatolians, may have been working metals even before Egypt. Or before Sumer had their little water problem.
 
So the question becomes....how would I machine this today, given my lack of a surface broach? The rotary table idea would work 'OK' but I'm not sure of the surface finish, and the radius of the mill would also create some issues at each end of the cut.
 
Willing to bet they used a form tool in a horizontal, probably rough and finish processes.

I think the real question is how you do it now, as high volume processes using custom machines were common back then well, still are


question is, how will you do it.

Interpolated in a VMC unless you know a tool grinder I guess.
 
So the question becomes....how would I machine this today, given my lack of a surface broach? The rotary table idea would work 'OK' but I'm not sure of the surface finish, and the radius of the mill would also create some issues at each end of the cut.

"Not at all" comes to mind?

I mean.. I could do the SHAPE on the H.P. Preis panto engraver.. but it would take all damned year to walk the tiny air spindle down the depth of it in steel.

Good as they were for their time, the motors aren't worth it NOW. Miles aside, too much TIME has passed. Notorious for long-term internal corrosion, shiney-wood to cast-in Iron cylinder liners. How d'yah fix THAT?

Yer friend-of-a friend should get an all-Iron one if it is to be more than a decorator. Otherwise the cost is going to go clear out of sight.

Or sub-in a modern motor. Some OTHER good ones have come along since.

"Period correct" even the venerable 292/318/340/360 V-8 were options on some of those MOPAR cheapsheets. We owned several Valiants, Darts,"Lancers", etc.

One white one even came with a toilet seat on its ass...thot it was an "American Standard" loo!

But it had no damned PLUMBING under it!

Go figure yah could trust an automaker's bullshit sales pitch ever again!

:(
 
I have seen them use giant broaches on things like that.

Just so

Likely horizontal with dozens of broach sections. P&WA had about a 40 foot long La Point when I was there sixties

A single pass gives you maybe 100 main caps
 
I think this is how I would approach it today

Drill and c'bore holes so I can bolt the caps to a fixture, then later finish drill them

Rough the ID for later line boring

interpolate the od with a decent sized end mill and finish with a smaller to get rid of most of the radius.

Rotate the part so you can clean the tool radius with a regular end mill.

MAybe draw that last bit in CAD and check to see if a tapered end mill might approximate the very beginning of the curve

All the fight here being to not have to modify the block[corner where the radius meets the bolting surface] to use the new caps, although it would be much easier to hit the block with a grinder, it aint your block

When my untrained eye looks at the part and its application, I think even a significant corner relief would not structurally matter, but again, aint your engine
 
So the question becomes....how would I machine this today, given my lack of a surface broach? The rotary table idea would work 'OK' but I'm not sure of the surface finish, and the radius of the mill would also create some issues at each end of the cut.

Use a small cutter (1/8" ?) to finish it...
 
All food for thought, thanks.

For the record....as I understand, the real issue with the aluminum blocks in 1960 was the lack of aluminum casting knowledge. The casting processes, and alloying, weren't good enough to produce the quality needed. So...as a manufacturer looking to sell millions of cars, they went with the 'sure bet' of cast iron. But you have to give them credit for trying, and for pushing the envelope a bit further. What was learned in 1960, you can be sure, helped later generations of engines come to be.

There is a moderate demand for the remaining aluminum Slant blocks. Like a lot of hobbies, no one 'needs' one but it has a cool factor which is the allure. I have one car running with a Slant (iron) engine and I can tell you when you drive down the road at 60MPH all of the past 50+ years of engine development melts away....the most hi-tech engine out there wouldn't do a better job.
 








 
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