Author Topic: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale  (Read 1767 times)

Offline eccentric

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I am starting a new build of the Offenhauser 97 Cubic Inch "Might Midget" Model Engine.


The midget motorsports craze of the 1930s and 1940s not only served as a launch pad for many drivers who'd go on to successful racing careers, it also ended up keeping afloat one of the greatest companies in American motorsports history via the Offenhauser midget engine.

Not a year after Fred Offenhauser took over the operations of his former employer, engine builder Harry A. Miller -- which consisted mainly of casting, machining, and selling racing parts, and rebuilding truck engines -- a new business opportunity arose in the summer of 1934. Midget racing had taken the country by storm, offering cheap thrills for spectators and drivers alike in the depths of the Depression. Organizers soon capitalized on the new motorsport's popularity by building dedicated tracks around the country.

One of those promoters, Los Angeles-based Earl Gilmore, grew dismayed with the frequent breakdowns and the unruly nature of the miscellaneous motorcycle, junkyard, and cut-down engines that powered the cars racing at his stadium, so he turned to Offenhauser for help.

"(The unreliable engines) made it difficult to run a show," Gordon Eliot White wrote in Offenhauser: The Legendary Racing Engine and the Men Who Built It. "His patience exhausted, Gilmore sent his manager, David Koetzla, to see Fred Offenhauser about building a real racing engine for the little cars."

Offenhauser didn't have anything on hand at the moment, but he and Leo Goossen, the longtime draftsman for Miller's creations and their successors, pulled up the plans for the 183-cu.in. straight-eight that Miller built for Harry Hartz's 1932 Indianapolis 500-winning entry and decided to cut it in half to make a 97-cu.in. four-cylinder. As White described the engine's construction:
The 183 was, as Millers went, relatively simple, that is, inexpensive. It had two valves per cylinder and was unsupercharged. Using half of the 183's crankshaft left the midget engine with only three main bearings but it seemed to work alright."

In addition, Miller had designed the 183 as essentially two four-cylinder engines sharing a common crankcase so, White conjectured, "Offenhauser could use 183 blocks already on hand, or at least casting patterns for the Hartz engine."

With not much turnaround time, Offenhauser had the first midget engine ready in time for Curly Wetteroth to place it in his midget chassis and subsequently hand the completed car off to Curly Mills for its debut in late September 1934. Mills not only won that race, he also reportedly won his next 16 races.

Though the five total engines he built that first year seems like small potatoes, he charged about $1,100 per engine, roughly the equivalent of $20,000 today. "Fred had a healthy profit margin on them, and with the help of those midget sales, the firm cleared $18,000 for the year," White wrote. "They kept him in business."

Perhaps just as importantly, the midget engine sales -- White counted at least 180 during the time that Offenhauser ran the company -- allowed Offenhauser to develop the larger engines that would go on to dominate Indy and many other forms of motorsport for decades to come.

When Offenhauser decided to retire shortly after the end of World War II, Louis Meyer and Dale Drake bought out his business in 1946 and continued offering the midget engine until about 1974. While it didn't sell in great numbers -- White recorded serial numbers up to 450 or so -- it remained popular and powerful enough to warrant continued development through the decades. Meyer and Drake, in fact, sold most of their midget engines as 102-cu.in. variants and even offered the engine in displacements as large as 111 cubic inches.
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I have created a 3D Model of the early 97-cu.in. version and am working on a set of plans for this historic engine.


I am building what is really a prototype of a never before built model. I am starting as is tradition with the crankcase. The crankcase is split into two halves held together with 4-40 screws hidden behind the crankcase side covers.
I started by squaring up the two work pieces in the mill, then moved them to the CNC router to machine the insides. I also machined what I am calling a "dummy crankshaft". It will allow me to check the bores in the crankcase. The crankshaft is supported at the ends by a pair of ball bearings and in the center by a bronze bush. Its fabrication was a straight forward set of operations on the lathe. I slowly brought the ball bearing surfaces to dimension to insure a tight fit. The front end of the crankshaft will hold the timing pinion and a starter dog. The rear end of the crankshaft will hold the flywheel.


Below is a photo of the top and bottom crankcase halves with the internal machining complete.


The crankcase is about 4 1/2 inches long and 2 inches wide. If you look closely you can see that the top left and bottom right screw holes have a locating hollow pin surounding the mounting screw that perfectly aligns the two crankcase halves. All of the machining on the inside, including the ball bearing holder surfaces and the locating pins, were performed in one setup. The machining time for the crankcase top was 1 hour and 32 minutes while the machining time for the bottom half was just under 2 hours.


When I machined the bottom crankcase half, I took an extra .010" off the top surface to allow for a Teflon (PTFE sheet) gasket. I then assembled the two halves with this gasket material in place. The rest of the crankcase outside machining will be performed with the two halves screwed together like this.



I plan to machine the front and rear of the crankcase next so I can test fit the dummy crankshaft. I may leave the machining of the sides and bottom of the crankcase for later as it will be easier to work with as a solid block.

More information can be found here:
https://gregsmachineshop.com/offy-build-draft/offy-build-introduction/

The process of developing the plans can be found here:
https://gregsmachineshop.com/offenhaser-model-engine-plans-development-home/offenhaser-model-engine-plans-development-intro/
« Last Edit: February 20, 2022, 08:23:26 PM by eccentric »

Offline steamer

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Re: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale
« Reply #1 on: February 20, 2022, 08:32:47 PM »
This will be amazing!!    Watching along!

 :popcorn:
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Damned ijjit!

Online Vixen

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Re: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale
« Reply #2 on: February 20, 2022, 09:37:52 PM »
This looks like it will make into a fine engine. I'm following.

Mike
It is the journey that matters, not the destination

Oh! sod the journey, lets hit the bar and pool instead.

Offline 90LX_Notch

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Re: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale
« Reply #3 on: February 20, 2022, 10:41:45 PM »
What happened with the GDB4? 

-Bob
Proud Member of MEM

My Engine Videos on YouTube-
http://www.youtube.com/user/Notch90usa/videos

Offline eccentric

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Re: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale
« Reply #4 on: February 20, 2022, 11:13:40 PM »
Bob,
Still plugging away :-)

Offline RReid

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Re: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale
« Reply #5 on: February 21, 2022, 12:23:29 AM »
Great project. I'll be following along with interest as well.
Regards,
Ron

Offline Roger B

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Re: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale
« Reply #6 on: February 21, 2022, 07:33:29 AM »
Looks to be an interesting project   :ThumbsUp:  :ThumbsUp: I will be following  :wine1:
Best regards

Roger

Offline eccentric

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Re: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale
« Reply #7 on: February 24, 2022, 04:56:47 PM »
Machining the Front of the Crankcase

Machining the features at the front of the crankcase requires precision because three gears and their bearings are mounted here. For the gears to mesh correctly they need to be precisely spaced from each other. The position of the front crankshaft bearing holder has already been machined and all of the features on the front need to be precisely placed with respect to it.
Below is an image of the front of the crankcase and it can be seen that there is a lot going on there.



Before starting the machining I go to the surface plate and carefully characterized all of the dimension of the assembled crankcase, using the center crankshaft hole as my master datum. I created a detailed sketch with the dimensions of the actual part, the actual size of the crankshaft bearing holder hole and its relation to all sides of the part.
Then the crankcase is mounted in the vise vertically and squared to the axis of the CNC router. I spent an afternoon checking and rechecking the alignment and touching off the part aligning it to all of the axis of the CNC router. I used the dimensioned sketch to check the alignment several ways using them to double check each other. Then I slept on it.
The next morning I rechecked the centering of the crankcase in the CNC vise, then ran the set of programs machining the front. The machining on the front took 25 minutes and the machining of the holes took another 8.



Below is a picture of the CAD model and the resulting machining of the front of the crankcase. One of the small bearing has been test fit. There is a second bearing not installed equidistant below the crankshaft.



Then I machined the two crankshaft main bearing holders, one is mounted on the front of the crankcase and another on the rear. Below I am test fitting the crankshaft ball bearing for a nice snug fit.



Then I turn the features on the outside of the bearing holder and test fit it into the crankcase, again to a nice snug fit.



Below are a couple of pictures with the dummy crankshaft installed in the engine with the two main bearing holders supporting it.




I heave a sigh of relief. The most critical features of the crankcase have been completed and I am satisfied with the precision of alignment on the different sides.

Offline eccentric

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Front Cover

The front cover seals the front of the crankcase and houses the oil pump and three gears, the crankshaft pinion, the oil pump gear and the valve timing drive gear. The features in the front cover hold two ball bearings for the gears need to precisely align with their partner bearings on the front of the crankcase.



Machining the front cover has several challenges, the foremost is aligning features on both the front and back sides. A secondary challenge is the tight quarters and fine details on the front face of the cover. The mating surface of the crankcase was machined in one setup, so the bearing holding features and the mounting holes for the front cover are well aligned. I will do the same on the front cover, machine the bearing features and the mounting holes in the back in one set up. Then I will load a piece of fixture stock in the mill, and machine holes matching the eight mounting screw holes in the front cover. I will then screw the front cover front side up with four of the screws, and machine as much as I can. Then I will put in the other four screw, remove the first set of four screws, and machine the balance of the front cover.

Below, the inside of the front cover is being machined. This is a straight forward set of operations because they are essentially a copy of the set performed on the front of the crankcase.



Below is an image of the finished inside machining.



I then use a band saw to remove most of the material on the front side of the cover, following up on the mill to provide an accurate surface to start with on the CNC router.



A fixture block is loaded into the CNC router vise. I perform a finishing operation on the top to flatten it and provide a known surface with respect to the CNC Z axis. Then the six mounting holes are machined and tapped. One of the holes was used as the X and Y axis zero set points and the top surface becomes the Z axis zero set point.



Below the roughing pass begins



I used a 1/4 flat end mill for initial roughing and machining of the horizontal flat surfaces. Then I used a 3/16ths ball end mill to machine the curved outer surfaces, and finally a 1/8 inch ball end mill to create the fillets around all of the features. I either used a dull 1/8th inch ball end mill, or did not properly match the spindle speed with the surface speed, but I was disappointed in the finish of the radius operation. Oh, and I hit the screw holes with a 3/16th flat end mill to create the counter sinks.



Below is the front cover mounted on the front of the engine.




The engine is smaller than the pictures let on.....

Offline Art K

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Making very good progress! I've got Ron's book and have most of the engine drawn in Alibre. But that's as far as I've gotten. To many projects not enough time.
Art
"The beautiful thing about learning is that no one can take it away from you" B.B. King

Offline Roger B

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Getting the magneto shelf there looks a lot of effort but it paid off  :ThumbsUp:  :ThumbsUp:  :wine1: I assume the original part was a casting.
Best regards

Roger

Offline eccentric

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Cylinder Block

The cylinder block forms the water jacket around the cylinder sleeves. It is a block of aluminum with most of the material machined away, then a lot of holes are drilled in addition to the bores for the cylinder sleeves. There are two side covers that mount to the sides of the block sealing the water jacket.

The block is shown in blue in the image below:



It was going to take two hours on the CNC Router to machine out all of the aluminum from the inside of the block, so I decided to use a drill bit and an end mill on the manual mill. I started by drilling 1/4″ pilot holes, then followed up with a 7/16th drill bit to remove the majority of the material. Then an end mill squared up the pocket. Finally I hit the corners on both sides with a 1/16th end mill for the small radii needed there to clear some screw heads.



On the CNC router, I machined as much as I could from the top of the block including the cylinder sleeve bores, then drilled the few holes on the bottom of the block by hand. I made the block with an extra .010″ of material on one side for some reason, and then touched off the bottom holes and the top holes on different sides of the block by accident. So the holes on the bottom are offset to one side by .010″ Fortunately I have not drilled the matching holes in the crankcase and can offset them by the same amount and no harm no foul.

An interesting feature of the Offy block is the taper in the sides. Instead of machining a custom fixture I simply clamped the block in the mill vise with a spacer so I could machine off the required .065″ from the bottom of the block and taper it to 0.0″ at the top. I used an indicator to insure my clamping was correct.



Then I ran an end mill around the edge.



Below the block sits on the crankcase in its intended position:



The block weighs just a wiff of what the squared up work piece did. I still have to drill and tap 64 holes for the 0-80 screws that secure the side covers. I will machine and drill the side covers, then match drill the holes in the block to insure good alignment. The side covers need to be flush with all sides of the block.

Offline RReid

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Quote
An interesting feature of the Offy block is the taper in the sides.
Does that taper in the sides have any function?
Regards,
Ron

Offline eccentric

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Ron,

The taper in the sides of the block, and the tapers that are a featue of the crankcase, really just boild down to making the engine as light as possible.  The width of the head dictates the width of the block at the top, but the width at the bottom can be less, so the material was removed.  This is a racing engine and every pound saved is important.  Of course, since these were castings in the real engine, saving weight also saved money.

Offline eccentric

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Offy Cylinder Head
The cylinder head is one of the most complex parts in the engine.  It has coolant passages, oil passages, not to mention the intake and exhaust ports.  The camboxes mount to the head and so its dimensions dictate how well the timing gears mesh.



I start with machining the internal features including the coolant passages and the coolant cavities.  The coolant passages connect the coolant water pipe flanges with the internal coolant cavities.  The coolant passages are drilled the long way through the head, they are .150″ in diameter, so drilling the 2″ from both ends to meet in the middle is relatively easy.



The coolant cavities are machined into the head with a 1/4″ roughing end mill.



Three small matching cavities covers are made from 1/8″ aluminum sheet.



The caps are secured in place with high temperature structural adhesive.


I set the adhesive aside to cure for a day and then fly cut the head top to final dimension.


I am happy with the way the sealed coolant cavities turned out. No one will know about them but us.


I drill the four spark plug holes.


Then I begin machining the bottom of the head.


Once the conical combustion changers are machined on the CNC router, it is back to the manual mill to spot drill, drill and tap the holes on the bottom side of the head.


The holes match up nicely with the mating ones in the cylinder block.


Then the head is flipped on its side and the intake and exhaust ports are drilled.


Then back to the CNC Router to machine the features on the top of the head, the water flanges and spark plug wells.


Below are a couple of pictures of the squared head with the features machined on all sides.


Can you see where the internal coolant cavities were sealed with the coolant covers?


But the fun has only just begun.  I need to machine four facets into the head for the camboxes and the intake/exhaust manifolds.  Before I can move forward on these I need to machine some fixtures to hold the head so I can machine off the corners.  Remember, it is supposed to look like this:


Stay tuned for part 2.