Offenhauser Mighty Midget Racing engine

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Cambox Caps

To make the cambox cap, start by imitating the shape of the top of the gear tower.

There is a radius on the front and back of the caps, but they are different. The rear of the cap mounts flush with the cambox cover.

The front however, has a raised lip and the edge has a larger radius.

There is a flange at the base of the cap and the stud mounting features that align with the holes in the gear tower. bolts mount through these holes securing the caps into place.


I worked on finishing up the gear tower sub assembly with the bearings, shafts and gears in place.



Above are a couple of images of the engine in its current state.
Oil Delivery System

Today I worked on the oil delivery system, including the oil pump and the oil galleries required to deliver oil to top end (Camshafts) and to the crankshaft middle bearing. The engine will have a wet sump oil system, that is oil will be stored in the sump of the engine and then pumped from there to where it is needed. The original engine was of a dry sump design with an external oil reservoir. A dry sump in a model engine is more complicated to implement; it would require two oil pumps, one to return oil to the reservoir and another to deliver oil to the engine bearing surfaces that need it.

Since I have chosen to use a wet sump I will provide an oil splash guard in the sump that separates the rotating crankshaft from the oil sump. The engine has two breathers on the side of the crankcase, I will use one of the breather top caps as a dip stick/oil-fill and the other as an operational breather.

The image below shows the flow of the oil from the sump, through the oil pump, up through a gallery in the crankcase bottom, through a gallery in the crankcase top. At this point the oil can travel one of two ways, continue straight up through the gear tower to supply oil to the camshafts or back through the crankcase to the main crankshaft bearing.

Below the image shows the channel on the back of the timing gear tower that routes oil up from the crankcase to the head. The block forms the other side of this oil gallery, an o-ring surrounds the channel in an attempt to minimize oil leaks as the oil is under a fair amount of pressure.

Below is a diagram showing how the oil makes its way from the crankcase to the internal camshaft gallery.


Today I figured out how get oil distributed throughout the engine. However, I still have a few design issues I need to resolve. Above I show a couple of items that still need to be worked out. Due to the direction of the engine rotation, the oil pump wants to deliver oil to the left side when viewed from the front, but it is difficult to bring oil to the top end on this side because of the gears that drive the distributor in the gear tower are in the way. So the oil pump needs to deliver pressurized oil to the right side of the engine when viewed from the front. There is a loopy path that creates an interference between the oil pump and the front crankshaft bearing holder mounting screws.

Below is a cutaway showing some of the issues still to be resolved. Again, as always, I would like to thank Terry Mayhugh as I have derived many of these solutions from his Offy build.

The Offy used a Bendix Aircraft Engine Magneto to fire the spark plugs. The engine did not require a battery as the magneto generated its own power. Today I research these and collected lots of good images. My current thinking is to make the magneto a project all by itself. It deserves good treatment, it sits right in the front of the engine for all to see.





I also cleaned up the design of the oil pump and resolved some design issues resulting from component interference. This is shown below

I started a Bill of Material spreadsheet capturing all of the hardware size and lengths for the engine. This will be a living document as I refine things moving forward.
Thanks for the kind words @johnmcc69.

I am planning to have the magneto house the hall effect sensors to trigger ignition and the distributor for the spark plugs. I had not thought to make it an actual functioning magneto and have it power the ignition system. That is an interesting idea.

When I said the magneto deserved to be a project by itself, I was thinking that due to its complex shape, internal bevel gears and bearings, positioning of the hall effect sensors and the desire to make the timing adjustable, that it could be a standalone project with its own 3D model, plans, and build log.

The Offy engine build is a bit overwhelming and it would be nice to spin off the magneto and discuss it seperately.

Just a thought
The Offy engine build is a bit overwhelming and it would be nice to spin off the magneto and discuss it seperately.

Earlier in the week, I was thinking that maybe you shouldn't call this an Offenhauser because of the enormous amount of design you're doing. It's in the range of saying things like, "inspired by the Mighty Midget" or something like that, but it looks to me like it's your design.

Which reminds me I was going to say in the last design picture you posted, where you removed the cooling fins in the center of the oil pump area, you could leave the fins there but cut them to some fraction (like a third or a quarter) of the depth of the ones farther away from the gear. With the engine fully built, someone would have to look really closely to see they're not full depth. Even more so if they're only the reduced depth in the area around the gear you're trying to clear.

I really like your suggestion for the fins on the bottom. I reduced the depth of the fins in the critical area, leaving them full depth where it does not matter. I am sure no one will know the difference, except maybe us.

Regarding your comment on calling the engine an "Offenhauser Mighty Midget", I am not sure of the semantics. Outwardly I want the model engine to resemble as much as possible the famous Offy, but I have take lots of liberty on the internals to make it an accessible design for model engine machinists like ourselves. I would like the name plate to say Offenhauser, and people who are familiar with it say, "Oh yeah, that's an Offy."

A purest might say, "that is not an Offy model because the Offy crank is nothing like the one you use." Ron Colona designed and built a quarter scale 270 cu in Offy that is incredibly true to the original. To install the crankshaft in the original engine, the crankcase had to be heated with a blow torch to expand it so the crankshaft could be installed through the end. Then these intricate bearing holder plates were installed and the bolts tightened through small openings cut in the side of the crankcase. This was very difficult for the original engine builders. Ron duplicated the way the crankshaft is secured in his model (he did not have to use a blow torch ;) ). He is an amazing craftsman and I know I do not have the skill to pull off building a model so detailed.


Heating an Offy crankcase to install the crankshaft
Source: Assembling A 270ci Offenhauser IndyCar Engine: Step By Step

As far as it being my design, I have to say that I am standing on might tall shoulders, Ron Colona for one and Terry Mayhugh for another. I am drawing heavily from Terry's build of Ron's Offy. I have learned so much reading and following his builds. I cannot express how much I appreciate the time these gentlemen have spent documenting their work. I am also using techniques that I have used successfully in the past and learned from other engine designers such as Westbury, Britnell, Howell and Hucks, to name a few. But in the end, I do want to have plans that I can freely modify and distribute.

Thanks for the help with the fins.
As far as it being my design, I have to say that I am standing on might tall shoulders, Ron Colona for one and Terry Mayhugh for another. I am drawing heavily from Terry's build of Ron's Offy. I have learned so much reading and following his builds. I cannot express how much I appreciate the time these gentlemen have spent documenting their work. I am also using techniques that I have used successfully in the past and learned from other engine designers such as Westbury, Britnell, Howell and Hucks, to name a few. But in the end, I do want to have plans that I can freely modify and distribute.

Well, add yourself to that list as far as I'm concerned. I'm overwhelmed reading this build, Foketry's Porsche 917, Mayhugh's Ford 300 Inline Six, and more.

Glad that idea on the fins worked out for you.
Oil Pump Assembly

Today I worked on the oil pump assembly. I am using O-rings to seal the drive shaft and the oil passages in and out of the pump. For pressure regulation there are two grub screws that can be used to limit the passage width to both the top end and the bottom end. I do not have any form of relief valve in the oil system, just a way to divide the oil flow between the top and bottom end.

There are still a lot of details to be worked out, but I need to start working on the actual mechanical drawings. The desired end result is a set of plans, after all. I have not given any thought to part numbering or descriptions standardization of the parts. I have a rough idea of how I want to maintain configuration control. I am thinking that once I start fabrication I will reset the version numbers to version 1, then increase from there as I find issues.

Below is a cut away view of the oil pump in the lower crankcase half showing how it is tucked into the engine.

When I kicked this project off on December 23rd last year, I gave myself one month to complete the design phase of engine. This means I only have one more week to wrap it up. I am getting antsy to get back out into the workshop and make some parts. I have learned the hard way not to start machining parts before the design is completely finalized. I have found many design errors after parts are made, that if I had been more thorough in the design phase, could have been prevented.
Finalizing the Crankcase

Today I worked on finalizing the crankcase which consisted of insuring the crankcase properly interfaces with all of the mating components. What this boils down to is making sure all the attachment holes match the mating components and that all the holes are accounted for. Also that the holes don't run into each other inside the part.

The breathers in the side cover will function, they are connected to large holes to the top of the inside crankcase and there are small holes that act as oil drains near the bottom of the breathers. Reviewing the above image, I think I can eliminate the block hold down screws behind the camshaft oil return holes.

Most of the above is self explanatory. The timing gear bearing relief insures that the bearing can be fully seated in the bearing pocket and that the inner race does not interfere with the inside of this pocket.



Above shows the crankcase side breather panel in place and below shows the detail under the cover. I like the way I was able to hide the crews that join the two crankcase halves behind the side panel. A ball end Allen wrench will be able to tighten these screws.


I also worked on some of the smaller parts highlighted in the next two images: The breathers, bell housing, fly wheel, front motor mount, right side block and crankcase covers.



And then on to the Honey Do list, sand blast and repaint the patio furniture. With a little help from Harvey.

Don't forget to allow for the thickness of the gasket or sealer between the two crankcase halves. It's non-zero (I used .004" vinyl) and will affect your gear meshes, oil passage transfers, and drilled hole locations on the front face. Nice work so far. - Terry
Thanks Terry,

I will incorporate gasket thickness into my design. I see that you use a wide array of gasket types and thicknesses. Vinyl .003" ish, Teflon in.004", .010" and .020". It also seems that some flexibility during assembly is required as tolerance variation may required different gasket thickness to resolve stackup issues.

I am researching the viability of fabricating a drag knife for my little CNC router. I am terrible at cutting out gaskets by hand, I would really like to automate the process. I'm thinking I should be able to adapt one of these:,office-products,124&sr=1-2

Post Script: I found many designs for drag Knifes on Thingiverse.
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Crankcase Plans

I am starting the process of creating the plans for each of the parts. The first few are going to be the most challenging. I am an engineer, not a draftsman, so I am learning as I go. On complex parts like the crankcase, the drawing is broken up into multiple pages and I am not sure how they should be organized. I think like a machinist and organize the prints in terms of order of operations, not sure if that is a good way to go. Should I have prints of the top and bottom crankcase halves individually, or the whole crankcase? As you can see, I am winging it here. Ultimately it may not matter as long as all of the needed information is present.

Eventually I will put these plans on my website for free download:




The other consideration I am pondering is: should I include instructions? how detailed should they be? What level of machinisht should I target with the instructions?

Thanks, Greg
Hello Greg, I think you're doing a pretty good job with all this.
I know from my experience, I liked to see only one view of the part as it was presented to the machine spindle, (I.E.=how many operations can be completed in the least amount of set-up's). Make as many pages as you think it takes to keep things CLEAR in the drawings. I've seen too many drawings that were excessively cluttered & very hard to read. Good machinists can read these cluttered drawing like a book & have no problems doing it, but for the hobbyist, try to keep them uncluttered & simple.

I think you're better off just doing good, clear, concise drawings. Dimension & tolerance features as required, for pistons & cylinders I've just added a note.."running/sliding/" fit to item "#XX. For everything else, "General" tolerances apply. Add a short note to items of importance "Lap valves item #X to valve cages item #X at assembly".

Leave any instructions for the end when you get it running, setting the timing, carb settings, ETC.
But, definitely keep notes on what you do. You may find that you have an opportunity to have this engine published & will be glad you scratched those notes on napkins...

I hope this helps in some way & look forward to other reply's from the builders here that do this all the time.

This forum is a valuable source of technical information, experiences, and knowledge from around the world that are fundamental to improving our projects, our hobby. I will take some ideas from your designs for my future model engines. I may have lost some information but I have not seen what system you use to push the valves, directly with the cams on the valve stem or will you use tappets?
You are doing a great job, thanks

You already know this, but it is worth repeating. The beauty of the overhead cam engine is that there are no lifters, push rods and rockers to rob the engine of power; fewer parts to expand and contract with the heat, to wear, and to mess up the vavle lash.

In my model engine, there is a simple cam follower that rides in the pocket of the cam boxes between the cam lobes and the valve. It is an upside down cup, with the valve riding inside the cup and the cam lobe pressing agaisnt the bottom of the cup. In the real engine the valve stems were manufactured over length and the engine builder filed down the top of the valve to adjust the valve lash. In my case the cam followers will be made after the valves and the thickness of the bottom of the cup will be varied to adjust the desired valve lash. A picture is worth thousands of my words:


Oh, it is a Valve Spring retainer, not Sprint.
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I am going to try my hand at developing plans for an Offenhauser Mighty Midget Racing Engine. There were many variations manufactured through the decades and my version will likely end up being an amalgamation of many of those. I will also be taking liberties for ease of manufacture/assembly and to increase the chance of it being "a good runner"--or at least a runner. Inspiration came from Terry Mayhugh's build of Ron Colona's Offy and I will be taking advantage of his engineering insights shared in his (Terry's) build log. Why don't I just build that engine? Well, it is a very complex model and I don't feel ready to tackle that level of sophisticated craftsmanship. Second, I want to have control of the plans and have the ability to freely distribute them if they ever get to that point. The variant of the 97 Cubic Inch Midget Offy I will build will have two valves per cylinder instead of the large Offy's four valves per cylinder, the crank will be supported in three places instead of the large Offy's five and the 97 cu in Mighty Midget Offy is smaller overall. I will design in 1/4 scale so the model engine will be about 5.375" long, 5.5" tall and 4 inches wide.

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Source: Fred Offenhauser Photos and Premium High Res Pictures - Getty Images

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Source: Offenhauser: The Legendary Racing Engine and the Men Who Built It: White, Gordon Eliot: 9781626540415: Books

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I will use the line drawings available in the book noted as a source for the attached pictures, then create 3d solid models of the major components, and I literary mean solid as there will be no internals initially. I will layout the moving components, timing gear train, cam shaft, crankshaft etc as simple stick models to define their geometry. Then I will add increasing detail to the engine, realizing there is a high degree of interdependence between all the components; a small tweak in one place will ripple through the whole engine. Once I am happy with the 3D CAD model I will begin developing the plans themselves. Most plans I have seen are manufacturing method agnostic, that is, they can be used for manual machining methods as well as CNC machining. My plans will be developed specifically with some limited CNC machining in mind. Home grown CNC routers are more common now and mine has given me greater flexibility in producing a complex part in a reasonable amount of time. What this really means is that I will not only produce a set of dimensioned prints, but some IGES files designed for specific machining operations. If someone chooses to build to my plans, they will not need to create their own 3D CAD model from a set of dimensioned prints, they can use the supplied IGES files.

I have scoured the internet and downloaded lots of pictures, enough I think to guide me in producing a realistic replica.

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I scale a 2D drawing and import it into my CAD program.

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Then I extrude the major components.

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And create an assembly.

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Then begin dimensioning the major components.

I will start nailing down the models specification's, for example I think it will have a bore of .75"
how are you planing on finishing valve seats?
how are you planing on finishing valve seats?

The way I currently plan to finish the valve seats and the best way to do it are probably not the same. In the past I have had good luck with a simple technique. I make the valve cage in one setup on the lathe, the last operation before parting it off is to set the cross slide to 45 degrees and take a small cut on the valve seats, less than .010". After parting off the valve cage, I use the finest polishing compound I have, put some liberally on the seat, insert the valve. Then by hand I rotate the valve with a little pressue into the seat, in random twists, lifting and reseating the vavle, then perform more random twists. I do this just a little bit to get a polish on the valve seat, I don't think I am removing material at all. Finally I perform a simple vacumm test through the valve steam of the valve cage. I pull the vacuum through the rear of the valve cage, past the stem of the valve. I seal the side hole and then take time measurements to see how long it takes to bleed down in different configurations. With the valve held slightly open it takes 3 seconds to bleed down--this is my baseline. This measures the leakage between the cage and the valve stem. Lightly holding the valve closed with my thumb I consider a pass to take at least 30 seconds to bleed down, this represents the valve spring holding the valve closed. The only time I had a failure with the vacumm test was when I was cleaning up the side port burrs inside the valve cage and accidently put a scratch in the seat with the X-acto knife. I didn't attempt to repair the valve guide as I made spares.

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