Offenhauser Mighty Midget Racing engine

Help Support HMEM:

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
Cambox Caps

To make the cambox cap, start by imitating the shape of the top of the gear tower.
1642025849752.png

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.
1642025900951.png

The front however, has a raised lip and the edge has a larger radius.
1642025917723.png

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.
1642025934819.png

1642025954345.png

I worked on finishing up the gear tower sub assembly with the bearings, shafts and gears in place.
1642025964452.png

1642025977808.png

1642026061650.png

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

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
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.
1642103161400.png



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.
1642103181995.png


Below is a diagram showing how the oil makes its way from the crankcase to the internal camshaft gallery.
1642103195483.png


1642103256534.png


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.
1642103279393.png
 

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
Magneto

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.
1642200514682.png

1642200523741.png







1642200581497.png

1642200590805.png

1642200601585.png


I also cleaned up the design of the oil pump and resolved some design issues resulting from component interference. This is shown below
1642200613127.png



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.
 

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
Thanks for the kind words @johnmcc69.

Peter,
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
 

CFLBob

Well-Known Member
Joined
Feb 10, 2018
Messages
877
Reaction score
257
Location
Central Florida
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.
 

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
Bob,

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.
1642269628887.png


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.


1642269648277.png

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.
 

CFLBob

Well-Known Member
Joined
Feb 10, 2018
Messages
877
Reaction score
257
Location
Central Florida
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.
 

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
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.
1642377074301.png



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.
1642377089307.png


Below is a cut away view of the oil pump in the lower crankcase half showing how it is tucked into the engine.
1642377106992.png



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.
 

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
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.
1642449487606.png


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.
1642449499187.png


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.

1642449513314.png


1642449537340.png


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.

1642449553910.png


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.

1642449564165.png

1642449576516.png

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

1642449589217.png
 

mayhugh1

Project of the Month Winner!!!
Project of the Month Winner
Joined
Oct 24, 2007
Messages
1,201
Reaction score
3,923
Location
Central Texas
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
 

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
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:


1642724750364.png

Post Script: I found many designs for drag Knifes on Thingiverse.
 
Last edited:

Eccentric

Well-Known Member
Joined
Dec 29, 2020
Messages
180
Reaction score
378
Location
Southern California
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:

GregsMachineShop.com

1642801758644.png

1642801770066.png

1642801779572.png

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
GregsMachineShop.com
 

johnmcc69

Well-Known Member
Joined
Dec 5, 2011
Messages
415
Reaction score
84
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.

John
 

Foketry

Well-Known Member
Joined
Sep 12, 2014
Messages
599
Reaction score
1,393
Location
Modena (motor valley) Italy
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
 

Latest posts

Top