Before deciding to build Ron Colonna's 270 Offy, I wanted to familiarize myself with the original full-size engine. I came across a link with a number of photos that describe the full-size assembly of a slightly later version of this engine:
https://www.hotrod.com/articles/assembling-270ci-offenhauser-indycar-engine-step/
Zeroing in on the model builders' major concern with the Offy is its use of a one-piece crankcase. The crankshaft is supported inside the crankcase by four main bearing webs. In the full-size engine, these two-piece split webs are assembled onto the crankshaft before it's dropped vertically through the rear of the crankcase. The webs have ears that are shrink fitted to notches machined inside the crankcase, and in order to finally seat the crankshaft assembly, the crankcase must be expanded with heat.
Ron's quarter scale Offy uses the same arrangement, but his three webs are four-piece assemblies. Using access ports on either side of the engine, the webs are assembled around the crankshaft while it's inside the crankcase. The access ports on the full-size engine are large enough to clear the hands and tools needed for final assembly. In the model, however, where even more assembly is required, the limited access provided by the scaled down ports have been the reason for its reputation as an 'engine assembled in a bottle'.
Being a visual person, I heavily rely on pictures and drawings to interpret a design, and I tend to get lost in textual descriptions. Since Ron's design is divided among all three, I decided to create SolidWorks models of some of the engine's key components in order to test my ability to follow the documentation.
For having just four cylinders, the engine is quite complex but a thing of real beauty. Even though the full-size engine was constructed from several complex castings, Ron was able to machine a faithfully scaled replica from bar stock using a basic mill, lathe, and lots of skill. Since my manual machining skills are not on par with his, I also needed to see if my Tormach was going to be of any help in machining some of the more complex parts that I had already spotted.
The photos contain CAD renderings of some of the models that I created. Except for a few liberties taken to fill in some missing minor dimensions, the models should be accurate representations of Ron's design. In order to avoid a copyright discussion, let me say up front that I realize these models are just another embodiment of copyrighted work currently available from Ron and that they're not available.
One of the critical steps in machining the crankcase is the milling of the pockets for the four ears on each of the three main bearing web assemblies. The design of a long reach shop-made fly-cutter is provided in the documentation to help with this. Mismatches in the depths of these notches will combine with machining errors in any of the four parts making up each of the three webs and create a misalignment of its particular bearing. Although this problem can be mitigated by line boring a pre-assembly of the webs inside the crankcase, their intricate in-place assembly around the crankshaft would still remain.
In addition to the connecting rod assemblies, the sixteen head bolts that secure the head to the block and the block to the crankcase must also be torqued through the 3/4" wide access ports. Even though I can't begin to visualize this very intricate assembly, I can certainly appreciate why it's been described as being done inside a bottle.
My next step is to modify the crankcase model to simplify the crankshaft assembly. In particular, I plan to split the crankcase and use conventional main bearing caps. I'm also going to look at a slightly different head bolt arrangement to try to come up with a bit more robust and serviceable head gasket. Before committing to the project, my goal is to model something with same outward appearance of the original model that I think I'm capable of building and assembling.
If I decide to go forward with the project, Ron has also offered to share his recommendations for improvements based on his experiences with building the engine and running it now for over a decade. - Terry
https://www.hotrod.com/articles/assembling-270ci-offenhauser-indycar-engine-step/
Zeroing in on the model builders' major concern with the Offy is its use of a one-piece crankcase. The crankshaft is supported inside the crankcase by four main bearing webs. In the full-size engine, these two-piece split webs are assembled onto the crankshaft before it's dropped vertically through the rear of the crankcase. The webs have ears that are shrink fitted to notches machined inside the crankcase, and in order to finally seat the crankshaft assembly, the crankcase must be expanded with heat.
Ron's quarter scale Offy uses the same arrangement, but his three webs are four-piece assemblies. Using access ports on either side of the engine, the webs are assembled around the crankshaft while it's inside the crankcase. The access ports on the full-size engine are large enough to clear the hands and tools needed for final assembly. In the model, however, where even more assembly is required, the limited access provided by the scaled down ports have been the reason for its reputation as an 'engine assembled in a bottle'.
Being a visual person, I heavily rely on pictures and drawings to interpret a design, and I tend to get lost in textual descriptions. Since Ron's design is divided among all three, I decided to create SolidWorks models of some of the engine's key components in order to test my ability to follow the documentation.
For having just four cylinders, the engine is quite complex but a thing of real beauty. Even though the full-size engine was constructed from several complex castings, Ron was able to machine a faithfully scaled replica from bar stock using a basic mill, lathe, and lots of skill. Since my manual machining skills are not on par with his, I also needed to see if my Tormach was going to be of any help in machining some of the more complex parts that I had already spotted.
The photos contain CAD renderings of some of the models that I created. Except for a few liberties taken to fill in some missing minor dimensions, the models should be accurate representations of Ron's design. In order to avoid a copyright discussion, let me say up front that I realize these models are just another embodiment of copyrighted work currently available from Ron and that they're not available.
One of the critical steps in machining the crankcase is the milling of the pockets for the four ears on each of the three main bearing web assemblies. The design of a long reach shop-made fly-cutter is provided in the documentation to help with this. Mismatches in the depths of these notches will combine with machining errors in any of the four parts making up each of the three webs and create a misalignment of its particular bearing. Although this problem can be mitigated by line boring a pre-assembly of the webs inside the crankcase, their intricate in-place assembly around the crankshaft would still remain.
In addition to the connecting rod assemblies, the sixteen head bolts that secure the head to the block and the block to the crankcase must also be torqued through the 3/4" wide access ports. Even though I can't begin to visualize this very intricate assembly, I can certainly appreciate why it's been described as being done inside a bottle.
My next step is to modify the crankcase model to simplify the crankshaft assembly. In particular, I plan to split the crankcase and use conventional main bearing caps. I'm also going to look at a slightly different head bolt arrangement to try to come up with a bit more robust and serviceable head gasket. Before committing to the project, my goal is to model something with same outward appearance of the original model that I think I'm capable of building and assembling.
If I decide to go forward with the project, Ron has also offered to share his recommendations for improvements based on his experiences with building the engine and running it now for over a decade. - Terry
