270 Offy

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Oct 24, 2007
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Central Texas
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:


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

Looks interesting, will be following along, If it is any thing like your other engines, it will be a masterpiece
I have a copy of Ron's pans recently purchased. I wil be watching this with interest. Love your work, you are brilliant if
I could be 1% as good I would be happy.

To create a model for the split crankcase, I started with two identical copies of the stock crankcase and, at the crankshaft centerline, literally cut away the top half from one copy and the bottom half from the other copy. This wasn't the most efficient way to begin since the CAD software will continue to remember and associate the invisible thrown-away portions with those actually retained, and this will waste precious resources on my ancient XP computer. However it was a quick way to get started, and it reduced the chances of making unintentional changes to the original design.

The stock internal features were cut away and replaced with three integral webs for the main bearings. Since I plan to machine the interiors of the unassembled halves on the mill, I enlarged the cutouts around the crankshaft counterweights in order to accommodate the fillets that will be left behind by a 3/8" ball end mill. These cutouts could be made smaller if, instead, they were lathe bored per the original documentation.

Mounting holes were added for cap screws to rejoin the halves and also for a pair of dowel pins to register them. The teardrop scars along the lower half's sloping sides created by counterbores for the screw heads should be the only outward indication that Ron's original design was altered.

Dados machined into the tops of the webs inside the lower crankcase half will be used to register the bearing caps to tenons machined into their undersides, and the final assembly will be line bored. The screw hole locations for mounting the circular rear housing were also altered slightly to accommodate the crankcase split. Grooves for o-rings to seal the side covers were added around the access ports straddling the split. Neither a gasket nor sealer will be used between the crankcase halves, and so a few more o-ring grooves were added to the ends of the mating flange on the lower half to seal a couple potential oil leaks.

Attention was focused next on the block. The original design uses long 5-40 studs running up through the roof of the crankcase to secure the block to the crankcase and the head to the block, requiring major disassembly for head gasket servicing. I originally planned to machine the block as an integral part of the upper crankcase half, but the risk added to the whole crankcase by so much additional machining caused me to reconsider. Instead, I kept the block and upper crankcase as separately machined items but joined them into a 'semi-permanent' assembly. The two are joined with a number of 3-48 SHCS's that are threaded up through the roof of the crankcase but stopping short of penetrating the wet interior of the block. I added a groove on the bottom of the block for an o-ring to seal the two surfaces against oil leaks. Once assembled, the pair shouldn't require separation. O-ring grooves were also added around the block's side cover openings to avoid dealing with sealant or gaskets pierced by the cover's forty 0-80 mounting screws.

The last component to be modified was the head. Eliminating the long studs through the crankcase and block allowed a bit more flexibility in the locations of the head bolts. The only change made to the head was a slight repositioning of the head bolts to provide a little more 'meat' for the head gasket. The head will be secured to the block using sixteen SHCS's threaded through the roof of the block and into the bottom surface of the head. The holes for the tubes that will return top end waste oil back to the crankcase were also moved slightly inward to gain clearance for the o-ring on the bottom of the block.

Since I don't yet fully understand the engine's lubrication system, I've not included the oil passages in either the crankcase or the head. None of these passages really have to change from the original design, but the elimination of the four-piece main bearing webs will create an opportunity for some simplification. This should become more apparent after I get some actual parts in my hands. The next step is to allow these models to simmer for a few days and then begin the machining of the crankcase. - Terry

Hi Terry,
Through the years of building engines probably the one job I disliked the most was line boring for the crankshaft. It seemed like every engine I built required a different bar and set-up. The larger engines weren't too bad but the smaller engines were just a flat out pain. We as builders take liberties in building our engines, just as you are making to Ron's original des
I apologize for the abbreviated response above. When trying to use my Ipad it loves to reload some web pages, this one especially.
I'll start again.
Through the years of building engines probably the one job I disliked the most was line boring for the crankshaft. It seemed like every engine I built required a different bar and set-up. The larger engines weren't too bad but the smaller engines were just a flat out pain. We as builders take liberties in building our engines, just as you are making to Ron's original design. It's virtually impossible to completely scale an engine down from the full sized copy and have it work.
While you're in the designing stage here's something you might look at. Rather than line boring my engine blocks I now cut a rectangular slot for the bearings. This does two things, first and foremost it eliminates the line boring process and second it adds greater accuracy to the whole main bearing part of the build.
The bearing slots can be made one of two ways, the first is to make the slot wide enough to accommodate the bearing and its mounting holes and the second is to make the slot large enough for the bearing with the mounting ears on top of the the split crankcase. To make the bearing I machine them to whatever configuration I'm going to use then make a fixture to hole them. Center is picked up and the bearings are bored to the required diameter. Who says that bearings have to be circular inserts. It's common full sized engine practice but for our model engines who cares. It's not like you're going to be replacing bearings every season, but if you did you just machine up some new blocks, mount them in the already made fixture and bore them.
Just a thought.


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Not to be critical, but did you consider using the original Offy crankshaft bearing design with the split crankcase? If you allow a little float it should be easy to align the bearings before firmly bolting them to the crankcase. You could then dowel them in place.

Lohring Miller
Thank you so much George, you just saved my V-8 block. Drilled it in the mill/drill. rear is higher than front. Will mill it out and use your square bearing idea. :):):)


Thanks very much for your suggestion on using square bronze inserts for the main bearings. At first I wasn't sure I fully understood how you were achieving the alignment without line boring, and so I spent time going over your straight-six build on the Model Engine Maker forum. I noticed that a lot of fitting was required at the end to get the crank spinning freely and that your final conclusion was that you'd probably not recommend the technique for journals larger than .437" (the Offy's are .500"). However, you've got me thinking about a hybrid approach that uses these bearings along with line boring to eliminate the typical inserts. I think I'll probably be able to re-use the boring bar and fixture plate that I made up for the Merlin (if I can find them).- Terry
Another Offy 270
I was fortunate in having Ron Colonna's "building the 1/4 scale 270 Offy" manual, as without it construction would have been even more difficult.
Whilst not having to exactly follow his methods it is a valuable starting point.
I used both metric screws etc and BA for the very small connections. This required redrawing many parts to ensure that they were not going to foul something or break thro where they should not.
I think it's a model worthwhile building.


Not quite finished.

I combined the comments from George and Lohring into an alternative split crankcase model using solid bronze circular main bearings. This approach is cleaner and a little closer to the technique used in the original design, and it does away with the need to deal with fragile shell bearings. I'd still likely line bore the final result, though. Both bearing halves are secured to the lower half of the crankcase with a pair of 6-32 cap screws with the upper half of the crankcase being machined for clearance around it. - Terry.

p.s. Thanks Petertha for the tip on the Offenhauser book. I've placed an order for one.

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Thanks for the credit, but my solution was cruder. I believe you will still need to accurately line bore both the seats in the crankcase halves and the bronze bearings. My plan was to make each of the bronze bearings separately and then face mount them to webs in the crankcase. That way there would be some play for alignment and you could then drill for dowel pins to maintain the alignment.

Lohring Miller
Construction of the crankcase began in typical fashion by band sawing a couple chunks of aluminum into rough workpieces that will eventually become the upper and lower halves. After squaring them up and leaving an eighth inch or so excess stock all around, their dimensions were recorded. Working from the centerlines of each, the locations of the two registration dowel pins were determined and their holes drilled/reamed. These dowels are primarily for convenience during machining since the ten close-fitting SHCS's that will eventually secure the halves together will also ensure they're consistently assembled. In the event that one of the workpieces has to be scrapped, the dowels can be helpful in creating its replacement.

Working on the bottom of the temporarily clamped-together assembly in the mill vise, the earlier recorded dimensions were used to locate and drill the holes and counterbores for the ten cap screws straddling the assembly's centerline. After tapping the holes in the upper half, the pair was assembled using all the fasteners. The remaining excess stock was then removed from the assembly while keeping the screws symmetrical about its centerline.

The upper half was then removed and set up in the mill with its interior surface facing up. After drilling and counterboring the holes for the block mounting screws as well as the top-end oil returns, its interior was completely machined. Excess stock was left on the bores for both the rear housing and front bearing to allow them to be finished in a later operation with the lower half. The bores through the three central webs, however, were finished to their final dimensions. These bores aren't critical since they only provide clearance around the bronze bearings that will be installed in the lower half.

Measurements were made on four of the five bores that I could conveniently access using a lathe-turned test bar. My purpose in doing so was to determine if the same machining technique can be used to create the more important bores that will support the three bronze bearings in the lower half. The results were better than I expected. Although all four bores came out .002" oversize, they were all in line within a couple tenths and, as best I could tell, were perfectly round. The test bar snapped into place with no light visible between it and any of the webs.

A bit of a disappointment, though, was that the bores' centerline was offset from the centerline of the workpiece by .003" due to a calibration error in the digital probe used to set the initial work offset. Almost the same error showed up on the x-axis causing its machining to be offset to the left by about the same amount. Fortunately, these errors have little significance in the top half since they only affect clearances.

Even though quite expensive and substantial looking, the probe I use seems overly sensitive to handling and periodically requires a painful alignment procedure. After recalibration, the errors were back to just under a thousandth.

The next step will be to machine the interior of the crankcase bottom half. The current plan is to use the same cutters and tool paths used for the upper half and to finish machine the bores for the bronze bearings. Similar to the upper half's machining, excess stock will be left in the bores for the rear housing and front bearing. - Terry