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Halfa, a DOHC inline Twin

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Completing my 2-Bits V-Twin and having it run successfully and well gave me the confidence and the desire to start designing another IC engine project. Thinking about what that should be, I eventually thought “How about a double overhead cam (DOHC) engine?” Why not? But what should I base it on?

My first car, bought while still in high school from money earned working as a “line boy” at the local airport, was a 1966 Alfa Romeo Giulia Spyder Veloce. Although it had been somewhat abused before I got it and the body had both some collision damage and some rust, it was ran well and strong, stopped well, and handled beautifully. Being lowered for Autocross (slalom) competitions, it could even be driven under a semi-trailer.

So that's what I decided to focus on, either a two or four cylinder DOHC inline engine, based on the general layout and outline of the Alfa Giulietta/Giulia series of engines. That means cast iron (or steel) wet cylinder liners in an aluminum block, and two fairly widely spaced camshafts driving two valves/cylinder in an aluminum head. There is even a precedent for a 2 cylinder version. In the early fifties, Project 13-61 was a proposal for an front wheel drive economy car using a 2 cylinder engine based on the Giulietta engine then under development. The project was rejected by management though and no prototype was actually built. If it had gone forward, it is possible that Alfa could have beat the Austin/Morris Mini to market by as much as two years!

Anyway, I started by drawing up a twin version, with 0.75” bore and 0.88” stroke, and perhaps an open crankcase as I had done with the 2-Bits. This I call the Halfa.

With that looking good, I decided to see about stretching it to 4 cylinders. That ended up with a block length of about 5 inches, which I wasn't sure wouldn't be just a bit too long for my Taig lathe if I wanted to use a between centers boring bar to line bore the main bearings. Not being at home to check, I decided to shorten it by reducing the bore slightly, which allowed closing up the bore spacing a little. This left me with a bore and stoke of 0.625” x 0.74”. This engine I call the Bialbero.

Now I figured it would make sense if the two variants shared most of their parts and key dimensions. So I redrew the first version of the Halfa by cutting the Bialbero design in half, keeping the smaller bore and stoke, etc. The only change was to relax the bore spacing just a tad. This is technically Halfa-v2, but as it is now the primary twin version , I just refer to it as the Halfa, and this is the version I'm going to start building.

For me at least, the design process is an iterative one, and this brief synopsis only touches on the biggest of the iterations! Enough for now though. In the next post I'll show first swarf.

Great looking project, should be a fun trip!!

One term I don't know, you mentioned 'wet liners' for the cylinders. What does that mean?

 :popcorn: :popcorn: :popcorn:

john mills:
the cylinder sleeve or liner goes through the water space  lot of older full size engines have wet liners  .separate from the engine block
standard engine co engines in Ferguson tractor is one.

Hugh Currin:
FUN. My dad had a Romeo Giulia Sprint Veloce if I remember the name correctly, (named Julie). Must have been early 1960s. Red and noisy. He received several ticket for being too loud. He'd go to court and be let off 'cuz it was stock and well maintained. Eventually the word went out to leave him alone.

Nice project, will be following as I can. Thanks.

--- Quote from: RReid on November 21, 2022, 02:21:42 AM ---1966 Alfa Romeo Giulia Spyder Veloce

--- End quote ---

Brendon M:
Exciting project :) I love the pun on the name too


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