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1975~78 Honda CB550 modification

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crazypj

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#1
In 1974 Cycle World magazine did a modification to a CB500 four and called it 'Gentleman's Express' using modified Honda CB750 pistons. (61mm vs 56mm)
The information is still 'doing te rounds' and I was recently asked to modify a set of CB750 pistons to 'my' spec
I bought a new CB550 F1 in 1977 and did similar modification using Honda CB750 pistons but nothing like the piston modifications given in the 1974 article.
The earlier piston mods were very simple, shorten piston skirt 7mm, machine piston crown, reducing diameter by 12mm then use a 15 degree angle to 'match' combustion chamber. Well, I checked and angle should be 12 degrees and it's only 6mm reduction in diameter. It results in much better combustion chamber shape with a very high cold cranking compression using stock cam(around 225psi) but more reasonable with 'fast road' cam. Te piston skirt is shortened by 3mm wit area under piston pin re-profiled to 6-3/8" diameter to clear flywheels. Other mods include chamfering inside skirt to direct oil scraped from cylinder wall to inside piston crown which helps keep it cool plus keeps small end lubricated better tan stock 'square edge on piston skirt. I know my mods work as I did over 70,000 miles after completion (a lot at 100mph + in the years before speed camera's) My bike was 'retired' (well, very tired) at about 106,000 miles although I still have it garaged in Britain (I've been in Florida 20 yrs) I will update with dimensional drawings (when I do them) Information will also be posted on SOHC4.net
 

dfsmoto

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#2
Sounds like a cool setup. I have a '79 CB750 in my shop now.
 

Buffalo21

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#3
I had a Honda 350F4, 4 cylinder 350 cc, not a high performance unit, but incredibly smooth power delivery. A local Honda road racer, did some kind of mod to the engine, It had about 25-35% more horsepower, but still very smooth. Unfortunately a nasty divorce separated me and the Honda, the guy who bought it, still has it.
 

crazypj

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I have a CB350f here in shed, been 'specialising' in CB360 twins for several years though so haven't really 'looked' at it. I have quite a lot of experience with the 400f, built a 500 'conversion' one around1988~89. It was one of the first '400's that would wheelie 'off the throttle' (without excess rpm and abusing clutch) Had a load of other mods, Yamaha 'mono-shock', CX500 forks, etc Also have a 'spare CB350f motor that 'threw a rod' through front of case. Going to see if I can fit a CB360 6 speed transmission some time plus upgrade rods to fir a 'big bore' conversion (probably around 55~60mm bore) It will need block bored and new liners fitted, don't know what I'll come up with when I start taking some measurements :wink:
 
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