QuestionI have a 1973 cb 350f it is a four cylinder obviously. all i can find is the 350 cylinders in standard bore. I am wondering if you knwo if you can use standard 400f cylinders in place of and bore otu to proper size . in other words does the 350 four and 400 four share the same engine block but wtih a different bore.
AnswerMatthew, Honda only supplies cylinder assemblies in STD bores for either model. After that you can bore them out .25, .50, .75 and 1.00 oversize. This goes for either the 350 size or the 400 size cylinder blocks/pistons. If you don't have a set of matched usable cylinders, then you would have to overbore them to get them round and true, by using oversized pistons and rings.
Read the middle section of this webpage:
http://www.motorcyclistonline.com/flashback/122_9612_honda_cb400f/
You will notice it mentions valve and combustion chamber changes.
Go to www.bikebandits.com and you can access the microfiche illustrations, used by Honda parts departments. Compare CB350F and CB400F illustrations and the part numbers used on that site (those are not OEM Honda part numbers, however), to see what was changed and what was left alone, when Honda made the 350 into a 400cc engine. When you install big pistons under an existing combustion chamber, generally the edges of the combustion chamber must be beveled out so you don't have piston/head interference and/or the compression ratio doesn't get too high. It may be necessary to bore the upper cases to install the 400cc cylinders, as well.
More info on 350-400Fs at www.sohc4.net
Basically, the engines are similar in design and the crankshaft stroke and connecting rods are the same between the two models. 400Fs have a 6 speed transmission, however.
Bill Silver
