Poor Boy Big Bore Kit

[Idler]

Hi everyone this is my first contact on this site - it's really good with some excellent posts - love flatheads tale....

My query is about a big bore conversion for CB77s using Hepolite pistons that was available here in the UK some years back.

I have a CB77 which I am rebuilding with a 64mm bore conversion which takes it out to 350cc but I am having great difficulty in sourcing replacement oil control rings which are 2.8mm wide but in two pieces (a top and a bottom half). Can anyone help source something suitable please?

Hastings list a ring set 2C4407 of 64mm diameter and one piece 3.0mm o/c ring (type501)which could be surface ground to 2.8 but it is for a Honda mower I believe so may not be suitable.

I have a few middle and top rings for this engine (top is a Dykes 'L' shaped ring as on two-strokes) in varying states of fitness for purpose but would like to source a complete set if possible.

Does anyone know if any other pistons would fit? 1970's CB350s have a 64mm bore with, I think a 2.5mm oil control ring, but I don't know the wrist (gudgeon in the UK) pin diameter or height relative to crown.

I know Bore Tech in the States make a 350+ of 65mm bore but it's not cheap, looks nicely manufactured though.

Any thoughts views assistance gratefully appreciated

[Phil-UK]

Hi Idler

The early CB350 pistons ( actually 325cc ? ) will fit and use the same gudgeon pin but there are differences in dimensions: I am doing the same thing and have measured them against STD CB77 pistons and found:-

1) Piston crown centre is @3.5mm taller than CB77 ( all measurements taken from top of gudgeon pin )
2) Outer edge of piston crown is 1mm lower than CB77.
3) No squish band but the crown is domed and MAY close off the outer edge of the combustion chamber.

As it is 4mm bigger bore it means that if the head is left untouched you have a flat surface at the extra 2mm radius "ring", then the original squish band and finally the original combustion chamber. So as the piston rises you have a domed surface coming up towards this mix of surfaces ... lord knows what this does to gas flow !

I haven't assembled it yet to check clearances and what is the best thing to do but would also like to hear from anyone who has used this set up. In an old MSQ article they featured fitting the Weco conversion and made no mention of cylinder head changes BUT the pistons in the photos look the same profile as the original Honda/ART ones.

cheers

Phil

PS - where in the UK are you ?

[Phil-UK]

Hi Idler

A sign of old age but I'd forgotten I'd asked some of these questions before - see previous thread :

Should a CB77 head be machined for a 350cc Kit ?

There were lots of useful recommendations in that.

Apologies to those who contributed to that thread for not remembering it straight away ...
why can I remember things from years ago better than last summer ?

Phil

[teazer]

I have made a few 350cc CB72/77 motors over the years and use the CB350 piston.

There are several ways to do this depending on your intended use but the simple way is to cut the pistons to match the step in the head.

An alternative is to cut the head with a new squish band at whatever angle you like - let's say 17 degrees. Then machine the block to get the right squish clearance after machining the piston squish band at 1 degree less than the head.

You will end up with a lumpy piston, but not as lumpy as the old race kit or Kenig pistons and the crown isn't so high as to harm combustion at higher revs.

Compression will end up anywhere from about 10:1 to 12.8:1 or maybe more depending on how much comes off. BUT too much off teh head exposes the edge of the cast iron skull and allows the gap to become pressurized and oil will leak out around the spark plug inserts.

Depending on the cam, you will probably have to machine valve pockets into the pistons and a small clearance for the plug as well which will drop the CR slightly. Al that machining doesn't come cheap, but back in the day that was our only option.

Oh and you really want to have new O/S liners as well, though it's possible to bore a 64mm piston into a stock liner. We use CB750 liners because a cylinder block used to be really cheap and then have them machined to fit as well as having the block machined for the new liners.

[Idler]

Thanks for the replies Teazer and Phil-UK, some good advice and help there. Sorry for long response time I have been away for a few days and not been able to pick up email.

My current plan is to get some new 64mm by 2.8mm rings made - I have ordered them and have a lead time of around 4 - 8 weeks - and use my relinered cylinders and ART pistons. Among the bits I have is a head which has been crudely "machined" (read "ground") to form a (sort of) squish which gives an indication of what should be done to make this right. My ART pistons are not too highly domed so I presume the compression ratio is not far off standard so I don't want to take too much out. Given that this will be a road / parade bike I am not looking for absolute power, just enough to make it a bit quicker / more torquey.

Once again, many thanks for your help and finally, Phil-UK, I am in West Staffordshire, where are you? Regards and best wishes.