Honda cb 360 Overbore questions?!

Speedfreaks

Speedfreaks

New Member
Hello Everybody,
I am a new member to "Do The Ton" I have been on this site for a multitude of learning curves with electronics and what not, so I decided to finally join. Today I am wondering if I could get some feedback with the question of an overbore on a Honda cb360. Let me give you guys some back round first. A year ago I bought 3 Honda cb360s in a farmers field and they all need a little work. I am very good with tools but( I would consider my self a moderately worthy mechanic) no actual experience with rebuilding engines. Although I am efficient to get anything done, as I have gotten them started and ran them around outside.

But getting to my main questions. Is overboring good? I have heard that original pistons are an oval shape and any new piston with an overbore is a circular shape. (is this information even correct?) And when you overbore them A guy was telling me that its hard to get them running right. (just looking for clarity)

The reason I ask this is one of my Honda cb360s rings is in pretty bad shape I think oil is coming up the wall and leaking out of the exhaust and spark plug threads. Im pretty sure this is the ring but could be wrong.

The next question : Should I try to hone and see if I can get an NOS ring ? I cant really see the wall because I haven't taken it apart but looking through the spark plug hole it looks a little rusty on the wall.


My concerns: what needs to be done after an overbore like carbs, valves, Head, thicker gasket, Shave the head down? things of that nature.

I would appreciate any feedback!
 
there will be zero problems with an overbore to the next usable bore size,as long as it is done RIGHT by a competent small engine machinist service
if you can feel a ridge or wear lip at tghe top and bottom of the stroke ring travel then you likely need an overbore also if it is scored,BUT your machinist can spec it out for you if you print out the service limits for her/him to use as a handi gide
the pistons and rings are a very precision deal and must be done right
get the download of the factory service manual it has all wear specs service limits
if you have the head and cylinder off, yes the valves/guides/seats/cam journals/rocker arms/cam lobes timing chain and guides/crankshaft assembly all need to be closely inspected as to service limits and looking for shit that IS woe out
all gasket surfaces clean as new
how many miles ? also there was a recall on the early ones i guess somebody will chime in and tell you about it
 
Thanks for the info! the bike only has 489 (actual) miles on it on it and it is a 75 do you know what yea they had the recall?
 
The original pistons are not oval. They are perfectly round.

With really low mileage, a hone and new rings may do. As XB said, though, a competent machinist needs to check roundness and dimensions.

No shaving of the head with an overbore.

No head work is really needed. the ports are plenty large and will not retrict performance. Going larger is likely to be detrimental. Howver, port matching is still a good idea, especially on the intake. Match the port to the manifold....

Talk to CrazyPJ if you are going to oversize. He has the best experience and background for that.
 
mydlyfkryzis said:
The original pistons are not oval. They are perfectly round.

With really low mileage, a hone and new rings may do. As XB said, though, a competent machinist needs to check roundness and dimensions.

No shaving of the head with an overbore.

No head work is really needed. the ports are plenty large and will not retrict performance. Going larger is likely to be detrimental. Howver, port matching is still a good idea, especially on the intake. Match the port to the manifold....

Talk to CrazyPJ if you are going to oversize. He has the best experience and background for that.
Click to expand...
Pistons are round at the top, the skirts are not, they are cam ground for expansion. The bottom of the skirt is larger than near to the piston rings.
 
Scooter trash said:
Pistons are round at the top, the skirts are not, they are cam ground for expansion. The bottom of the skirt is larger than near to the piston rings.
Click to expand...
Not oval. Same shape as oversize pistons.

Sent from planet Earth using mysterious electronic devices.
 
Even in the 70's the pistons were far from round.
The sides are reduced diameter because of the pin bosses - more material,more expansion.
The should be round and parallel at operating temperature.
Normal measuring point is about 10mm up from bottom edge, the thickness of piston crown plus extra material so rings can be fitted means the top will be at least 0.50mm smaller and possibly as much as 1.00mm
The intake side will probably be different from the exhaust side as exhaust is going to be hotter.
Modern pistons are often more of a pear shape when cold
 
Pears are round. Diameter changes, but they are circular, not oval. OP was told stock pistons are oval, oversize are round . My point is the Pistons, whether stock or oversize, are the same shape.

Nothing is perfectly tound, and no 2 items are the same size either. But close enough. :)


Sent from planet Earth using mysterious electronic devices.
 
Yep, ;D
Modern pistons are pear shaped top to bottom and front to back ;)
It's only been possible to make them like that since high precision CNC has been available. (quite often older CNC was good to 0.0005" ~0.001", now, 0.00001" is often used during stock production, robots do the checking as human/hand inspection isn't accurate enough)
Means that much tighter clearances can be used between cylinder and piston, even 'standard' motors have a much higher design life (200,000 is common even on 'small' engines) The timing chains or belts are first thing to wear out, plus all the ancillary components
It's kinda possible to modify pistons for a twin by hand but it's just as much art and guesswork to get things right (but if you get lucky the performance difference is noticeable 8) )
 
The reason for an overbore is because I have a loss of compression its at 140 (or lower). What should I do to the carbs???
 
Everything needs to be measured. There is a possibility it is overbored already, or that the cylinder is scored beyond overbore, or that your low compression is a valve problem or head gasket issue.

Take it apart, have the cylinder inspected, measured...It may be possible the rings are just stuck, and all you need is the cylinders honed and new rings.

The increase in displacement on a 1 size overbore is not enough to add any measurable horsepower. Only bore if it needs it...
 
Crazy pj, I've read a few of your post a few months ago ... So I know your the most CB360 proficient on this site. Can you give me a few tips to get the barrels off and pistons. I need to take them to the motorcycle shop I found in my area for measuring. The motor is completely together now.
 
Easy to do. You can do it with the engine in the bike still. Start at the top and start removing bolts and nuts and you'll eventually get the rocker cover and head off.

Grab a shop manual for more detail.
 
It's a simple enough job to get apart. remove valve caps and loosen adjusters before removing cover as it's easier to do when you not chasing it around the bench.
Exhaust and carbs off first, spark plugs out, top engine mounting off frame, points cover on left side and points plate. Advance/retard stays on cam extension.
Undo all the bolts and remove cover.
Remove 2x6mm bolts on chain tensioner then slide tensioner up and out (turn it 90 degrees to clear frame) Use a long extension and 10mm socket to remove cam sprocket bolts, slide sprocket sideways and remove chain then slide out cam and sprocket. Remove the 6mm bolts under spark plugs and 14mm nuts, tap under intake with dead blow hammer and head lifts off. Don't worry about cam chain, you can fish it out after cylinders are removed
It's probably quicker to do than for me to type out using two fingers ;D
 
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