350 big bore blow out

G-Man · Post by **G-Man** » Wed Nov 14, 2012 2:54 pm

Ricky

You are suggesting by your measurement that there is 1mm clearance between piston and cylinder. One of your measurements is wrong and you should check again. If the pistons say +3.00 then the piston diameter should be around 63mm

You have created a lot of expense and bother for yourself by taking insufficient care when you built and set up this engine. If you just put another piston in then you are heading for a repeat of the problem.

If it were my engine, I would want to know that the carburetion, and ignition timing are spot on. You need to do plenty of reading to understand engines in general and this engine specifically. The air screws you mention only adjust the mixture at idle. For high speed running the main jet and needle control the mixture.

Learn how to check piston and ring clearance. Don't take any shortcuts. +3 pistons in regular liners will leave them very thin and prone to distortion. It is much more important to have the right clearances with this engine.

G

jensey · Post by **jensey** » Wed Nov 14, 2012 5:22 pm

Hi G,

Well spoken, and he will learn the hard way, or he puts the bike together, start it once, and sell.

Jensen

teazer · Post by **teazer** » Sun Nov 18, 2012 12:47 am

Honda did not sell 3mm O/S pistons, we they are not OEM.

Where did you measure the piston - above the top ring, bottom of the skirt, somewhere else?
Where matters because piston skirts are neither round nor parallel.

The bottom end has to come apart to remove the oil baffle plates to get the debris out of the bottom end. Failure to clean it out will result in a destroyed pump, crank and valve gear.

Do you have a picture of the crown and inside the skirt of that blown piston? The reason I ask is that it almost looks as if the crown is very thin and skirt thickness may give us a better idea as to who made them.

I suspect they may be Hap Jones 3mm o/s which were usually OK and at least as thick as OEM.

Timing and jetting need to be spot on.

davomoto · Post by **davomoto** » Mon Nov 19, 2012 8:59 am

I have a couple sets of these pistons. They are ART, and while not sold by Honda, they are obviously from the OEM manufacturer.

Davo

CJ · Post by CJ » Wed Nov 28, 2012 1:05 pm

You don't mention what motor oil you are using? Although others may disagree, I would NOT use synthetic oil with these motors... On a rebuild, synthetic will retard the 'break in' and the piston rings seating correctly, posssibly causing greater oil consumption and smoking. Synthetic could also cause clutch/shifting problems with the 'wet' clutch.

CJ

'66 CB77
'66 CP77

jensey · Post by **jensey** » Fri Nov 30, 2012 1:38 am

Hi,

Any experience with the above ?

Jensen

CJ · Post by CJ » Fri Nov 30, 2012 1:41 pm

Hi Jensen:

Yes, two of them. Both with rebuilt motors.

One was having smoking problems (the bike, not the owner) and thought he had broken a ring putting it back together. Also had some issues with the clutch slipping under hard acceleration.

Second one had the clutch seem to 'jump' under accelaration. Both were using high quality synthetic oil. I suggested they go back to standard oil, and although it took some time, problems went away.

CJ

'66 CB77
'66 CP77