I have my engine in the shop and they are replacing all of the gaskets, cam chain, chain roller and tensioner, and piston rings...
The vin number on the engine showed that it was a 1963 CB72, but when I got the head off and took the engine to the shop, they saw that it said ".75" on it, so they figured it was .75mm oversized. I found the appropriate rings on ebay and bought some CB72 .75 oversized piston rings, but when they went to put them on, they were far too small.
He measured the piston, and is getting a 60.75mm measurment. I googled really quick, and it looks like the CB77's used 60mm stock pistons, so the .75 is oversized for a CB77. So I have CB77 .75 oversized pistons on my CB72 engine. I didn't think that you could go that big on a CB72 engine. Am I wrong? Is the vin wrong? Are those parts interchangeble? Will that change any other parts I may be looking for in the future (like the carbs)?
I found the .75 oversized for a CB77 on ohiocycle.com, and I sent payment yesterday (really wish they had an online ordering system, but what can you do). I'm hoping these rings will go right on. I'm also hoping that once the egine is all back together with all these new parts the oil leak and smoking problem will be gone.
Hopefully my questions make sense, let me know if I need to clarify anything.
What engine do I have (pistons not matching with vin)?
My 61 CB72 has CB77 pistons and barrels, they are basically interchangeable, though work has to be done to the head and some other stuff that others here can probably elaborate on better than me.
Probably not too uncommon a discovery on CB72's... Matt
Also into Alfa Romeo Giuliettas and Giulias! Looking for a Giulia Super.
Re: What engine do I have (pistons not matching with vin)?A couple of things to look at as for size of cylinders.
To check from the outside. At the front bottom the 250 will show 247cc and the 305 will show 305. When looking at the pistons there will be cast in it CB77 if it's a 305. .......lm
Displacement equals (number of cylinders) * pi * bore radius squared * stroke all divided by 1000 to convert from mm cubed to cm cubed. The table below has displacements based on oversizes. Not sure if this is perfectly exact, but it should be close. See attached picture for table.
Example ( 2 (cylinders) * 3.14159 * 30(mm bore radius)^2 * 54(mm stoke) ) / 1000 (mm cubed per cc) = 305.4 cc's. Good times with math... A better question is what is your compression ratio. This is cylinder volume at BDC divided by combustion chamber volume at TDC. Supposedly a CB72 head correctly chamfered on a CB77 lower end is higher compression than a CB77 head on a CB77 bottom end. Anyone?
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||