Topic: Cb400f Clearances

[Tom R]

A production race bike that will live at 6k to 11k most of the time.
It’s a production class , so stockish cruzin image .050 pistons.
What should the piston/bore clearances be? 
I broke a top ring last year… the clearances on the original stock cylinders measured between .001 and .0012.
What about ring end gaps?

[HondaMan]

Yeah, that clearance was a little much for heavy-duty hi-RPM usage. I rebuild them to 0.0004"-0.0008" (same for the 350F) clearance.

When the clearance is too large the rings will have less tension to the bores. This can develop into harmonic vibration in the 3-piece oil rings, letting oil slip by: this then applies very high hydraulic pressures to the upper rings. If the top ring isn't chrome, it should be for racing - at least, that's how I built the racers I did. I don't do them anymore, though...

...although I still use the small bore clearances. They work fine on the street, too.

[Tom R]

That goes against conventional wisdom that you open up the clearances for racing.
I was thinking I’d hear .0015” or .002”.

[HondaMan]

That goes against conventional wisdom that you open up the clearances for racing.
I was thinking I’d hear .0015” or .002”.

That was 1960s "wisdom" that came from speed shops who built GM V8 engines for weekend dragracers. It didn't work beyond 1 season of racing (like, 500 miles tops) and got the engines back into their shop for another rebuild over the next winter.

When there is too much clearance, yes, there is little break-in needed. There is also little lubrication going on. The oil film, at temperature, is about 0.0002" thick, so factor that into your bores and you'll see how quickly the metal-meets-metal will happen when the piston reverses to head the opposite direction. Then, no oil is even in the midst.

[Tom R]

What ring end gaps?

[HondaMan]

What ring end gaps?

Don't go tighter than 0.004" for [all] the 3 rings. If I get to adjust them (because they were made right) I usually go for 0.006" on the 350F/400F, while Honda calls for not more than 0.012". If your ring set has the honeycomb oil ring type, those can be pretty loose on end gaps with no issues. Many times those seem to come out of their packages with more than maximum end gaps.

The side-of-ring-to-piston-land gaps are pretty important on the tiny Fours, too. The top ring gets more clearance than the others, with 0.0012" being Honda's minimum spec and 0.0022" being the max. The 2nd ring (usually cast iron type) needs to be tight at 0.0006"-0.0012" while the oil ring will be tight if they are the 3-piece version, so you don't usually need to worry with those.