These O-rings seal the joint between the intake manifolds and the cylinder head on the Honda CB550 four. When they dry out and shrink, the engine pulls air past the carbs, which leans out the mixture and gives you a high or hanging idle. A fresh set of four reseals all four intake ports.
The intake manifolds bolt to the cylinder head, and these O-rings seal that joint. After fifty years the rubber hardens and shrinks, so air sneaks in around the seal. The bike runs lean, the idle climbs and hangs when you roll to a stop, and no amount of carb tuning quite settles it down.
You reach these O-rings whenever the carbs and intake manifolds are off, so it's worth resealing all four ports at once. While you're in there, check the rubber intake boots and replace any that have gone hard.
Note: These are the intake-side O-rings that seal the manifolds to the head. They are not the O-rings on the cam-end side covers at the ends of the cylinder head — those are a separate part.
Difficulty: Intermediate
Tools needed: Screwdriver, carb sync gauges (to reset the carbs afterward)
They seal the four intake manifolds to the cylinder head on the CB550 four. They are not the O-rings on the cam-end side covers, which are a separate part.
Four, one for each cylinder. They're sold as a set of four.
Often, yes. When these O-rings shrink, the engine draws air past the carbs and runs lean, which shows up as a high or hanging idle that won't tune out. Reseal the intake and re-sync the carbs.
The rubber intake boots are the usual companion. If they've hardened or cracked, replace them while the intake is apart.
Last updated: June 2026
