Topic: Should I replace my piston rings and hone the cylinder walls? 1978 CB550K

[aryv]

Hello all - so I have a 1978 CB550K that started having bluish smoke on start up and would go away after riding for a while. The plugs would be fouled black - one had oil on it, they were not flat black like just carbon. I decided to take a look at the top end, and looks like its a valve stem leak (oil around the bottom of the head where the valves sit) and the head gasket had also gone out. After taking the head off, I also noticed the pistons had carbon and oil buildup. I also thought that it could be the piston rings, so I pulled off the cylinder bank.

I am fairly certain the cause of the fouling was the valve stem leak, but now I have pulled the cylinder bank off as well. Should I replace the piston rings, and hone the cylinder walls, or just add new valve stem seals, gaskets, etc and put it back together? Any feedback is appreciated. Thank you for reading.

[denward17]

With the cylinders and head off, I would perform measurements on everything and replace / machine as needed.

How many miles on engine?

What's your piston to cylinder clearance? 
Are your pistons still in spec?
Does your cylinders show signs of oval-ing (egg shape)?

How about valve/valve guide clearance? 

Once these questions are answered you will know what direction to go.

BTW, welcome to the forum!

[calj737]

Service the head. Resolving the oil leak will help with the fouling on the piston and the plug.

If you have the means, measure the bore in 3 locations down its depth and compare it to service limits. Re-honing normally involves new rings and might require new oversized pistons depending upon how aggressively you hone the bores. Do the cylinders still show cross hatching? How many miles on the bike?

If the cylinders present the need for service, boring oversized, new pistons and rings will cost you >$500 between parts and labor. And you might as well have the head milled flat too. So new gaskets, new valve seals, maybe have the seats recut (and oh did you check the valve guides for wear?). See the slippery slope of costs building up?

But if you love the bike, intend to keep it for several years and want a solid running little beast, do the work once and never look back.

[denward17]

^+1 to what calj737 said....

[aryv]

Sorry, I should have specified: I have not had the chance to measure anything just yet but for context, the bike has around 14500 miles. Cylinders have some faint cross-hatching. I have never honed cylinders before so it would definitely be a nerve-wracking process but am willing to give it a try.

[calj737]

Sorry, I should have specified: I have not had the chance to measure anything just yet but for context, the bike has around 14500 miles. Cylinders have some faint cross-hatching. I have never honed cylinders before so it would definitely be a nerve-wracking process but am willing to give it a try.

Don’t. You are more likely to cause an issue than to improve the condition. Even using flexible ball hones, there’s more to it than folks understand. Take the block and head to machinist and have them accurately and properly measure the guides and cylinders. Then go from there.

[Deltarider]

Hello all - so I have a 1978 CB550K that started having bluish smoke on start up and would go away after riding for a while. The plugs would be fouled black - one had oil on it, they were not flat black like just carbon. I decided to take a look at the top end, and looks like its a valve stem leak (oil around the bottom of the head where the valves sit) and the head gasket had also gone out. After taking the head off, I also noticed the pistons had carbon and oil buildup. I also thought that it could be the piston rings, so I pulled off the cylinder bank.

I am fairly certain the cause of the fouling was the valve stem leak, but now I have pulled the cylinder bank off as well. Should I replace the piston rings, and hone the cylinder walls, or just add new valve stem seals, gaskets, etc and put it back together? Any feedback is appreciated. Thank you for reading.

Seen the mileage, most likely the oil stem seals are aged. Some oils are better than others in preserving rubber parts in your engine. My CB500 has 140.000 km on the odometer and still has the original pistons, be it with new rings. Yours has just done the break-in period...

[Geoff Hastings]

Blue smoke after idle or after the over run is definitely stem seals, rings will cause smoke on acceleration as well as excess crank case pressure. As you have barrels off I’d definitely hone the bores and then gap the new rings to the honed bores. New stem seals, check the tensioner and cam chain, rebuild and enjoy.

[Little_Phil]

I have a 78 with only 20000 miles. Some of the valve guides were shot. I think it may be common on the 78s. Replaced the head with an earlier one with double the mileage.

[bryanj]

If the block is off have 3 to 5 thou machined off the top surface, this will hep the O rings that fit in the recesses in the ends seal better as modern gaskets with no asbestos are thicker.
As to rings only probably not just de-glaze the bores unless worn then rebore.
Valve guides have been known to wear quickly when its a 77/78 for some reason, fitting is a specialist job as they require reaming to size and seats recut. If you can afford it send the head to M Reike a member on here, he does fantastic work

[calj737]

If you do happen to have the cylinder deck milled, you must mill the locating dowels an equal length else the head will not sit against the block.

If you replace valve guides, you must have the valve seats recut.

Have a competent machinist examine it and advise you.

[BenelliSEI]

Given the low mileage and you can still see cross hatching, why do pistons/ rings? I’d just do the head. Check guide clearance carefully. Lap the valves and replace the valve seals and head gasket……

[HondaMan]

Given the low mileage and you can still see cross hatching, why do pistons/ rings? I’d just do the head. Check guide clearance carefully. Lap the valves and replace the valve seals and head gasket……

That's my vote, too. DON'T hone it, and don't disturb the rings: they will clean the carbon off of them by running, once the oil leak(s) are fixed.
Otherwise, get larger pistons and rings (CrusinImage makes good 0.5mm oversize kits for this engine) and if you do pull off the head, PM me for some oversized O-rings for the oil ports, or else it WILL leak oil on reassembly (due to modern head gaskets being thicker than OEM).

[Tracksnblades1]

What 737 and 17 said….🤔 ^^^

You’re right there. The cylinder block and head is off..

Fix the head right, check your guides and valve stems for excessive wear…too much wear overworks the seals..

“Taper” is what you want to check for on the cylinders..a taper gauge can check for this in a hurry…what 737 and 17 are both recommending..

The normal compression/combustion process wears the cylinders at the top by forcing “loading” the rings against the cylinder wall and rings lands…where the Big Bang occurs has higher working pressures than at the bottom of the cylinder farther away. This wear is called “taper”. Think funnel. Rings don’t do funnels very well.. especially at 10,000 rpms…
.”blowby” is an indication of poor ring seal…

You can’t make a cylinder straight again with a ball “cross hatch tool”..it not a hone..
A hone is a precision tool like a boring machine it can make cylinders straight again..the boring machine removes the larger amounts of material quicker for the hone saving the stones for the precision work..

Honing is a science too.. The grit of the stones and plateau size determines how well the rings seal up and how much oil is retained on the cylinder wall to aid in the sealing and prevent excessive wear…

A good machine shop does this every day…

If you’re not going to ride it much or it’s just a garage ornament then I would concentrate on the leaks dripping on the floor..

[bryanj]

"Honing" nearly always makes the piston to bore clearance too big, Glaze busting does exactly what it says and can be done using what in UK is known as a flapwheel

[M 750K6]

I'm not recommending this, but here's my experience. CB750K6 with 40,000+ miles. I had to replace my head and cover due to oil leaking. A previous owner had put in new piston rings and damaged the gasket surfaces between the head and cover beyond repair. Chisel gouges a couple of mm deep fully across both surfaces.

The cylinder bores were completely glazed. Very beautifully shiny! I suspect new rings were used without a hone of the bores. My local engineering firm checked the bores and despite a small step at the top, they were round, not tapered and notvtoo worn. They would have preferred to re-bore, but were happy to just hone them. I checked the existing ring gaps, which were in spec and just put it all back together, after myy work on a decent used replacement head and cover.

If I had the money I would have done a re-bore, pistons set and rings. I'd just bought the bike, was spending more money on getting it running and roadworthy. Having been struggling to get it running, I didn't have enough riding to know if I enjoyed the bike and if it would be a keeper.

I was lucky. It now runs and rides beautifully. I've put about 3,000 miles on it this year, compression and sealing seem fine. No smoke, no leaks and plenty of torque, nice idle set at 1400rpm. It feels 'happy'! Old Hondas don't want to die.

I may be lucky and get another 20,000 miles before a rebuild, maybe less. Next time I'll be ready and know I want my bike back on the road.

So, from my experience, if it were me, as the bores are not glazed, I would just put it back together with a bit of care and enjoy it.