Topic: I don't understand...

[Keith]

When overhauling a sohc engine, why are the barrels dropped over assembled piston assemblies? That requires lowering the barrels over the pistons, installing two pistons at a time, while not disturbing the gasket. Seems like a disaster waiting to happen. Why not split the case, and pull the rod/piston assemblies out, check the bearing condition while you are at it. Then, install pistons one at a time, leaving the rods connected. Slide each piston to the top of the bore, then install the barrels onto the case, loaded with all four pistons. Rotate the crank to line up, and tap each piston assembly down, one at a time, install bearing cap, and torque to spec. The barrels could be secured with spacers and nuts to prevent movement when crank is rotated. Why not? Yes, it would require splitting the engine case, but if the rings are shot, what shape is the rest of the engine in?

[BomberMann650]

You can do that.  But most "rebuilds" don't involve bottom end work.  Cb550's can have top ends done with the lower case still bolted in the frame.  Avoids a bunch of additional work and makes the job simple.

[DaveBarbier]

The bottom ends of these bikes are pretty robust so I hear. It can totally be done that way and I'm sure many people do. But if the bottom end isn't giving you issues, why mess with it? It's not hard at all to drop the cylinders over the pistons. Rubber band the base gasket to the bottom of the cylinders so you're not disturbing it. It's really not that hard. Plus, it's really best to replace the connecting rod bolts with new and that gets more expensive.

Take all this with a grain of salt, I don't really know what I'm talking about


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[scottly]

That method would require at least three people and six hands; four hands to position the rods, and two hands to drop the crank into place.

[Keith]

Actually, the crank would be already installed, tap one piston at a time into place. Simple, really. Far less chance of broken rings. Spent many years in the auto repair business, this method was standard procedure.

[754]

It is standard with autos especially with the tapered ring compressor.
 What was your block holding  idea ?
 All this to save a 15 minute cylinder install..  they are actually fairly  easy.

[Bodi]

There are lots of good reasons to do it "our" way. Car engines are very different:
- car engine main bearings have caps, ours use the lower case for that.
- car bores are in or part of the block, ours are a separate assembly - the head holds the barrel section on.
- car engines usually have massive oil pans and removal gives access to all the bearing caps - ours have poor if any access to the crank without splitting cases.
- car blocks have one or two shafts and the transmission is separate, ours have several shafts for the gears in the "block"... usually at least one at the case split. Flipping the engine is not a good plan before the cases are mated.

It just makes sense to assemble the bottom end and close the cases first, then flip it, install the pistons on the rods, then slip them into the sleeves. Many motorcycle engines had roller cranks ... every 2-stroke and quite a few 4-strokes - there is no good alternative to putting the barrels on "last" with those. Possibly it became a habit before 4-strokes took over.
I recall changing several ring sets a season on hard ridden 60's smokers, and rebores or new top ends weren't uncommon (or, actually, very expensive). You do not want to remove the engine and split the cases unless absolutely necessary... and it isn't, to replace pistons.
"Our way" isn't difficult at all, most if not all bike sleeves have forcing chamfers to help get the rings in without a lot of difficulty and without a ring compressor.

[BomberMann650]

... tap one piston at a time into place. Simple, really.

Except thats not how the pistons are staggered on an 550 crank.  Gotta go 2 at once.

[754]

He means put one in , put cap on, then put second in, before turning crank.

[DaveBarbier]

I get what you're saying Keith. You'd have to tap in one pistons at a time into the cylinders then you have cylinders with pistons and rods sticking out. Have the upper case upside down hanging off the bench and hold the cylinders up against the case. But you'll be trying to hold the cylinders as you tighten the cap bolts. Not too hard. I just think it's pretty easy to do it the way Honda intended. The taper on the sleeve makes it pretty easy.

Plus, for the 500-650 motors one of the nice things is the ability to do a complete top end job while the motor is still in frame. 750 too if you have a frame kit.

Your way you'd turn a quick top end job into a full rebuild with the need to buy all new case and crank seals.


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[lrutt]

I've done several SOHC's and just not had a problem dropping the cylinder down on the pistons. Certainly way easier that splitting everything up.

[MikeSimon]

That's why Honda has ring compression tools. Having the right tools for a given job makes everything sooooo much easier

[Bodi]

I've tried the ring compressors and for me just finger-fiddling the rings through the chamfer is easier than installing and removing the compressor sleeves. Never broken a ring. With a hefty overbore the chamfer gets rather small though, and the compressors are worthwhile then.

[754]

Did you get 3 pce  oil rings in with no compressor?
 One piece are easy, use a popsicle stick to help it along.

[Keith]

I know I'm not comparing apples with apples, but in the air cooled VW's heyday, our shop rebuilt these on a continuous basis. We line-bored every engine, including the thrust bearing, and flycut the cylinder heads, new guides and valves, Neway cutters for the valve seats. It was a complete, thorough job, or nothing...never a "valve job"  or rings on a used engine. We repaired LOTS of engines that went somewhere else first. So, I am a bit surprised to see rings being replaced without a complete overhaul, risky business for a professional shop...try to explain to a customer "we only rebuilt the heads...how could we know you had a bad main bearing?" ... A week later. We averaged one  VW engine a week on top of other scheduled work!