Topic: to overbore or not to overbore, that is the ?

[nigel1114]

Ok.  So, I was a frequent reader of Greenspun, and finally stumbled upon this.  Here's my current situation:
  I have a 78 cb550k that I've put a good bit of work into.  It's now in mostly good working order EXCEPT,  I'm no longer in denial that the pistons and rings need to be replaced(it's blowing white smoke and clacking with no load).  I would gladly pull the engine out and apart myself, but I live in NYC and have no workspace for such an operation.  This leaves me with the costly alternative of giving it to a mechanic.
Questions:
Do the sleeves automatically need to be rebored to accomodate a new piston?  I assume that replacing with the same sized pistons and rings won't do much. true or not true?  If so and I go .25mm over, the sleeve needs to be rebored, yes?
Now if I am reboring, why not go even bigger? will this add significant machining time?
More questions to come, but let's leave it at that for now.
thanks a bunch,
Neil

[kghost]

Maybe. Depends why the Engine is smoking and clanking. It really can't be determined if new overbore pistons are needed until disassembly. Usually they are, due to wear and scratching(grooving) of the cylinder walls. If however the cylider walls look good you can get away with honing them and installing new rings.

If you are going to machine oversize, going really oversize doesn't appreciably alter the machining time. The thing to consider when really punching one out are as follows.

1. If you go max oversize you will not have enough cyl. wall material left if you ever have to do it again.
2. As the wall thickness of the cylinder gets thinner there is less of a heat sink (less metal in the walls) and cylinders tend to run hotter. Walls also are more prone to heat distortion.
3. Max overbores remove the chamfer at the bottom of the cylinder and make it tougher to get the piston/rings in on reassembly.

If your smoking don't forget to check valve guide seals.

[nigel1114]

thanks for the info, kghost.
has anyone seen the "605" kit for the 500 on ebay? do you think that would work on a 550 with the existing sleeves?  I'm assuming that the sleeves that come with it woudn't fit without reboring the block.  probably not getting it, just asking.  you know, cosidering my options

[tomasz]

the "605" kit that you mentioned is in reality a 592cc, i emailed the folks who sell it yet for some reason they won't change the add... 

[nigel1114]

ok, so does anyone know if the sleeves for these various 500 kits will fit in a 550 without modification?(seems like there's plenty of kits available for the 500 but not the 550) in other words, do the 500 and 550 cylinder walls share the same shape and diameter minus the sleeves?

[tomasz]

 sleeves in the kit mentioned are larger diameter :
OD on the piston is 60mm
OD on the sleeve (bottom)  70mm
OD on the sleeve (middle)  72mm
ID on the sleeve (inside)  61mm
than the stock so you'd need to bore the upper crankcase as well;

 

[scottgarland]

Interesting reading, I wiuld like to know where I could find information on speed tuning and aftermarket parts for my newley aquired 1970 CB750. I have 35 years experance on my 1955 FLH and other assorted english bikes. The CB750 is my first Japaneese bike. Still gitting used to the ride. The carbs are sure a son of a #$%* though, four cables into one what were they thinking. I do like the bike, and am slowly sorting out the bugs. But nothing on news stands that reflects vintage Japaneese bikes.

[Terry in Australia]

G'Day Mate, well you don't necessarily have to re-sleeve for a bigger capacity, you can buy replacement sleeves to take your stock pistons, however the sleeves still need to be bored once they are installed in your block. If you intend to keep the bike forever this is a good option, however not really that much cheaper than the alternatives.

New rings might fix the problem (often just measuring the ring gaps and if still within the serviceable limits, honing the bores will fix the problem as it may well be that the cylinders are just "glazed") but if the pistons really are "slapping" (and they rarely slap very much when they're not under load) then I'd suggest your pistons are too far gone and will need to be replaced also.

My advice (for what it's worth) is to get a shop to do a hot compression test, and if it's under par, get them to do a top end rebuild with a .25mm overbore, new pistons and rings, gaskets, valve seals, new cam chain (can't hurt, the old ones been rattleing around now for 27 years) and give it a tune, and it'll probably last at least another 28 years! (you should have the repairs paid off by then!) Cheers, Terry.

[nigel1114]

thanks for the info guys.  I'm still weighing my options and I'll keep you updated on what happens.  My dangerously slippery slope logic at the moment, however, goes something like this:
If I'm going to replace the rings, I might as well replace the pistons.
If I'm going to replace the pistons, I should do it right and bore out the sleeves.
If I'm going to bore out the sleeves, I might as well overbore while I'm in there(Less reliability vs. more power? uuuuh what the heck why not) maybe I'll replace the sleeves while I'm in there, chopper imports has that 610 kit for only a lot of money....
If I'm going to do all that, might as well rebuild the top end.
If I'm doing all that I might as well start a charity rebuild fund.
I'll be accepting donations on the corner of 42nd street and Broadway, New York City, New York, USA starting today for the next 1-3+ years.  No contribution is to small!  All are welcome! No, seriously though.....