Topic: When to bore cylinders?

[Tim.]

Ok, so as near as I can tell in the Service Manual, the outside threshold for piston size to cylinder size for a CB550 is 0.04 mm.

If I'm at 0.05, and have the top end of the engine opened up anyway, should I spend the extra bucks and bore the cylinders / buy new pistons?

I have to replace 1 piston, and plan on replacing all the rings while the engine is open.  New gaskets of course as well.  The cylinders will be honed.

So, the incremental cost of doing a bore is 3 more pistons and the bore itself.  This amounts to $72/piston (CAD$ Honda parts incl piston, pin, clips), and an extra $40 / cylinder for boring vs. just a hone.  So, $216 for extra parts and $160 for the bore.

Given that the pistons are all at/past the limits in the manual, and compression is relatively low at 140 lbs, and I LOVE the bike and plan to keep it for the long-term, is the bore worthwhile?  Is 0.01 mm a big deal? (sure sounds small).

[kghost]

Bore it you'll be happier long term.

You don't want to redo it in a year or two do ya?

[Tim.]

Yeah, I think I'm going to bore it.  I want years of nice riding after this so it's worth the investment.

I've searched eBay for CB550 pistons etc., but there are none to be had.  Lots of kits for CB500's.  My cost from Honda for new pistons will be $60 each, plus $10 for the pin and $2.40 for the clips.  So $280 for the bank of 4.  This is all Canadian.

The cheapest I could get a kit on eBay for is probably 1/2 the cost.  I kind of think the extra $140 is worth it to have Honda parts inside the engine, where I NEVER want to go again.

[frostypuck]

One of the kits that shows up on ebay quite frequently for the 500 will work with the 550 and give you about a 605cc motor when done. I believe it was the largest oversize for the 500. Worth looking into, I bought mine for around $150 complete with gaskets and rings, pistons, pins clips, everything. Try putting "Henry Abe" into the ebay search, that was the manufacturer.
Chris in Boston

[Tim.]

But what size are the pistons in the 605cc kit?  The ones on eBay come with new sleeves, and don't mention the piston size.  I need something like a 59mm piston.

Another issue though - should I go with the first overbore (.25) or jump to .5?  The largest Honda sells pistons for is 1mm over.  Is there any advantage to going .5 over instead of .25?  I want to make sure I get a good solid bore, so I thought perhaps going bigger, i.e. deeper into fresh material, might be better.  I'm not a machinist obviously, so I'm looking for opinions.

At least after all this, I'll have a completely fresh top end with the exception of the valves.  I'm not doing anything to them.  The seals are good.  Maybe in a year or two I'll have the valves lapped and seals replaced.

With fresh carbs, and a fresh top-end, I'm really banking on this thing running like a swiss watch and giving me years of pleasure.

[don_m]

Unless you want to increase the displacement for more torque do not bore the cylinders just because of excess piston clearance.  Replacement rings can usually accommodate up to .005in. taper of bore.  Use accurate guages or take the block to someone who has them & compare the bore at the bottom, (where wear is minimum), & at the uppermost point the rings travel.  The difference is the taper.  A rule of thumb is if there are two steps, (from the top & middle rings), or if a ridge can be snagged by a fingernail, the taper is excessive.  Of course this is a judgment call, based on experience.  If taper is acceptable, excess clearance is due to piston wear.  Best way to check is a ribbon guage, a long feeler strip, inserted between the thrust face, (right angle to the pin, approx. 1/4" above the bottom edge), & the bottom to the cylinder.  Make sure there is no damage to bore or piston when you check them.
Don Madden.

[Tim.]

So, if the clearance is due to piston wear, new pistons / rings and a hone should do the trick?

I'm going to go to the shop on Monday and see everything for myself.  So far they just told me the difference between the pistons/cylinders was 0.05 mm, which is in excess of the limits in the service manual.

Out of curiousity, if money wasn't an issue, would you bore?  The incremental machining cost of boring vs. just a hone is $160 ($60 vs $20 per cylinder).  If you're suggesting I buy all new pistons and rings anyhow, and if boring is going to get me more life out of the engine, then I certainly don't mind dropping the extra $160 at this point.

I suppose the bottom line is the difference between one new piston and a hone vs. all four pistons and a bore is about $340.  To have all four cylinders/pistons exactly the same seems worth the extra $340, and if that buys me a few years extra before doing another rebuild (if I ever have to at all) then it seems worthwhile.

[bryanj]

Look on ebay for the 550/555 overbore kit its 12th oversize for a 500 which is 2nd oversize for a 550. If they really dont show I have a set here in UK along with a copper head gasket

[Tim.]

Shoot me an email at taysan@hotmail.com to discuss - I might be interested in buying your set if you're selling.

[don_m]

All the books will tell you that if you bore 1 cylinder, bore them all.  Balderdash--unless you are building an ultimate race motor.  On your 500, boring a cylinder to the next size will add about 1cc or so, insignificant for normal use.  The slight difference in weight is also not an issue.  I once bored a badly scored Harley +0.090 on one barrel & left the other standard.  Couldn't tell the difference when I plowed the south 40 acres. 
If you go for the BIG bore be sure the liner is big enough.  If the kit comes with big liners you may have to open up the cases the fit them, requiring a complete teardown.  Big bore mods are great for increasing the torque & engine response but don't boost power much as the head will only flow about the same amount.  Now big bore with gas flowing--That's where it gets to be fun, but expect to modify air filter, carbs, exhaust, clutch etc. to make it right.  All the above & more went into my 350F out to 425cc, but worth it to me.
Cheers, Don Madden

[Tim.]

The Service Manual indicates the serviceable limit on the cylinders is 58.6mm.  It also says the serviceable limit on the pistons is 58.35mm

If I combine the two serviceable limits, we're talking about a serviceable clearance of 0.25mm - am I correct on this?

The original Standard Value for the cylinder is 58.50mm to 58.51mm.  For the piston it's 58.47m - 58.49mm.  Basically a standard clearance of 0.01mm to 0.04mm (which is exactly what the guys in the shop told me on the phone, so at least they're not using a BMW manual).

How can there be such a difference between the standard clearance range of up to 0.04mm and a limit of 0.25mm?  And further, over the phone I was told my clearance was 0.05mm (unless he said 0.5 and I misheard him!).  Seems like I should be fine with a new set of rings, a replacement piston for #3 that was damaged and a hone.

I'm not adverse to spending the extra $350 on a new set of pistons and a bore if it means I won't have to do this again in my lifetime.  But, if just a hone and rings will last another 10,000 miles or something, I'm happy too.  Or I go down the middle with 4 standard size pistons, new rings and a hone.

Suggestions please!  Correct me if I'm wrong on the clearances.  On Monday, I'm going to the shop to see the cylinders and pistons myself, and will measure them.

[Tim.]

Ok - I'm confident I now understand serviceable limits on Honda cylinders and pistons 

I checked the CB500 model information in the manual, and downloaded the CB400F manual and checked it too.  I wanted to confirm the serviceable limits I was seeing for the CB550 were in line with other models.

Not only are they inline, the various models all seem to be exactly the same.

CB550   Standard Values      Serviceable Limit
Cylinder   58.50-58.51           58.60
Piston      58.47-58.49           58.35
Range      0.01-0.04               0.25      
         
CB500   Standard Values      Serviceable Limit
Cylinder   56.00-56.01           56.10
Piston      55.97-55.99           55.85
Range      0.01-0.04               0.25   

CB400   Standard Values      Serviceable Limit
Cylinder   47.00-47.01           47.10
Piston      46.97-46.99           46.85
Range      0.01-0.04               0.25

The standard clearance on all these models is 0.01 to 0.04mm with a max Serviceable clearance (meaning both piston and cylinder are worn) of 0.25mm.  All identical.

So, if I truly have 0.05mm clearance, I'm obviously fine and just need a hone and rings and one piston because it got damaged.  No point in boring when I have a ton of clearance left.

[kghost]

Humm,
 Ya might want to look at the math on that one. Remember as the engine wears the Bores get bigger and the piston gets smaller.

Max service bore 550 = 58.60
Min Piston 550 = 58.35

That could leave a clearance of up to 0.25 mm (service) which is quite a bit bigger than .001 to .004
That clearance will only increase as the engine runs. And .25 mm sounds like allot to me.

Recommend you get the measurements from your shop. Have them measure the bores and the piston then compare. Make sure thier talking mm and not Inch.

[Tim.]

I'm a little confused, because your number 0.25 is the same as mine - perhaps I'm not understanding something (which is very likely!).

The manual says to measure the cylinder at bottom, mid and top on x and y axis.  Rebore the cylinder if beyond Serviceable Limit (58.60mm) at any point.  Standard oversizes are 0.25 ,0.50 etc. etc.

It then says to measure the piston and replace if beyond the Serviceable Limit (58.35mm).

So, let's say the cylinder is at 58.55mm and the piston is at 58.45mm - both worn, but within their respective limits.  This would result in a clearance of 0.10mm

As the extreme example, the cylinder would be at 58.60 and the piston at 58.35.  Both right at their serviceable limits, resulting in a clearance of 0.25mm

Considering the initial maximum clearance of 0.04mm, I agree, 0.25mm sounds like an awful lot.

Bottom line is I agreed to this addtional work with my mechanic because he said an additional 3 hours labour to take the top end off and then put it all back together after machining work was completed.  This also means they will re-tune the bike when back together, which is what I was originally there for.  They already did 5 hours labour on the bike initially rebuilding the carbs, replacing points/condensors, doing the timing and tappets and then trying to synch the carbs and get things going.  Another 3 hours labour isn't much $ wise, and I'd have to buy all these parts and have the machining work done regardless.

I think for the extra cost of 3 new pistons and boring vs. honing, I'm best off boring to the next oversize based on the maximum cylinder clearance measured in any of the 4 cylinders.  Then at least I won't have any issues for years and years to come (with the cylinders anyhow).

[bryanj]

Your maths is correct but in 30+ years i have NEVER seen a smallest piston in a biggest bore, in fact apart from one PF50----dont ask it was my full time job--- i have NEVER seen a 4 stroke honda that needed rebore through wear, only when seized. I have mailed you direct with answers to the questions you possed me.Personally if It was mine and i planed to keep it for decades I would get the 555 kit and bore it now as parts are getting scarcer.

[don_m]

In all machining & manufacturing operations thereis a thing called tolerance.  The finished parts will vary from piece to piece & bore to bore.  What is important is that the clearance between them be within specification.  At factories they measure the parts prior to assembly & select those that fit.  In a block there may be considerable variation between bores or pistons but the selective fit assures that they have the tight clearance needed.  ALWAYS  rebore a cylinder to fit the piston to be installed.
I agree with the durability of Honda cylinders with one exception.  Here in SoCal in olden days Honda 90cc & 70cc trail bikes were often ridden flat out in sand washes until they quit.  What little air reached the engine was already hot & the horizontal layout cooled the head a little but not the cylinder.  The fine dust didn't help either.