Topic: Strange piston measurements - correct? Possible cause of my grief?

[Tim.]

Last spring, I bought a 555cc 'big bore' kit off eBay to rebuild my '76 550F with.  They were sold to me as 59mm pistons, making them 0.5mm over stock for the 550.  Perfect to rebuild with.  Made by ART, $100 for pistons/rings/pins/clips and gasket.  A bargain to be sure.

The seller still sells this and other kits on eBay.  Here's a current listing for the kit I bought:

http://cgi.ebay.com/ebaymotors/Honda-CB500-Four-BIG-BORE-555cc-Performance-Rebuild-Kit_W0QQcmdZViewItemQQcategoryZ35595QQihZ012QQitemZ220020290362QQrdZ1QQsspagenameZWDVW

What's confusing to me now are the revised measurements the seller provides in the auction description:

"4 New High Performance ART Pistons - S12 (3mm over - total 58.4mm at the Top of the Psston and 58.95mm at the bottom of the piston.. the 58mm shown in the pic is incorrect)"

How can the piston be 0.55mm bigger at the bottom than the top?  Is this normal?  My bike started smoking 2000 miles after the rebuild, and after a leakdown test suggested it's the cylinders, and now I see this on eBay, I'm wondering if the bore job on my cylinders could have been done correctly at all.

Thoughts?  Is the skirt of a piston bigger than the top normally?  Is this to reduce piston slap and allow the rings room to work?

I know nothing about piston design obviously, so please forgive my ignorance.

[Tim.]

On a related note - if I get another set of stock cylinders bored properly to match these pistons, assuming they're still in good shape after only 2000 miles, will a stock head gasket work with the 0.5mm overbore?

Alternatively, would it be safe to think the pins from the new pistons would work with stock pistons?

I have 2 sets of stock pistons/cylinders on the shelf.  I plan on taking them to a machine shop to get them checked out / cleaned up.  I'm either going to put on a good stock used set of pistons/cylinders, or have one of the stock sets bored properly to work with the new pistons/rings in the bike now.  I don't have pins for the used sets.

Either way, I only have a stock sized aftermarket head gasket to replace the current one.

One last question - any decent source for stock sized piston rings?  I'd like to hone the cylinders and use new rings if possible, but in the past have had no luck finding stock sized ring sets anywhere.

[dusterdude]

yes,all pistons are bigger on the skirt than the crown(top)

[Tim.]

Well, that's a relief!  At least there's hope for the newer pistons/rings.  I'm sure hoping it's just a poor bore job.  The leakage was pretty consistent across all cylinders (23% or so) which suggests to me a consistently bad bore across all 4.  We'll see.  I'm going to re-do the leakdown test (bought my own snap-on leakdown tester) tomorrow and will confirm it's the cylinders and not valves.

Then I'll tear the top end off and take all three sets of cylinders/pistons to a machine shop and get one good set out of the bunch to reassemble with.

Any tricks to removing the wristpin clips?  I need to crack open my manual.

[dusterdude]

sme pin clips can be a real #$%*,but it depends on what kind you are dealing with.

[MRieck]

sme pin clips can be a real #$%*,but it depends on what kind you are dealing with.

Small valve keepers are a REAL #$%*. 4mm valve stem stuff plus there are 16 of them.

[bryanj]

TinTin, not only is the skirt bigger in diameter BUT it is turned oval by a fraction just to confuse you! Wrist (or gudgeon) pins will be the same with the circlip grooves machined slightly different depths from the edge so pins will be OK . I have no idea where to get standard 550 rings except Honda at prices you hate( and NO idea at all about 3mm over rings!) and as to the head gasket lay one on there and as long as the steel inside ring is on the liner and not hanging over the bore it will be OK. I do have a couple of copper head gaskets but to get one to you would be fairly expensive (I also have a set of 3mm over 500 pistons somewhere as well!)

[Tim.]

It's only 0.5mm over, as I'm using it on my 550F.  A copper gasket sounds interesting - I'm unfortunately stuck with an aftermarket one from a versah kit or something.

One other question - what about cleaning off the surfaces for the gaskets?  Some sort of chemical product?  The cylinders and head will be cleaned up at the machine shop, but the engine case surface for the base gasket will likely need to be cleaned.

Anything I'm supposed to put on the surfaces with the gaskets?  Sorry for all the silly questions - nothing is mentioned in my Honda manuals.  Maybe I need a Clymer or some basic engine know-how!

[Jerry Rxman Griffin aka MuthaF'er]

Guys,

I was on Total Seal's web site checking out availablity of rings for a 900 kit and that seem to have a very large inventory. Check them out.

[clarkjh]

I tried to get stock rings for my 550K from my honday dealer, N/A on all sizes, standard, .25, .5, .75.  I hope my machine shop did a good job ($180 CAD) on my sleeves when its put back together.  Keep us posted as for the condition of the innards when you take it apart.

James

[bwaller]

Two questions.
When you received your new pistons, you must have sent one along with the cylinders to be bored. What piston to cylinder wall clearance did you ask for?  Somewhere around 0.002" would be good.

 Secondly did you by any chance use synthetic oil when you reassembled and broke in? Not to get into a discussion on synthetic oil, I've used it, but I have seen cases when rings wouldn't seat properly when using it in a newly rebuilt engine.

I have a  scraper dedicated to gasket removal, the trick is not to scratch or gouge the surface. There are spray gasket removal products available. Just make sure whatever method, you keep the junk out of the base.

[Jonesy]

Although it has already been answered, here's why Honda makes them narrower at the top:

[Tim.]

The work was done by a shop, who farmed out the machining.  They had all four pistons and should have bored each hole to match each piston.

This time around I'm doing it myself.  I'm taking my cylinders/pistons and head to http://www.conceptsonwheels.com/home.asp

They're local to me, and as you can see by their site, do some serious work.

I'm taking them three sets of pistons/cylinders and two heads with hopes that I get one good set back.

[puppytrax]

Somewhere around 0.002" would be good.

That sounds too tight. The last cycle I had done was a 500 Triumph twin; it called for .003" - .004" piston to wall clearance, measured at the skirt perpendicular to the pin. Your piston instruction sheet should give the desired clearance. Remember, if you're doing any racing, they need to be looser.

[bryanj]

Tintin, as it hasnt been together too long the base gasket will probably fly off and you shouldnt use jointing compound on gaskets.

[Tim.]

So, I pulled it apart this morning (took 2 hours to having the pistons off the rods).

No big surprises - cylinders look good as do the pistons.  The pistons were shiny silver on the sides, with some carbon build up on the top surface - it's been running rich and burning oil of course.  The valves/head are completely coated in carbon, and had been cleaned to new condition before the assembly last spring.

Then, I was looking at the pistons/rings to make sure the spaceing of the ring gaps were good (perhaps a little close on #1 but nothing silly) and the oil ring on #2 caught my eye - it was a three peice ring, and all the others were a one peice ring.  The one peice was what came with my new piston kit...  #$%*E!

The gap is MUCH smaller on this ring, but it does turn freely in the groove.  It doesn't appear to be marked, while all the other rings are marked 3 for 3mm over the stock 500 the kit was built for.

Now, the leakdown test didn't show any particular problem with #2, but still - I'm PISSED.  Obviously the shop working on the bike broke/lost the new ring and put this on instead.  It likely isn't the right size for the 0.5mm bore.

That being said, I'm now boxing up my 3 sets of cylinders / pistons and 2 heads to take them to the machine shop to get one good set out of the bunch.  This screwy oil ring might be a problem, if I end up using these new pistons with one of the other cylinder sets.

The other problem is one of the cylinder studs (one of the ones threaded from top to bottom) is stripped where the nut tightened down.  I'll need to replace this.  Anybody have one laying around ?

I'm also going to order Honda valve stem seals etc. etc. to replace the rubber bits on the top end.  The head gasket seems in perfect shape and wasn't stuck to the head or cylinders.  Tempted to reuse it, but I know I shouldn't.  The base gasket broke apart.

Will post some pics later.

[bwaller]

I can send you a stud, let me know which one. Are you saying it was threaded top to bottom? If so this isn't original.

Three piece oil rings are superior to the older one piece, so you might try to find a new set of rings for that overbore and just get the shop to run a deglazing hone back through the cylinders quick. Are these pistons 2nd overbore 550's? Surely those rings can be had from Honda somewhere. Pick up a new head gasket too.

[Tim.]

Thanks for the stud offer - will confirm which one it is.  The piston kit was a 3mm bore kit for a 500 - 59mm pistons which work out to 0.5mm over on the 550.  No way to get matching rings near as I can figure.

We'll see what the machine shop says - worst case scenario I'm into another 59mm piston kit from the eBay guy to get another ring    $120 US shipped for 4 pistons/pins/rings/clips/head gasket.  If I had to buy one new ring set and a head gasket, I'm into most of the $120 anyhow.

I could go with this kit:

http://cgi.ebay.com/ebaymotors/Honda-CB500-Four-BIG-BORE-555cc-Performance-Rebuild-Kit_W0QQcmdZViewItemQQcategoryZ35595QQihZ012QQitemZ220020290362QQrdZ1QQsspagenameZWDVW

Or go nuts and go for the 605cc kit:

http://cgi.ebay.com/ebaymotors/Honda-CB500-Four-BIG-BORE-605cc-Performance-Rebuild-Kit_W0QQcmdZViewItemQQcategoryZ35595QQihZ012QQitemZ220020287277QQrdZ1QQsspagenameZWDVW

[bwaller]

These pistons show Honda product code 323, so must have been produced by ART?? At 59 mm these should use 550 2nd over rings or am I missing something? I read that clarkjh couldn't get stock 550 rings from Honda but surely some could be had somewhere. These would be ticket and I think I would try to find at least one set and check the fit. Even if someone here had an old used set you'd have your answer.

 I have several engines 500 & 550, but the only one apart at the moment has 61mm pistons and I'm using 750 rings on those.

That 605 cc kit isn't. At 61 mm the displacement equals 592cc.

[clarkjh]

When I went looking for rings, I had the Honda Stealer look up the part numbers.  When they cam up N/A, I started thinking 750 pistons, if I remember right (not very often ) you could still get the 750s.  Also ask the machine shop if they can get custom rings, never know they might be able to get some that fit or machine the pistons to fit what they can get.

James

PS: Sort of off topic, but where would one get a copper head gasket?

[bwaller]

Cometic sells copper head gaskets. They offer at least one size for a larger bore 550. 

[jsaab2748]

If you guys wouldn't mind, a quick question: I bought one of the e bay kits like tintin's (haven't installed it yet) and, I haven't been able to tell a difference between which are the tops of the rings and which is bottom.
In other words, how do you keep from installing the rings upside down? Either I'm going blind(possible ) or there aren't any marks on the top side of the rings indicating which way is up.
I've always been told if rings are installed upside down it can cause oil consumption problems.
I also seem to remember some sort of rule of thumb as to slant or chamfers of some sort on the ring faces that are suppose to face upwards(?)

[clarkjh]

Somebody correct me if this is wrong.

Top ring has a number or S, this goes up
Scrapper ring has a mark or groove, mark goes up/groove goes down - This I am not 100% sure on
Oil ring (One or Three piece), doesn't have a top or bottom

Hope this helps

James

[Terry in Australia]

Somewhere around 0.002" would be good.

That sounds too tight. The last cycle I had done was a 500 Triumph twin; it called for .003" - .004" piston to wall clearance, measured at the skirt perpendicular to the pin. Your piston instruction sheet should give the desired clearance. Remember, if you're doing any racing, they need to be looser.

Nah mate, .002 is normal, I go for .0015 - .002 when I rebore, .003 -.004 is what we call a "racing clearance" for quick break-ins, and the pistons feel "sloppy" in the bores with these kinda numbers. Cheers, Terry.

[Tim.]

So, the stripped stud is the one in the front right corner.  The two front corner studs are threaded from top to bottom on the 550.

Now, I'm thinking that I might be able to extract it, flip it over and then the stripped 3/8" of threads will be in a position where they aren't used.  Worst case I suppose, unless someone has a stud for me (which really sounds odd).