Topic: CB500K1 piston problem

[meloni.racing]

Hi to all,

I'm doing a complete engine rebuild of my K1 500, but I have a doubt about one of the piston (#3..)
I'm going to replace a lot of stuff :
Primary chain, timing chain, clutch, probablu spring valves, all the seal etc... etc....
All the 4 pistons/sleeves are good talking about tolerances, so I'm going to replace just the ring series with a new one, but I have found the top ring of the piston Nr. 3 broken,  the slleve is perfect, but the groove in the piston is now worn a little (infact If I put a old ring in it, there is some play..) What I should do about that?
1) replace just the rings in all 4 pistons
2)replace all the rings and oonly the piston N'3
3) Buy a new pistons/ringa/pin kit?

Let me know what you think.
Thanks in advance for the help

Regards
   Daniele

Thanks in advance for the help

[1timduke]

I believe there's supposed to be a slight play in the ring groove, this should allow for the ring to slide slightly back and forth with the movement of the piston.   If everything else is in tolerance, than a re-ring on all 4 cylinders would be my suggestion.   Some may say just a re-ring on #3, but if that one broke, then the others may not be far behind.

I'm assuming that if you were to examine the other pistons, you would find a similar groove upon removal of the rings, however, if this was not the case and one ring groove was not like the other pistons than you may be in for a new piston, maybe even go .25 over.

Welcome to the forum.

-Tim

[bryanj]

If you are a bit short on cash 1 piston with 4 ring sets, if you put new rings into a worn groove it will flutter and shatter again.

better to put 4 piston ring kits in and as oversize is similar in cost to standard we always used to rebore to guarantee size and nil wear.

Your cash---your call do check out www.davidsilverspares.co.uk as he did have piston kits at a reasonable cost

[turboguzzi]

Hi Daniele

nice to see another italian here!

the last two ones stayed for just  a short while and then dissapaeared.......

If you have the engine open I would spend a lsittle more money and go with new pistons all around, with David silver in the UK, ti shouldnt cost that much.

Also do some honning before you refit, otherwise the rings wil not run in well.

TG

[meloni.racing]

Thanks to all for the help,
Also sorry if I start to ask question from my 1st post here...

In the next days (I'm working out of the town now..) I will ceck better the worn ring groove, and evetually go ahead for a new piston kit (in case .25mm more).

Other than the stock ones are available somewhere some kind of "high compression" pistons?? (with the original dimensions) I didn't had much luck searching in internet..

Thanks again
   Daniele from Italy.

[turboguzzi]

XL125 pistons wil give you quite a lot more compression, you will actually need to lower the crown.

the main problem is that the CB500 was the only honda model to have 56mm stock bore, most of the other pistons that could fit a 125 cc cylinder will be 56.5mm standard size, so it means that you will have to go to +50 in termrs of reboring.

second option for highhr compression is to simply mill the block and use stock pistons. you could go up to 12:1 that way if you wanted....

TG

[meloni.racing]

good to know about the XL125 pistons, I will look for them too.
How them have to be milled? is a lathe enaugh to mill the top of the piston or I need a milling machine? How much they have to be milled?

Thanks
   Daniele

[Bill/BentON Racing]

CHECK your ring end gap no matter which way you go!!!,Bill

[turboguzzi]

A lathe should be good, you will need to make a special holder,

will post pics later

TG

[meloni.racing]

hi TG, will be great to have some photos of the holder, so I can eventually made one.
 About the ring gap,  I did ceck the old ring gap and they where loose, don't remember at the moment how much , but they where a bit out of the limits  infact the bike was burning some oil (just a bit..).
Regards,
   Daniele

[turboguzzi]

Answering to your message in public, if you are from Roma, then there are quite a lot of guys who are very into 500's there, the Vitalli brothers who were italian vintage champions for a few years and also Carlo Murelli from Samoto Roma, who later became the only non-japanese NR500 (oval pistons) mechanic.

Samoto was the best honda team in Italian production racing in the 70's.

We come to Vallelunga every March for the first race of the season, the "Trofeo d'Apertura".

Will post pics later.  

TG

[meloni.racing]

Hi,

I knew that Samoto was very good, I didnt know that the only non-japanese mechanic of the NR500 was a Italian, that is a very respectful thing.

On my bike I dont wont to do anything special, but because I have already the engine teared down , I want to make it a little "stronger" (like little higher c/r, some porting and polishing of the head...thats it)

I have seen on internet (e-bay, classic motorcycle..) a 555cc big bore kit for the 500, it is good quality stuff or is just crap?? (I dont see it anymore on the internet....)

Can I put a 550 series of pistons (stock size, in the 500  after a adequate reboring )  this is because I guess the 555 was just a +0.50mm reboring of the 550 engine) but I dont know very well all of the differences between the CB engines....

Thanks
   Daniele

[turboguzzi]

meloni, I see that you are already getting greedy with your "little more power search",

first a little bit more compression,

now 555 pistons,

Next a 718cc monster? It is doable, ask paulages  

not sure you will be left with much sleeve after boring out to 58.5mm (550 bore), some people here should know better.

Would be way more easy to find a 550 block and pistons in ebay and simply bolt it on. no problem to do that

Anything more will require messing around, new sleeves, higher compression, etc.

now back to your original request, here is the piston holder I built. All the pistons have a circular machined step that is used to manufacture them so you need to measure it and build something similar. it should fit precisely inside that step.

the internal part holds the piston from it's pin by pulling on a threaded rod.

one picture also shows the difference between the crown of a stock piston and a modified XL125

enjoy and start learning about these little motors, this site is full of info.

and its also much more interesting and challenging getting 10 hp more from a 40 hp 500cc motor than from a 180hp 1200cc one 

TG

[bryanj]

You can happily put standard 550 piston kits in IF YOU CAN FIND THEM!! they aint easy to get and cost a lot more, best way to get more power is a good bore with new pistons, a head job my Mr Reik (member on here) and some say a 650 cam.

The 500, if tuned, needs regular primary chains and the clutch operating mechanism is not the best.

Easiest upgrade is drop a 550 engine in, all you need extra to the complete engine is the RH rear engine mounting as it has the clutch cable stop on it

[turboguzzi]

So bryan, how big can you/he go on stock 500 sleeves?

Just for your knowledge, 550's are almost non existent in Italy, 500's rule here for some reason and they were sold well after the 550 was being sold elswhere.

maybe because of some tax laws.

TG

[meloni.racing]

You're  right TG:

I start with a normal engine rebuild, then looking for High compression piston, and after that for a 550's kit.....I just like to tweak engine, no matter witch tipe...
 But for this one is better to stay more stock as possible, Also like you said there is no much more room to bore the stock sleeves, and I dont want to put bigger sleeves or make the stock one too thin. I have to make this bike more original as possible so I cant do "radical mod's .

I like the idea of the XL125 pistons, also they look to have a small skirt (less friction and weight) , do you confirm that?
and the special holder is not going to be a big deal to make, so I will probably go ahead with that.

[turboguzzi]

the XL piston has the same skirt as the original, see the last picture

the piston you see on the holder is NOT an XL's, it's from a CG-125, it's shown only to explain.

The CG125 piston is good and light but requires milling the block and other heavy modifications to use

Cheers

TG