Piston size - Urgent Advice Appreciated!

NSR125-Kid-UK · Attention Whore Joined: 03 May 2003 Karma :

Well folks,I've finally talked myself into going and getting a rebuild as well as a load of other work done to the NSR. Problem is, while looking at prices for parts, the dude in the parts department noticed there were 5 piston sizes, to compensate for the wear on the barrel.

I don't know the size of my piston, and neither do the mechanics. As far as I am aware however, the bike has never had a top end rebuild, and so the piston should be the one fitted at the factory, this requiring the fitting of a #2 piston.

To be sure, the chief mechanic wants to take off the fuel tank and airbox, and look at the engine tomorrow, to see if it says the size of piston that's been fitted anywhere.

I'd find it a huge help if anyone knows where to find any such identifying marks - maybe stamped on the barrel? or on the face of the piston itself?

Allhelp gratefully appreciated Very Happy

!
____________________
https://www.bikepics.com/members/nsrandy/96rs250/ My Bike!

"I'm either going to teach andy to get his knee down, or I'm going to get him killed. One of the two" - Teaman

Adam_P · World Chat Champion Joined: 23 Jan 2004 Karma :

Err, surely if he's a qualified and experienced mechanic he'd know what to get and fit? Is it a dealer or an independent? If it's the latter and he's asking you that sort of question, I'd consider taking it somewhere else. It wouldn't fill me with that much confidence.

Sorry, don't have any advice on the piston! Embarassed

____________________
Colin McRae MBE 1968 - 2007 RIP Sad

Orwell Rolls in his Grave
God is imaginary

sickpup · Old Timer Joined: 21 Apr 2004 Karma :

I like this mechanic. He should really strip the engine to see how much wear there is. I once had an RS250 on its original piston that had to go to 3rd oversize.

cagiva gezzer · World Chat Champion Joined: 17 Mar 2003 Karma :

The original "family" of the cylinder will be stamped on the cylinder somewhere. I think the cagiva one is under the cylinder, so you have to take it off anyway.

If they are going to do it properly, they should measure the clyinder diameter as specified by the manual. Normally 10-15 mm down from the top.
____________________
"because one stroke isnt enough and four strokes waste two"

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

The sizes are not so much to account for wear, more to account for production inaccuracies.

You need to really find the size of the barrel / piston currently fitted (think it is on the underside of the NSR barrel as well), or better still measure it (VERY accuratly).

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

hugo-a-gogo · Trackday Trickster Joined: 13 Feb 2004 Karma :

I'd have thought measuring the bore would be sensible
____________________
"There are some things nobody needs in this world, and a bright-red, hunch-back, warp-speed 900cc cafe racer is one of them - but I want one anyway, and on some days I actually believe I need one. That is why they are dangerous."
Hunter S Thompson
clicky 4 bikey

sickpup · Old Timer Joined: 21 Apr 2004 Karma :

Sorry should have said CB250RS.

Keith is right but I'll explain further. A lot of bikes have 3 different standard piston sizes due to tolerance then 2 oversizes.

I would take the top off the engine to measure the bore and see how good or bad it is.

NSR125-Kid-UK · Attention Whore Joined: 03 May 2003 Karma :

Would this require a new gasket once the top's been off?
____________________
https://www.bikepics.com/members/nsrandy/96rs250/ My Bike!

"I'm either going to teach andy to get his knee down, or I'm going to get him killed. One of the two" - Teaman

DM · Scooby Slapper Joined: 16 Oct 2004 Karma :

Usually not, neither at the top of the cylinder nor the base. Runs fine afterwards.

Spiral · Traffic Copper Joined: 17 Jul 2004 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

mr jamez · World Chat Champion Joined: 04 Aug 2003 Karma :

There is a letter stamped on the bottom of the barrel, then you just need to get a piston with the same letter (A B C D or E). Not a very good idea to re-use a head gasket, so just replace any gaskets that you have to remove. Remember to check the barrel for damage, A new barrel with a OE piston is £85 from david silver spares.

They should be able to tell you what piston you need from the engine number anyway.

NSR125-Kid-UK · Attention Whore Joined: 03 May 2003 Karma :

Well, after much encouraging from BCF dudes, my bike is finally booked in next saturday to have the piston, rings, small end bearing and gaskets replaced.

Also, the dude said he's going to de coke the barrel, the exhaust, generally sort out the exhaust port, bleed my brakes, and sort out my slightly-slack throttle.

The total cost is £230, unfortunatley while I was chatting to a cool dude about the problem with my sliders, the mechanic (who is an acquaintance of mine) comes upstairs, hands me the keys and says with a grin "your steering head bearings are knackered" After I murmered "oh good Confused

" He says "I am the bearer of good news" Laughing

.

Ah well, that's something else to sort out, but at least the bike's no longer a worry Smile

. Well it won't be, a week on saturday.
____________________
https://www.bikepics.com/members/nsrandy/96rs250/ My Bike!

"I'm either going to teach andy to get his knee down, or I'm going to get him killed. One of the two" - Teaman