Honda NSR125 - Top End Rebuild - Performance Upgrades

[DarkLightFX]

Right, So I've purchased an imported Honda NSR 125 with 16,000 miles on the clock. I Fear that it has not had any top end matience done so therefore I am going to do a completely top end rebuild.

I ordered all the parts from DavidSilver - Here is the breakdown:
2 Antifreeze / Coolant 1 litre £9.90
1 ***OEM*** OIL SEAL 9x18x7 £2.50
1 GASKET,CYLINDER H *A £9.95
1 GASKET,CYLINDER *A £2.25
2 CLIP,PISON,PIN *A £3.00
1 BRG.CON-ROD S-END *B £8.50
1 PIN,PISTON *A £9.25
1 RING SET, PISTON (ST*B £32.00
1 CYLINDER SET PISTON *B £155.00
£232.35
Delivery £5.85
inc VAT
(20.0%) £285.84

So anyway, I have read up as much as I possibly could and I think I'll be able to do it. I'm most likely over worrying and over complicating things. But considering I've never done it before any instructions/tips/advice would be highly appreciated.

My plan is to strip the motorbike down and drain the coolant. Once I have got access to the top end I will Remove the top end and dissemble. Then I will clean up any marks/carbon using carburettor cleaner.
I will also remove the old gaskets from the cylinder head e.c.t
Once all clean, I will reassemble the new barrel,piston,rings,clips and small end bearings ensuring to lube up with 2 Stroke Oil. Last of all I will then put on new gaskets and put back together. Last of all, before running I will fill up with new coolant and top up the 2 Stroke Oil.

On the first run I will just let her idle for about 10 minutes then turn off and cool down and repeat two/three times. As for the breaking in I guess I will ride gently with the odd burst of power here and there until I clock some miles up.

In addition to the engine rebuild I will be de-coking my exhaust and upgrading the jets as well as new carbon reeds in the intake.

As I mentioned previously, any feedback or advice would be much appreciated.

[salem1987]

https://www.nsr125.co.uk/Downloads/Honda_NSR125_Manual.zip


Should help if you haven't got it already. Its for the older models but the engine is pretty much the same. Rebuilt mine following it.

[DarkLightFX]

Awesome, I'll have a look through it.
From your experience with your rebuilt, did I miss anything vital from my procedure? Any help from experience would be great

[salem1987]

The barrel was awful to get off on mine. Have to remove the powervalve/RC valve assembly to get one of the nuts off and they were rather rusty aswell!
Also took a lot of tapping with a wooden mallet to free the barrel from the bottom end. Don't use a screw driver to pry it off as it will scratch the surfaces.
Other than that its pretty straightforward, just follow the manual word for word and you can't go wrong.

[DarkLightFX]

Once you had rebuilt your top end engine how did you go about running it in? Did you take it easy for a certain amount of miles e.c.t?

[chris-red]

You have ordered a piston without stripping it down? Did you even do a compression test?

If you don't know the history of the bike it could have had a an oversize piston already meaning the one you have bought is too small. The piston could also be absolutely fine (likely) and you have just wasted the best part of £300.

A piston is a lump of metal and as far as I know doesn't wear, the rings do, but the piston would only get replaced if damaged.

The only top end maintenance the manual states is de-coking.
____________________
Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.

[DarkLightFX]

I have ordered both a barrel and a piston. I am not going to mess with purchasing an oversized piston. I will be doing a compression test anyway because it's useful.

Are you suggesting I shouldn't rebuild it - depsite not knowing any of the history of 16,000 miles. Surely it's better to rebuild the whole thing and have to security and reassurance that it's solid.

[stirlinggaz]

hi,
why did you go & spend all that cash on parts you might not need?
like i said in your other post, heres is the service intervals straight from the owners manual :
7500 miles (12000 kms) change rings
15000 miles (24000 kms) change piston & rings
22500 miles (36000 kms) new cylinder/top end kit including small end bearing,gaskets etc
you might just likely need a piston kit & gaskets, no cylinder. however the bike will feel like a new engine has been fitted when you've replaced all that stuff.
so take it easy for 1st few hundred miles to "break it in"
well if you've paid for it, you might as well do the job.
its a fairly easy job, just make sure you have all the right tools, including a spanner that will get that 1 nightmare cylinder nut off

theres a few guides available on line, so theres no reason why you couldnt do it.
just be methodical, put all nuts,bolts & other parts in tubs so you dont lose them or get them mixed up. (i use 1L ice cream tubs for this very purpose & label with masking tape & marker)
take some pics every so often, in case you forget which way parts go.
have you even done a compression test? compression tester imho is one of the most useful tools when you own a 2 stroke.
i have a few & use the same gunson compression tester i got years ago

just dont drop anything into the crank case, its a good idea to stuff a rag around conrod/crank case.

cheers,
GAZ

[chris-red]

Were it me, I'd do a compression test, the manual states 142 +/- 28 PSI.

Compression test should be done when warm and with the throttle wide open.

If it were low, I'd inspect it and see what is worn. (likely rings) and replace what is needed

Either way if you don't know the history do everything listed in the maintenance schedule.
____________________
Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.

[DarkLightFX]

You are correct but if I'm going to the trouble to change the rings and gaskets then I might as well just do the whole lot. It's not cheap, but it'll be worth it because I'm going to be riding the NSR for 2 Years. I rely on my motorbike as transport and I don't really want the hassle of engine problems when I'm going to be relying on it.

I have no documents to prove that any top end engine work has been done what so ever. However in saying this, I could open the top and it may be pristine. I really have no clue.

But If it's in good condition I will have spare parts or could potentially sell them to repay the £300 cost.


I guess the advantages is I will have a reliable engine and better performance. But feel free to let me know your opinions

[stirlinggaz]

[DarkLightFX]

I have now cancelled the order. I will test the Compression when the kit arrives and figures cross who ever previously owned it would have look after her. What PSI am I looking for?

Worse case scenario then maybe I'll open the top end, clean and put some new gaskets, small end bearing and rings in?

Sound better..?

[DarkLightFX]

[chris-red]

[DarkLightFX]

[stirlinggaz]

[stevo as b4]

Sorry Chris but have a think about what you said about a piston does not wear?

Then think about the temperature gradient across the piston, the hotter running two stroke cycle and what happens to the piston if the engine ever gets too hot, runs a bit lean or detonates.

It's my preference if doing replacing nothing else after measuring the bore of a 2stroke motor, to change pistons with the rings together every single time!

Did you think about ring locating peg issues, rings fretting in the port windows, and the effects on the ring lands of blow by etc?

A Piston is not expensive, and I've had bad experiences (should have known better) from putting new rings onto an old piston with worn ring grooves.

Change the top end as a set, do it properly, and it'll be long lived and you won't be back in there for another 8000mile+ on the bike mentioned in thread.

[stirlinggaz]

hi,
agreed i would prefer to change the piston & rings.
rings at an absolute minimum, but previously i've just bought a piston "kit" containing piston,rings,gudgeon pin etc. & i usually go for the best piston i can afford.
for the nsr, i would remove cylinder & look for the code (a,b or c) & buy the coresponding vertex piston kit. double the price of some other pattern pistons but the piston & rings isnt something i would skimp on,
cheers,
GAZ
EDIT : just looked at yoiur list of parts you were going to buy. £32 for rings etc. you can buy a top quality vertex or wossner piston kit for around 70 quid. granted, thats a bit dearer than other pattern parts but vertex & wossner is top stuff & worth paying for imho.

[salem1987]

just to add regarding the piston and rings etc.

The stuff i got from davidsilvers were all honda oem bits.

Definetly do piston and rings together. Would you use the same rings on a new piston?

[DarkLightFX]

Okay well I've decided to scrap the full rebuild as it was costing me almost £300 and I feel it was a little excessive. Instead I am planing on just having a look and up the main bits. My question is what do I need?
I know I will need a piston and (x2) rings. Should I get new gaskets too or?

Last of all, I will be de-coking my exhaust but I plan to get a TYGA exhaust system in the near future. But for now, a nice clean standard will run smoothly.

So my question is, what parts do I need and could you possibly link me? I get a little confused with all this different sized piston stofff.

Thanks guys!

[DarkLightFX]

Fair point

[DarkLightFX]

I have located a Piston kit by 'Vertex'.
They have three different sized pistons - how would I know which one I want?

Honda NSR125F NSR125R Vertex Piston Kit
Vertex Quality Piston Kit To Fit The Honda NSR125R And The NSR125F
Piston kit contains piston, piston ring, gudgeon pin and 2 circlips.
Please Choose Piston Size
53.96mm Vertex A Grade
53.97mm Vertex B Grade
53.98mm Vertex C Grade

Price: £59.60 (£71.52 Including VAT at 20%)


Also will I need a small end bearing and gaskets also?

[Fidge]

[DarkLightFX]

[salem1987]

yes.

Before purchasing though check the cylinder bore for smoothness. Shouldnt be able to get your finger nail stuck in any scratches for example.


If there are some serious scratches, its nikasil so will need replating or a replacement barrel. It might be wise to take the barrel to a local bike garage and ask them their opinion if not sure, 30 second job for them, im sure itll be for free!

And post us all some photos for the old piston and barrel when you've got it in bits!