Hi all, newbie here. I recently purchased a very used 2003 TXT Pro and quickly proceeded to torpedo the top end. Valuable lesson learned not to pin a 18 year old trials bike too hard up a long hill in the heat

After pulling it apart it seems the cylinder is still smooth, only one thin vertical scratch. However you can see where the nikasil coating has worn through. 

General question: I only use this bike for practice and enduro skills development, as such I have a limited budget and would like to keep repairs as affordable as possible. Do you think a new piston & rings could be sufficient for low & slow riding? 

Any advice or vendors (US based here) appreciated! Piston kit, base gaskets, etc.

Video showing more detail here: 

 

Needs re-plating or find a good second hand one.

you will scrap another piston if you use that bore again! 

 

Edited September 9, 20214 yr by suzuki250

Suzuki250 is right, but first find out why it pinched up in the first place and fix that.

Then get a new piston and a replate or a look for a decent second hand piston and barrel.

Thanks @suzuki250 & @Goudrons2 for the advice! I think you're right, it'll probably just cook another piston without correcting the cylinder first.

It's hard to say what the culprit was here. The bike is new to me and I don't have any history on it unfortunately. It appeared to have good compression and it ran well, I was running 60:1 motorex crosspower 2t.

Looking at the crank there is a bit of play vertically, does anyone know the recommended tolerance? I'm sure it would benefit from new bearings & seals. 

I'm looking forward to getting the bike running again, I've never had so much fun riding so slow

 

change the bearing and seals, they only a few quid from bearing suppliers

I run my GasGas at 50:1 never had any issues with smoke or fouling.

Was the fan working correctly, also check the pump impeller

Edited September 10, 20214 yr by suzuki250

agree. its a good looking mount you have.  overhaul it. new everything-   there are shops in western usa that will do this. yes its costly. bike looks like a keeper to me-- agan, i stress the importance of running non ethanol gasoline, the photo of the cylinder & piston  sorta looks like phase seperated ethanol gasoline hammered it.---  just a wild guess--  pardon spelling mistake-- D.B.

Check you actually had coolant in it, and the fan / thermostat are working.

The water pump impellers very rarely fail on Gas Gas. The shafts and seals wear allowing bypass of coolant into the gearbox, but this you can tell super easy by the condition of the oil in the sight glass. if coolant is leaking past the seal the oil will go milky and not stay transparent.

That sort of nip up around the exhaust port (the hottest part of the combustion chamber) is a classic case of the fan probably not operational, and the bike was running hot, or it had little or no coolant in the system.

Get you cylinder re coated, chuck a new slug in it, but test that thermo switch is working, and when it switches the fan is coming on.

If everything is working as it should you can "pin it" up any hill!

 

Edited September 11, 20214 yr by arnoux

5 hours ago, arnoux said:

The water pump impellers very rarely fail on Gas Gas

They do, especially if the bike has already overheated!

The melt and become detached from the shaft

There has been an upgrade of of the water pump, The Shaft, the Impellor and the bronze bush are all made from better materials.

 

Water Pump information, Pro 2002~07:

 Beginning in 2004, there are three changes to the waterpump.

1. The centre shaft has a new heat treatment. The part number remains the same.
2. The impeller is made of a new material. The part number remains the same.
3. The outer bushing (drain cap) is made of a new alloy. It has a much longer life than the original model. The new cap uses a “Torx” bit for removal, and installation. The original one used an Allen wrench. (See photo) The part number remains the same.  

                       New                                                                        Old

 

 

You should be able to purchase a matched cylinder / piston set.  with a return of your cylinder for a "core".

easier than finding a place to replate etc.

I recommend new crank bearings and seals if you feel vertical play. (you are halfway torn down anyway) Pro crank bearings are lubricated with transmission oil, so be sure the hole in the races match the hole in the case.

 

My 0.02

Thanks everyone for the advice! 

- The fan has been working well, it's nearly on all the time but it's been hot over here (northern california). No boiling over or smell coming from burning coolant. 

- Plenty of coolant came out when drained thankfully.

- The gear oil looks good, not milky nor any unusual particulates. 

- I took another look at the piston/head and it looks like something was banging around up there, so looks like I'll need a new cylinder head. I'm assuming you can't replace the insert in these stock ones? Perhaps it was a piece of the broken ring, I didn't see anything fall out when I disassembled. 

- There's very slight vertical play on the crank, it was hard to measure but less than 1mm.

I'm leaning towards a "core exchange" swap with Jim at http://trialspartsusa.com, should be around $500 USD all said and done, including the piston kit.

 

The head is wasted. So is the main bearings-seals-upper & lower rod bearings on crank shaft.  Bet there is a 1/4 tea spoon of metal in the bottom end floating around.   Time for a total overhaul.  I wonder if something entered tru the air filter side- or the screw-s that hold the reeds came loose.    sad to see this. some of the marks on piston crown look like broken piston rings. 3 of the marks on head look like some other metal part that came apart.

Some of those marks have a distinct curve that I've seen before.

I think a wire wrist/gudgeon pin clip has done that.

One of these https://www.pedparts.co.uk/product/2241/14mm-gudgeon-pin-piston-circlip

The large score on the inside of the barrel looks about the same diameter as a wire wrist pin clip as well.

Have you still got both clips, one from each end of the wrist pin?

 

Edited September 14, 20214 yr by Goudrons2

While waiting for the new top end I dug around a bit more in the bottom end. The reed cage is still intact with 4 screws holding in the reeds. I tried to clean the crankcase through the crank opening with a microfiber cloth and long screwdriver, there were some tiny metal shavings but nothing significant. I used some compressed air to try to uncover any particles but nothing could be detected. 

The wristpins were there and intact. Perhaps it was a piece of broken piston ring. 

Does anyone know of instructions or a video on disassembling the bottom end? I'm unsure of the process or what tools I'd need to replace crank bearings & seals. Might be better left to professionals  

Looking forward to getting the new top end on there.

10 hours ago, EnduroDad2k said:

While waiting for the new top end I dug around a bit more in the bottom end. The reed cage is still intact with 4 screws holding in the reeds. I tried to clean the crankcase through the crank opening with a microfiber cloth and long screwdriver, there were some tiny metal shavings but nothing significant. I used some compressed air to try to uncover any particles but nothing could be detected. 

The wristpins were there and intact. Perhaps it was a piece of broken piston ring. 

Does anyone know of instructions or a video on disassembling the bottom end? I'm unsure of the process or what tools I'd need to replace crank bearings & seals. Might be better left to professionals  

Looking forward to getting the new top end on there.

 

this video shows the new style crank bearing (w/ seals) and how to retro fit to older Pro engines.

more videos;

https://www.youtube.com/c/TRIALSPARTSUSA/videos

there used to be a good video by Jim of tearing down a GasGas pro engine, I am unable to locate at this time

 

good resource for parts diagragms

https://www.thehellteam.com/technical-support/gas-gas-parts-and-service-manuals.html

click parts books and find your year.