Yamaha TRX850 Enthusiasts Forum

On the OEM motorparts there are 3 types of TRX pistons listed:

003 YA00157796 PISTON (1.00MM O/S 90.5MM) 90.5 MM 2 74.69
003 YA00157792 PISTON (0.50MM O/S 90 MM) 90 MM 2 74.69
003 YA00157788 PISTON (STD 89.5MM) 89.5 MM 2 74.69

If i´m not mistaken the Over bike had a 90,5mm bore at 870cc!?

Theese are bloody cheap...so watcha think??
Not high comp pistons? ...or just not forged ones?
...Ohhh, listed pirce is EUR€

So give or take £50 a pop plus new rings - lookign at around £200 including the rebore versus around twice that for JE Hi comp pistons with a 1.5mm overbore.

Of course it will give gains over stock but I have no idea how much is attributable to the higher compression?

Certainly worthwhile if you need to hone / bore teh barrel

phuk72 wrote:So give or take £50 a pop plus new rings - lookign at around £200 including the rebore versus around twice that for JE Hi comp pistons with a 1.5mm overbore.

Of course it will give gains over stock but I have no idea how much is attributable to the higher compression?

Certainly worthwhile if you need to hone / bore teh barrel

Exactly my thought, i´ve asked them a couple of Q´s so we´ll see if they´re serviceminded!
....also, i think that "Peder" on here skimmed both head & cylinder and got a nice comp with his standard pistons.

You'll get standard pistons, just a tad bigger. This in itself will increase compression ratio, but not with much. JE pistons have domes, hence more increase than just the bore size would indicate. However, domes are mostly not good for flow, so...

You'll get standard pistons, just a tad bigger. This in itself will increase compression ratio, but not with much. JE pistons have domes, hence more increase than just the bore size would indicate. However, domes are mostly not good for flow, so.......your damned if you do damned if you don´t!?

Killerwhale wrote:On the OEM motorparts there are 3 types of TRX pistons listed:

003 YA00157796 PISTON (1.00MM O/S 90.5MM) 90.5 MM 2 74.69
003 YA00157792 PISTON (0.50MM O/S 90 MM) 90 MM 2 74.69
003 YA00157788 PISTON (STD 89.5MM) 89.5 MM 2 74.69

If i´m not mistaken the Over bike had a 90,5mm bore at 870cc!?

These are bloody cheap...so whatcha think??
Not high comp pistons? ...or just not forged ones?
...Ohhh, listed price is EUR€

just been on to Yamaha for the 1mm oversize ,, 2 pistons ,rings ,gudgeon pins and circlips is £138.66 +vat..... will the rods take a bit more compression

I think you can get Wiseco pistons quite a bit cheaper than that.

burty wrote:I think you can get Wiseco pistons quite a bit cheaper than that.

Checked with wiseco a while back, don´t do TRX anymore.

bugger,, i just e mailed wisco, no reply yet but Yamaha seems right money ,,,,how much do we pay for a rebore and who's the best ?

Killerwhale wrote:On the OEM motorparts there are 3 types of TRX pistons listed:

003 YA00157796 PISTON (1.00MM O/S 90.5MM) 90.5 MM 2 74.69
003 YA00157792 PISTON (0.50MM O/S 90 MM) 90 MM 2 74.69
003 YA00157788 PISTON (STD 89.5MM) 89.5 MM 2 74.69

If i´m not mistaken the Over bike had a 90,5mm bore at 870cc!?

Theese are bloody cheap...so watcha think??
Not high comp pistons? ...or just not forged ones?
...Ohhh, listed pirce is EUR€

Thought about this meself. Seems like 2nd oversize is 878cc. Do it and let us know eh? Much cheaper option than JE, but will it do any good?

steve speed wrote:bugger,, i just e mailed wisco, no reply yet but Yamaha seems right money ,,,,how much do we pay for a rebore and who's the best ?

Wasn´t there a post about an english company who bored and then coated with nikasil or something.....quite cheap also!

can recomend these guys they are very good and local to me

http://www.fjpayne.com

CBA to quote but to answer previous questions

878cc is 91mm bore (that is 1.5mm over)

Yes the standard rods can cope with the extra power. I have only ever heard of one instance of rod failure (and that was mine after cold running the piston for 60 miles to avoid being stranded in France with a blown coil !!).

Yes, the rods (as stated by phuk) will take the extra power.

To increase compression you can also use a copper head gasket. The stock unit is a fair bit thicker than a copper shim. It will up your compression maybe 0.3 or possibly 0.5 at MAX i would suspect.
You wouldnt really want to go much higher.

Phuk has a skimmed head from memory, dunno what his final compression is.

Also note if you DO up compression, make sure you dont have a hugely lumpy cam else your valves smash into your piston.. BUT i dont this this will be a problem with the TRX. Many people use "Kent" cam grinds and thats 8.3mm lift (from 8.0mm standard) so i dont think using a thinner head gasket will be a problem at all.
BUT thinner head gasket AND a kent cam would need inspection just to be 100% safe.

has anyone checked the weight of the pistons je versus stock, I know the je kit is bigger and of a higher comp but that dont mean it will be heavier you could end up buying an oe piston and it will be heavier than the je kit so will not rev as quick just a thought as it all sounds to good to be true