Yamaha TRX850 Enthusiasts Forum

I've got a lead on some Arias pistons to replace my shot Wossner items which were standard bore but high comp. The Arias pistons are 868cc so I need to do an overbore but I've been told that the black motors aren't good for overbore. Is this true? If so, can I fit the silver motor barrels/head or do I need to find myself a complete silver motor?

redkiwi wrote:I've got a lead on some Arias pistons to replace my shot Wossner items which were standard bore but high comp. The Arias pistons are 868cc so I need to do an overbore but I've been told that the black motors aren't good for overbore. Is this true? If so, can I fit the silver motor barrels/head or do I need to find myself a complete silver motor?

I would be very surprised if the specification differs, what is the source of this info?

The guy with the pistons. Used to have an OV15A customer bike

These are the two sets of pistons

Black engines are perfectly capable of being over bored without any issues. Anyone who tells you different is wrong!

Honks motor was a 'black' and that was taken out 2mm overbore

Can the motor go to 868 and still retain enough metal between cylinder and water jacket??

Yes, 868 is max bore available direct from Yamaha. JE pistons are larger bore & can be run with no issues.
I've never heard any issues over boring black engines - sounds like a ridiculous bit of advice to me

Cheers, guys I think thats enough to let me know that I'm safe to go ahead

I'll let the guy know he was misinformed. He's got a six speed gearbox setup there too, what do you guys reckon it is worth? It's all new, still in packaging. A 260kph top speed is rather inticing for the tracks I'll be running the bike on.

check out Kedo.com for 878 piston kits.(249pounds x2) around $700 aust for both pistons. They have oversize head gaskets to suit also. Cheap base gasket/valve seals etc and postage

I will personally prefer to order it from TTS performance - it costed me 388 pounds posted (pistons + head gasket).

Regards

Radek

redkiwi wrote:These are the two sets of pistons

IMG_1543.jpg

Feck a rook, look at those valve cut-outs, what compression ratio are those Arias meant to give redkiwi?

Mincehead wrote:

redkiwi wrote:These are the two sets of pistons

IMG_1543.jpg

Feck a rook, look at those valve cut-outs, what compression ratio are those Arias meant to give redkiwi?

All he said was that one set was "high compression" at around 12:1 and the others are "yeah, higher compression"

I`m guessing around 13.5 looking at those crowns and valve cut outs, FECK, pump fuel will struggle at that!
If they are direct copies of those fitted in the OV15A they should be 12.8 to 1. Best you find out in advance of using them.

Tesa wrote:I will personally prefer to order it from TTS performance - it costed me 388 pounds posted (pistons + head gasket).

Regards

Radek

Bingo glad I haven't ordered yet

redkiwi wrote:Cheers, guys I think thats enough to let me know that I'm safe to go ahead

I'll let the guy know he was misinformed. He's got a six speed gearbox setup there too, what do you guys reckon it is worth? It's all new, still in packaging. A 260kph top speed is rather inticing for the tracks I'll be running the bike on.

I`m intruiged, one of Over`s 6 speed creations redkiwi?
Would be likely to have been at least 1,500 English quids worth if it is truly Over stuff, oh and a top whack of 160 mph for a TRX? Even a higher final drive ratio and tuned one might hit that on the 5 speed box.
According to the calculations a 17 / 33 sprocket arrangement might see you achieve it but don`t expect a clutch to cope with the racing starts on that gearing!