Big bore kit

I had planned right from the start to eventually install a big bore kit. I read about the Thumper Racing kit and almost ordered it, but at the last minute, came across the Athena kit offered by Yamaha as a GYTR kit. That settled it for me; Athena was the one. The only problem was the price - I heard $1100 and $1300; ouch! But then I stumbled across South Seattle Sports Plaza and their 25% off pricing. They had the kit for around $720, so I ordered it. I'll install it over the winter when snow closes the mountains and bike maintenance and mods are in full swing.

The reasons I got the Athena kit instead of the Thumper Racing kit are the GYTR tie-in, the larger bore, and the cylinder swap instead of liner install (been there, done that, tedious). I do note that Travis from Thumper claims a bit better performance from his kit, but I'm stickin with Athena for this go-round. Riders have tried the Thumper kit and are happy, but I read one good report from a happy Athena customer also. We'll see.

The Athena cylinder showing the spigot end. The spigot wall thickness is a little over 3 mm; I measured 3.0 - 3.05 at different spots around the circumference.

The Athena piston. The black coating on the side burns off quickly. The top of the piston is actually shiny aluminum; I angled the piston so the glare from the aluminum wouldn't flare the picture.

Athena rings, pin, and clips. Note that there is a top ring and an oil ring set, the same as all the new high-performance 4-stroke engines. Curiously, the stock WRR rings are a top ring, a 2nd ring, and an oil ring set.

Athena gaskets and washers. The large black gasket is metal.

This is some kind of electronic control unit that is connected to the fuel injector and the throttle position sensor. There are no external controls nor does the kit include any software to adjust or set the unit. There are two 'maps' controlled by whether the gray wire is grounded or not: grounded = stock exhaust map, ungrounded = racing exhaust map. Kinda crude, but there it is. I will not be installing this unit, but will depend on the Power Commander V and Auto Tune to pick the best map based on AFR tables.

The label on the unit reads: GET  PE00100014  WR250

	Stock 250	Athena 290
Bore	77 mm	83 mm
Stroke	53.6 mm	53.6 mm
Displacement	250 cc	290 cc
Comp ratio	11.8 : 1	12.7 : 1
Cylinder wt		1043 g
Spigot wall		3+mm
Piston wt		220 g
Pin wt		48 g
Clips wt		1 g
Rings wt		10 g
Piston assy wt		279 g

GYTR / Athena Big Bore kit 290cc - about $720.  South Seattle Sports Plaza
Thumper Racing Big Bore kit 277cc - about $550.  Thumper Racing