tri-z cylinder bore

[jmax857]

okay so i have to send my cylinder to get bored, i am taking it to the machine shop tommorrow. my question is, my bore is 69.8mm, and has some light scratches. do i just get it bored to 70mm right on? and order a 70mm wiseco piston such as this?

http://cgi.ebay.com/ebaymotors/NEW-YAMAHA-PISTON-FITS-ALL-TRI-Z-250-70-00-MM-_W0QQcmdZViewItemQQhashZitem33633f7243QQitemZ22070 8434499QQptZMotorcyclesQ5fPartsQ5fAccessories

[swampthang]

Prolly your only choice. How bad is your cylinder? Im on my final bore on my tri-z too kind of sux that in in line for a resleeve soon thats some extra cash.

[jmax857]

its not ery bad, the bike was running and had good compresssion also, i gave it a hone and it looks like it might be okay but i dont know if i can put a 68.75mm piston in a 68.82mm bore.

[swampthang]

hummm, idk i'd think that that would actually lower you compression.

[cr480r]

.



its not ery bad, the bike was running and had good compresssion also, i gave it a hone and it looks like it might be okay but i dont know if i can put a 68.75mm piston in a 68.82mm bore.

is 68.75 the actual piston measurement? or just the size stamped on the top? if the cylinder clearances are good get another 68.75 piston. If it needs bored get a 69mm.. you should have the piston before the boring is done so the machinist can fit the new bore to the piston to get the clearances right.


hummm, idk i'd think that that would actually lower you compression.

?

[Ol Deuce]

Just did my Tri-Z to 69mm and the machinist said I still had some left.........I was happy

[Bryan Raffa]

thers 69mm,,,69.50mm,, and 70mm,,, you will need a 70mm if you go bigger.

[jmax857]

thers 69mm,,,69.50mm,, and 70mm,,, you will need a 70mm if you go bigger.

I meant to say I have a 69.82 mm. Bore not. A 68, but if I go to the shop I need it bored to 70mm and the 70 mm or .80 over piston is designed for that bore

[cr480r]

i gave it a hone and it looks like it might be okay but i dont know if i can put a 68.75mm piston in a 68.82mm bore.


okay so i have to send my cylinder to get bored, i am taking it to the machine shop tommorrow. my question is, my bore is 69.8mm, and has some light scratches. do i just get it bored to 70mm right on?

so is it 69.8? or 68.82? either way... its best to bore the least amount needed to get a fresh round bore with proper clearances.. `

[WilliamJ]

The running clearance between the piston and the bore is 2.5 thou or 0.0625mm (from the Yamaha manual) your figures would give slightly more at 2.8 thou or 0.07mm.

But you need to get the machine shop to check your cylinder bore is round and not tapered. Usually about 1 thou out-of-round and 1 thou taper is within specs. Book says up to 4 thou (0.1mm) total piston to bore clearance is okay / within spec. You also need to check the ring gaps but if the piston isn't worn then they are usually okay.

If you need it bored you have to have the piston first as they vary in size slightly.

A slightly bigger bore actually raises the CR a little bit but nothing to worry about - for a stock piston its 7.7 to 1, for a 1mm over its 7.9 to 1.

[cr480r]

The running clearance between the piston and the bore is 2.5 thou or 0.0625mm (from the Yamaha manual) your figures would give slightly more at 2.8 thou or 0.07mm.

.0025 is for cast pistons.. thats not gonna fly with a wiseco..


If you need it bored you have to have the piston first as they vary in size slightly.

this is very important.. especially on final bore... you can always hone for more clearance, but once its excessive thats it for the sleeve.

[Bryan Raffa]

Yes, stock pistons run alittle smaller than a wiseco

[jmax857]

okay, i have a spare cylinder, i have no idea of the bore size, if its smaller i will get it bored to match an oversized piston, and just get the piston first.

[WilliamJ]

.0025 is for cast pistons.. thats not gonna fly with a wiseco..

I have read that before. Can you say why that is? Is it because the Wiseco has a different expansion rate, or because it gets hotter, and expands more as a result? I am very interested to know as I build 4 stroke race engines (as a hobby now but used to be for work). For a race engine we always specced an additional 25-30% clearance as the pistons have less drag and potentially run a little hotter. With race engines they get torn down frequently so bore wear is not an issue like it is for stock.

Bill