DIY 760 Big bore on a 61x preparation...

[sfab]

hi folks,
ive did a couples of search on the subject but im still a bit confused. what i want is to put 760 stock sleeve into a 61x cylinder (have access to a machine shop, been a machinist help :biggthumpup: ). i thought all ill have to do is to bore the cylinder casting to the size of the 760 sleeve minus fit tolerance and bored it to 85 and here it is, a 771cc beast! (with some minor carbs ajustemnt, of course). i do know it can have some ports timming issue and was wondering if i only want bottom end will it be critical or i can just leave the ports stock. thank you in advance!

 

[robertg]

Why wouldn't you use the 760 casting?

 

[Mouthfulloflake]

No kidding, its a beefier cylinder, and you wouldnt have to swap sleeves?

 

[Matt_E]

I would just get a 760 62T cylinder.
The point in sleeving & big-boring an X cylinder is the better port timing. If you resleeve with stock T sleeves you loose that advantage and spend more money than you would buying a 760 cylinder.

JMHO.

 

[#ZERO]

I would not use 760 sleeves because the exhaust port timing is 2.5mm higher witch is going to lose some bottom end and you will be close to the max bore at 85mm. Use the Northwest Sleeve part number 1588-BB because they're thicker and then you can bore to 86.5mm with the correct exhaust port timing. Bore out cylinder so you have -.004 (+ or - .0005) interference fit, heat cylinder in pre-heated oven at 400 degrees for 45 minutes. Set the cylinder block up on 4x4's and just drop them in and line up the transfers as close as you can, don't worry about the exhaust port as that can be matched up very easy. Machine the sleeve flange -.005 on the register, then machine top of deck to match when done with sleeving and then finish bore for piston clearance and hone out last .002 thousands for a perfect bore finish.

 

[sfab]

I would not use 760 sleeves because the exhaust port timing is 2.5mm higher witch is going to lose some bottom end and you will be close to the max bore at 85mm. Use the Northwest Sleeve part number 1588-BB because they're thicker and then you can bore to 86.5mm with the correct exhaust port timing. Bore out cylinder so you have -.004 (+ or - .0005) interference fit, heat cylinder in pre-heated oven at 400 degrees for 45 minutes. Set the cylinder block up on 4x4's and just drop them in and line up the transfers as close as you can, don't worry about the exhaust port as that can be matched up very easy. Machine the sleeve flange -.005 on the register, then machine top of deck to match when done with sleeving and then finish bore for piston clearance and hone out last .002 thousands for a perfect bore finish.

Exactly the infos i was looking for!! thank you! btw, should i use Blaster 760 pistons or ill have to buy specialized ones that fits the sleeves?

 

[#ZERO]

A nice set of wiseco or pro-x 760 pistons will work fine with those northwest sleeves.

 

[shepster]

A nice set of wiseco or pro-x 760 pistons will work fine with those northwest sleeves.

Just make sure you set a decent piston to wall clearance

 

[#ZERO]

That depends on your piston type and the application. Most watercraft pistons will run between .0035-.0045in for stock engines running cast pistons. Most Wiseco forged pistons recommend about .004-.005. Most engine builders will add some clearance for added heat from more compression, porting, pipes, timing, etc.

 

[shepster]

Exactly, id be happy with .005 for most freeride motors

 

[sfab]

thanks for all the replies!
one last question: i want to buy a brand new ADA girdled head with my BB 61x (780cc) but i cant pick up the right dome for this particular setup. theres 2 different models: one for the 62T (i think) and another for flattop pistons. which one should i choose? they also state on their website that i must use 38cc domes for 180lbs (only have 94 octane fuel where i live) and thougth it was a bit too much volume from what ive read on a couples of posts here.
my current setup included a factory B pipe, 61x case with a single 50mm carb (bored) and everything else is stock (1995 superjet).
thank you!

 

[Matt_E]

The 62T 760 domes should work for bore sizes 84-86 mm. Verify this with ADA - and check your squish.
(I am assuming you're going to run stock domed pistons, and not flat tops)

 

[#ZERO]

Using the Yamaha 760 domes for a big bore engine without flat top pistons would be the cheapest way out. The maximum bore size on these domes are 86mm and if you wanted to increase the size to 86.5mm you will have to use the R&D head. If you want to use the more expensive flat top pistons you will have to deck the cylinder down about 2.5mm or use a stroker crank along with new port timing and a spacer plate.

 

[sfab]

Using the Yamaha 760 domes for a big bore engine without flat top pistons would be the cheapest way out. The maximum bore size on these domes are 86mm and if you wanted to increase the size to 86.5mm you will have to use the R&D head. If you want to use the more expensive flat top pistons you will have to deck the cylinder down about 2.5mm or use a stroker crank along with new port timing and a spacer plate.

thank you again!! one last Q, ive read somewhere that i will need to bore the case for the bigbore sleeve, is that true?

 

[SUPERTUNE]

Yes! You do need to bore out cases for sleeve clearance.