Cylinder Boring...yes another question

I have an 81 XJ 750 SECA and the motor is going under the knife right now but since I have the head and cylinders off I was thinking how far could I bore my stock cylinders? my dad told me I could go 30 over and be fine and take like 10 thousands off the head, I cracked a piston so 4 new ones need to be ordered but has anyone done this before?

I know I have a lot on my plate with all the mods im doing right now but I wanted some info before I go to ordering more parts. thanks

 

I think the block has to be sleeved in order to bore. I'm not positive, maybe someone else will chime in.

 

All the XJ-Bikes are Steel sleeved.

You have the option to go to a First or Second Oversize.
Boring the Cylinders out to the Oversize is a piece of cake.

Finding Over-sized Pistons to fit after boring the Block out is a whole different story.
I'd wait to bore-out the mill until I had all four new Pistons In Hand!

 

Bako:

Original pistons were available in 5 sizes:

Standard bore
1st oversize (+.25mm)
2nd oversize (+.50mm)
3rd oversize (+.75mm)
4th oversize (+1.00mm)

So you can actually bore, acoording to Yamaha, up to 1.00mm diameter oversize.

Only the 2nd and 4th Oversizes are still available on the PISTONS.

Obviously, there are matching piston RING sets for each size piston; on this item, all ring sizes are available EXCEPT the 3rd oversize.

I can get you prices on any of these items whenever you get ready to perform the work, just drop me a PM, however.......

The rule of the game in boring cylinders is, as Rick points out, to get the pistons first! The reason for that is a couple:

a) although mass-produced pistons are "supposed" to be all the same size, they usually aren't. Thus a "2nd Oversize" piston that is supposed to be .50mm larger than stock (which, btw, is 65.00mm) may or may not be EXACTLY 65.50mm, it might be 65.51mm, or 65.48mm, and the next one you get might be 65.47mm diameter, etc.

When boring a cylinder, the machinist has to calculate how much piston-to-cylinder wall clearance to allow (for the 750's, it's .03 - .05mm)...therefore, he must know the actual diameters of the pistons that he's going to use IN EACH HOLE, and then bore each cylinder appropriately to the correct "actual piston for-that-hole + wall clearance" size(s).

If the machinist is telling you anything different, then pick up you engine from that shop and leave!

Normally, what is done in this situation, is that the engine will be dis-assembled, and the cylinder bores measured for specifications, inspected for damage, pistons examined, etc. and then a determination of whether boring is needed. If boring IS needed, then you start seeing what size oversize pistons are available (those are listed above for you). Then you order the pistons. The the machinist measures the actual new pistons, and from that determines how much each cylinder needs to be actually bored.

If your other 3 pistons and cylinders are in good shape, you may just need to replace (with a used one, since new "standard bore" pistons are NOT available) just one piston, and of course 4 new standard bore ring sets. This might save you a boatload of $$........

Also, the amount you take off the head or the top of the block should be enough to restore the compression ratio to stock, any more and you could run into problems. Generally, the less the merrier. I don't know what type of valve-to-piston clearances there is on stock motors, but much care must be taken whenever you're bringing the valve closer to the piston (as will result from surfacing the head and/or block).

 

There isn't a lot of material that can be cut from the Mating Surface of the Cylinder Head. You should only have the Head shaved to relieve a warp.
And the minimum amount of material shaved to make the surface flat; at that.

The Piston Crowns are designed with relief for how closely the Valves open during the Intake and Exhaust stroke.
Bringing the Valves down closer to the Piston Crown can really put the Engine at risk for damage if you shave the head too much.

The amount varied from Model to Model ... be sure you research the Technical Section of the Repair Shop Manual for information SPECIFIC to your Bike.

 

on the 550 seca head I have milled .050 off of it and only had to slot the cam sprockets to make sure the timing was correct. Anything can be done but before you go that far put some clay on the piston and bolt the head up using the old gasket. Rotate crank a few times and then measure the clay thickness. That should tell you how much you can go with it.