Cylinder boring question

[willis]

I am in the process of tearing down my top end due to a defective cam chain guide. I have owned this bike for several years but it has never run since I've owned it. I have decent mechanical skills, but not much knowledge or experience in rebuilding engines and have limited hand tools. The motor has about 18k on the clock. Since I have it apart I am going to freshen it up so I know what I am working with. The Pistons and head cleaned up pretty well. My question is, how do I tell if I can just hone and re-ring, or if I need to bore and go oversize? Should my local Machine shop be able to advise if I take the cylinder and Pistons in? I considered sending it to Hoos Racing to bore but thought that might be a waste of money if I can get by on just honing and rings. I have a little money to work with but on a tight budget.

 

[Paulrxs650]

You can tell if there's excessive wear in the bore and on the Pistons, visually. If there's gouging, linear grooves piston ring gap is put of tolerance etc, then you can be fairly confident a rebore is required. An internal micrometer and venier / micrometer, will confirm if your bores / pistons are out of tolerance. If everything is intolerance, you'll get away with a rebore. I'd have thought with 18k miles, you may be ok but check anyway. If you do get away with everything in tolerance, give the bores a quick home and always use new rings to ensure they seat properly.

 

[willis]

the pistons have some wear, but the cylinders look pretty decent in my opinion. No significant scoring or ridges. When I go to the machine shop should I provide factory specs, or will they just advise me what the numbers are?

 

[Paulrxs650]

There is some wear evident on those pics but I looks reasonable. If you get a manual or search, you ll get the tolerances for your engine. So you can see if you are in or out of spec. The machine shop will measure and give you your current state of play which you can then use to decide whether you need to rebore or not

 

[650Skull]

Pistons are matched to a bore when they are first installed. There will always be different wear in each bore so if the pistons are going to be used again with a light hone then it would be recommenced they go back in the same bore they come out of.

There is some scoring in the bore and on the skirts of the pistons. The manual will have tolerances for the piston to bore

Page 68 in my Clymer Manual and page 38 in my Haynes manual

 

[5twins]

A "quick and dirty" check can be done by inserting the clean piston back in the bore (rings removed) and fitting a feeler gauge between it and the cylinder wall. The original (new) clearance spec is about .002". .004" is about the most you'd want to see to just do new rings and not over-bore.

There is a 3 digit number stamped on your piston tops. That was the fractional portion of their size when new, as in 74.XXXmm. There is another 3 digit number stenciled on the sides of the sleeve that hangs out the bottom of the cylinder block. That was the fractional portion of the bore size when new, as in 75.XXXmm. The new clearance spec is .050-.055mm so subtracting new piston size from new bore size should give you a number in that range. If you measure those parts now (or get them measured), you can see just how much wear has occurred.

 

[TwoManyXS1Bs]

A "quick and dirty" check can be done by inserting the clean piston back in the bore (rings removed) and fitting a feeler gauge between it and the cylinder wall. The original (new) clearance spec is about .002". .004" is about the most you'd want to see to just do new rings and not over-bore...

+1 on 5twins. Here's a procedure:

http://www.xs650.com/forum/showthread.php?t=40227

 

[scabber]

Light scoring not a problem they fill with oil. Most likely a hone and new rigs is all you need.

 

[willis]

Pistons stamped with "96" & Cylinders are marked with "004"

 

[hardrock785]

what thay said,make sure the pistons go back in the hole thay came out of.

 

[willis]

Yes I made sure to mark them right away once removed. Everything was bagged, tagged and marked very meticulously. Carbon's tear down video was tremendously helpful!!!

 

[XSLeo]

Stock pistons are marked with three digits. The digits are the piston size after the decimal point. As in 74.965.
The three digits on the lower cylinder sleeve is the bore size. As in 75.004. The difference between these two numbers is the stock clearance. As in 75.004-74.965=.039. This example is a bit tight for stock. Most run right about .052 The specs call for .050 to .055.
These are all metric measurements.
So one your bore being 75.004 the piston size should be more like 74.952.
Leo

 

[willis]

Interesting. The Pistons have the Yamaha logo stamped inside. Any reason these would only have 2 digits? Trying to upload pictures but having some issues

 

[scabber]

They measure 96 exactly.