Rebuild top end

[Downer]

Ok tore the top end off did a few measurements and found that the pistons are out of spec and the bore seems fine besides needing a honing so my first question is do just the pistons wear on these things? And is there a way to just buy stock pistons and put oversize rings on them to fill the gap? Do they even make oversize rings for stock piston what other possibilities do I have I took it to a machine shop they said they could bore it 1 over for 200 bucks!!! But by the time I pay for that a gasket kit and 1 over pistons I'm already pretty close to mikes stock 650 kit I'm pretty new to rebuilding so far it's pretty easy but the whole measurements thing is throwing me off so I'm not sure what parts I should be measuring for or really what to do next I'm at a standstill thanx

 

[5twins]

The pistons wear 1st, obviously because they're softer metal, then eventually the bores wear. Yes, you can buy stock pistons but there's no telling if they will be an exact match for your bores. The pistons and bores are matched as a set at the factory. Every bore and every piston is a little different in size. The trick is to match them up so you get the proper clearance between the 2. There is a 3 digit # stamped on the piston top. It is the fractional part of that piston size, I.E. 74.xxx mm. There is also a number stenciled on the side of the cylinder sleeve that sticks out of the bottom of the alloy cast part. That is the fractional part of the bore size, I.E. 75.xxx mm. Subtracting one from the other will give you the clearance. Ideally it should be between .05 and .055 mm.

But remember, you're dealing with used parts now so the factory #s probably don't apply anymore. Basically, they are just a reference for you to see how much wear has occurred. I go mostly by cylinder condition. If it's all scraped up with vertical lines, that indicates too much clearance which allows the piston to rock back and forth on the rod as it goes up and down. That makes the scrape marks. If there's little or no scraping, I usually just hone and install 1st oversize rings, filing the end gaps to the minimum spec.

If it's too scraped up to hone clean, I consider a bore job. I would shop around, $200 seems pretty high for boring a twin. I thought my bike dealer was high @ $140.

 

[sreimer]

What do the bores measure? Just looking at them is not an effective way of measuring

 

[catastrophe]

The bore was measure with a micrometer. There @ 75.02mm before a hone. Its going to take up a few thousandths so the question is if the bore ends up at 75.05mm can we still run a stock piston and ring set? Can we get and oversized ring to fit a stock piston? Or does it need a one over bore and piston ring set???

 

[sreimer]

Getting the next size ring and filing the end gaps to get the proper gap is a common thing in the racing world. Also I doubt it will take a few thou to clean up honing. Maybe a few 10ths. i would use a rigid hone, something that a local engine shop would have

 

[5twins]

The wear limit for the cylinders is 75.10mm so .05 would still be in spec. I hone mine with the bottle brush or Flex Hone type of hone. The stone hones will enlarge the bore if you're not careful and also don't give as good a finish for ring seating. Yes, you can just buy the 1st oversize rings, the ones that go on 1st oversize pistons, and file the end gaps to fit. They'll fit on your stock pistons just not in the bore (unless you file them).

 

[catastrophe]

Yeah saw the wear limit but it gives the skirt gap of .002. Which is leaving me a little jumbled. By the bore being bigger its gonna leave a bigger gap. Are the rings gonna fill the gap. I work in a machine shop on precision cutting and forming tool and to let sreimier know its going to take of about .002 to break the glaze and clear out the wear on his cylinder wall. What I need to know is... Ok the wear limit is 75.10mm a standard stock piston is anywhere from 74.945 to 74.965. Are stock rings gonna fill that gap and will a stock piston be to loose? Off those measurements there is a .006 gap between piston and wall. In the book it says if its .004 in that the piston is to small.

 

[5twins]

I guess the only way to put your mind at ease is to have it bored. Grab your wallet and go for it, lol. Personally, as I said, I don't bore them unless the cylinder is all scored up. Are they perfect? no but they run fine for several 10s of thousands of miles. Then they need bore on the next rebuild for sure.

 

[retiredgentleman]

Maybe I can add to the post.

Measure the clearance between the piston and the cylinder, with the piston at the bottom of the cylinder. Use feeler gauges.

You should not be able to insert a 0.004" gauge on the thrust side. 0.002" is good................0.005" is bad.
I measured 0.003" on my re-build ( I just installed new stock rings on my stock pistons) and my engine runs very well.

My cylinders were 2.9535" (75.02 mm), the same as you measure. My pistons were 2.9514",and as mentioned I had a 0.003" clearance.

Cylinder taper should not exceed 0.002" top to bottom.

The rings are not used to "fill the gap". Cylinders are matched to the piston for the correct clearance. If the cylinder wear limits are in spec. and the cylinder to piston clearance is in spec. ,then reuse your stock pistons with stock rings. You could also use the first oversize rings, as 5twins mentioned, and file for the correct end gap.

If not in spec then go with first oversize piston and have cylinders bored to match the pistons.

Use a flex-hone (ball hone) as 5twins mentioned. It will deglaze without removing much material.

 

[XSLeo]

If your local machine shop charges $200 for boring. Thats rediculous. The local speed shop that does engine work for many of the local racers charged me $35 per hole. I supplied the pistons. They were NOS second overs from Ebay, $50 for the pair. $35 x 2 = $70 + $50 = $120 for the rebore.
Mike's stock bore kit is a nice kit. It is built from the same big fin cylinders the 750 kit is made from. The extra cooling is a good thing.