Top end tear down, how do you determine ring and piston wear limits?

[Evilclone13]

Hey all,

I've got a 79' and a 75' that are both burning too much oil. The 79' is so bad, it goes through 1/2 quart after 80 miles of riding! Both engines have about 11000 to 13000 miles on them. The bottom ends are good, but they are both smoking out the exhaust and leaving oil on the plugs. (even fouled one plug because of the oil build up) I wasn't looking to have two bikes at the moment, but the 75' only cost me $150, so now I have two engines that need the same work. I'm looking to tear down both of the engines and do the usual stuff to them, cam chain guide, cam chain, seals, gaskets, re-lappig valves/seats ... etc. Since this will be my first time doing this, the thing I'm not sure about is, what are the specs to determine what the limits are for needing new pistons, and how to figure out what size rings I'll need after honing?

Also, where do you guys prefer to get your engine parts from?


Any information would be greatly appreciated, Thanks!

 

[retiredgentleman]

Hey all,

I've got a 79' and a 75' that are both burning too much oil. The 79' is so bad, it goes through 1/2 quart after 80 miles of riding! Both engines have about 11000 to 13000 miles on them. The bottom ends are good, but they are both smoking out the exhaust and leaving oil on the plugs. (even fouled one plug because of the oil build up) I wasn't looking to have two bikes at the moment, but the 75' only cost me $150, so now I have two engines that need the same work. I'm looking to tear down both of the engines and do the usual stuff to them, cam chain guide, cam chain, seals, gaskets, re-lappig valves/seats ... etc. Since this will be my first time doing this, the thing I'm not sure about is, what are the specs to determine what the limits are for needing new pistons, and how to figure out what size rings I'll need after honing?

Also, where do you guys prefer to get your engine parts from?


Any information would be greatly appreciated, Thanks!

The specs for piston/cylinder/rings etc. can be found in the Yamaha Service Manual. Its available at www.biker.net and should be read thoroughly.

1/2 quart in 80 miles....................head gasket is in bad condition and is passing oil from the camchain tunnel into the cylinders. That may be due to the large engine studs losing their proper torque (30 ft-lbs) over the years. The rubber coated acorn nut washers were known to back off over time. That's why most lads use copper washers nowadays, on the 4 outside acorn nuts, for engine re-builds.

When you open up the top end you will see a ton of carbon build up on the pistons and on the head.

 

[bikefreak69]

Poor mans down and dirty way to check taper in cylinder:use rings to check end gap-larger gap means more wear. I have checked this method against using telescoping gauges and it is damn close. Ideally everyone would use a nice dial bore gauge to measure the cylinder and an outside micrometer to measure the piston itself.

 

[XSLeo]

I shop around Ebay. I got lucky and picked up NOS second over pistons for $25 each.
You can often find them there for batter prices than Mike's or other places.
On the specs your repair manual has all that info in there.
If you don't have a manual you should get one. I like real books, downloads are ok but I like the feel of a book.
The down loads you can print out the pages you need and take them to the bike.
You can take the cylinders and pistons to a machine shop and get them measured for a small amount. They have the tools and experience to do it accurately.
Leo