On the inside of my Yamaha piston it says Yamaha in one place and also IZUMI 7. What does the 7 stand for? I've found one that looks to be identical except that it says IZUMI 6.

I'm pretty sure "IZUMI" is the foundry/manufacturer. The numbers are probably some internal factory mold numbers or something.

The critical number is the size stamped in the crown... keeping in mind that there are some minor variances in size... for example all 0.25 mm oversize pistons (stamped "25") are not going to measure exactly the same. This is why the machine shop needs the new piston to measure so they can size the bore correctly to fit that specific piston.

The exception is Yamaha standard pistons which are sized accurately... on the production line they measured the finished bore and matched it to a piston to get the correct clearance.

DT360 pistons are a little rare... if you haven't contacted Enduronut yet you probably should. He has a supply of pistons and rings and probably will be cheaper than some of the crazy prices out there sometimes... and he likely has whatever you need.

Thank you,Mark, I'm trying hard to figure this stuff out on my own without bugging folks but having some trouble doing it on my own. All knew to me so I do appreciate any help I can get.

I contacted Enduronut but he doesn't have any std. sizes left, just oversize. Still not positive which size I need yet.

I bought a telescoping gauge set and a caliper this morning at Harbor freight in an attempt to get measurements on my bore so that I would know what piston I need. I was surprised to come up with measurements showing that the wear isn't nearly as bad as I had thought it would be. I've never took bore measurements before but assuming I didn't mess it up it looks like I may be able to go with a std size piston. I've sent my measurements to Enduronut to get his opinion on that.

Right now. I'm waiting on my rod kit which has a delivery date of Oct. 29th and in the meantime I'm trying to get the piston size figured out so I can purchase it and be able to send everything to Bill Bune all together.

My old piston is shot, looks like the top ring broke and took part of the top groove with it. The numbers stamped on it are 977. The one I'm looking at as a potential replacement is hard to read on the pic seller has put up but it appears to be 96., definitely not 977.

You can use a Honda FL350 piston if you want to make a spacer for the cylinder base. 4mm/.160" thick aluminum spacer, two .030" thick gaskets and your deck height will be right on the money.

The FL350 piston is available from Wiseco from 80mm on up. I prefer these to old stock Yamaha pistons, never been a fan of dykes rings.

The concern I have with whatever your bore measurements are that they need to be compared to a piston measurement in order to determine clearance... that's the missing piece.

Can you post a picture of the numbers on your piston? Sometimes there are a few stamps and it can get confusing.

Standard bore sizes are "nominal" and very close to the size in the specs. So if bore was exactly 80mm, the standard piston will be smaller than 80mm to get the required clearance.

* A piston marked 96 should be 79.96 mm

* Clearance spec new is 0.04mm to 0.045mm. Maximum allowed clearance is 0.102mm

So that means the 96 piston would be a perfect fit in an 80.00 to 80.05mm original bore. (80.00 - 79.96 = 0.040mm)

If your piston is a 977, then even if your bore had zero wear since new, using a 96 would add 0.017 mm clearance beyond the original 0.040 clearance.... but then it's very likely the bore is worn so you'd probably be very close to maximum allowed clearance. Would it be worth it to install a new standard piston and have it rattle and have to pull it apart again soon no matter to have it bored and another new piston installed?

If you found a standard piston and rings super cheap... then maybe... but most likely it would be better and cheaper in the long run to bore and install an oversize piston.

Or if you'd like to use the FL350 piston turbodan is recommending... since you're replacing the rod, just use the 5mm shorter rod and should be very close without a spacer.

P.S. I think the FL350 piston might not have the windows in the skirt... can't remember for sure.

Oh thank you for the detailed explanation! After reading your post I think going oversize and boring to fit is the way I should go. I'd been thinking of that anyway since Enduronut only has oversize and I was having a lot of trouble trying to find a std and not even being sure a std would work. There are quite a few std pistons for sale on ebay but most are obviously aftermarket and not genuine Yamaha pistons and even the few that look to be original are questionable as to whether part numbers give are correct. I've seen several that claim the piston fits both 250 and 360 cylinders and I don't see how that's possible. Anyways, I'll wait to hear back from Enduronut on what he thinks of my measurements but thinking that first oversize is probably the way to go. Thanks again!

Thanks turbodan, to be honest I need to keep everything as simple as I can and I'd rather just go with Yamaha pistons if possible. I don't even know how I'd go about cutting out the aluminum spacer either so I'm going to try to go with Yamaha pistons if possible and if not then I'll probably be back to bug you for more info about building the spacer and such. Thanks again. I appreciate your input.

You are correct. Comes with no windows in the intake side. I want to try one like that one of these days, just to see what difference it makes.

There are a few ways to make a spacer. I did it the hard way, hacksaw, drill and a dremel. Whatever works. The material is available on ebay for a few bucks. I used fel-pro karropak .030" to cut out the base gaskets. Stock gaskets are .020" and for the price of one of those you can get enough gasket paper to make a heap of them.