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Discussion Starter · #1 ·
how is bore size measured? I bought this 2006 Rhino 660 used and was told it was a 720 big bore. Didn't take long to blow head gasket. I wanted to get back to stock size. I bought a replacement cylinder & piston and was told it was considered one bore size over, so almost stock.

I measured the top of cylinder with my digital caliper. My OLD head is 101.6mm (4"). The replacement head is 100.7mm (3.96"). So how do those convert to bore size? I thought the 101.5 = 686.

ALSO, what does a blown head gasket look like? Mine looks normal to look at it, no obvious blow-out marks. I don't know what a blown head gasket should look like. I know there is all kinds of coolant on piston, but my oil level never went up, so wherever it leaked, I think it went into combustion chamber and burned off. In fact, my oil level would go down, and of course, I went through coolant like crazy. It would create an air lock on a short ride it was so bad.

when Yamaha manual describes disassembly, they say to take off cam sprocket. This is second rhino 660 I've done piston replacement on, and both times the cam wiggled out once the tension was off and exh side tension rail was removed, so no sprocket removal.
 

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I'm not sure if you can go 720 without getting it stroked... so the piston size could be the same, but the stroke is longer.

Apparently you CAN get the cam out without removing the sprocket, but I never tried that. I've always removed it, but it can make reassembly take a few minutes longer.

Not sure if you can 'see' the blown head gasket where it was leaking...
 

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102mm bore is 686. 102 bore with a +4mm stroker crank is a 720 (actually 719).

100.5mm bore is 666 when using stock 84mm stroke :eek5:
101mm bore is 673 when using stock stroke.
105mm bore is 727 when using stock stroke.

You likely have a 4mm stroker combined with a 102mm bore. Make sure you check the compression height of your new piston (compare it to the old one). Google compression height to see what it refers to.

As for the head gasket, sometimes they are torn and are easy to see. Soemtimes they are torn slightly and difficult to see. Soemtimes they are not torn and still leak. You need to make absolutely sure the head and cylinder mating surfaces are completely flat. Call us tomorrow if we can help with anything. Typed explanations can get quite lengthy.
 

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Discussion Starter · #4 · (Edited)
thanks guy. I was going to take it to machine shop.

Is the 4mm stroker achieved by a longer conrod? which means I can't go with the replacement stock jug and new piston for that jug? I assume with a stroker, you also replace with a taller cylinder?

both cylinders are off right now. would I be able to measure the length of both and compare them that way?

I googled it like you suggested. The top of the piston looks confusing since there is a raised area on the top of piston, and a lower area (on side view). Almost like a shelf. I'll phone you.

thanks Chris (& Canucklehead).
 

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MOST 4mm Rhino strokers use a shorter rod, allowing the use of a std type piston. SOME 4mm strokers use a stock length rod, requiring the use of a piston with a shorter compression height. If the pistons have the same general height when lining up the pin, then the engine probably has a short rod.
 

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Discussion Starter · #6 ·
I noticed the conrod says "Hotrod" on the side, HR 613 on the other side. So do I need to re-use my old cylinder and match up a piston? I had got a stock cylinder, bored once, and wanted to get it back closer to stock.

And I couldn't see any obvious weld areas inside the crankcase.
 

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I noticed the conrod says "Hotrod" on the side, HR 613 on the other side. So do I need to re-use my old cylinder and match up a piston? I had got a stock cylinder, bored once, and wanted to get it back closer to stock.

And I couldn't see any obvious weld areas inside the crankcase.
Obvioualy that is a HotRods crank. I don't know what the 613 means. You should call them to find out. Or, you can compare the piston compression height like I already told you about.
 
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