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YZ cyl and head identification

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I bought a basket case 2001 YZ and it came with 2 engines out of the frame. I'm trying to figure out what's what. I figured out the bottom ends one is a 2001 and the other is 2000. Can i tell by the casting # on the cyl. and head?

One head is (4JY)---Y-2 the other is (5MV)---G-1. I only have one cyl. and it is (5HD00)---Y-2 It also has some stamped #s near the power valve but cant make them out. Thanks for any help

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the numbers for the heads:

4YJ head 1999-2000 yz125

5MV head 2001 yz 125

cylinder:

5hd00 2000 yz 125

from what i can recall, hope this helps

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Here's the Casting stamp to determine the year model of the cylinder and head . The first 3 digits determine the year model the rest is production line order etc. The stamps on the head are on the top. On the cylinder, it's found where it says the capacity of the engine i.e. 124-cc stamped on the side.

97 & 98- Head: 4SS, Cylinder: 4SS.

Cylinders run a open water jacket on the base. The coolant hose fitting on the head is located on the rear, next to where the bracket bolts onto from the frame. These models ran a male type expansion chamber with o-rings on the outside. They fitted inside the exhaust port of the barrel. More basic powervalve design.

99- Head : 4JY, Cylinder : 5ET.

Cylinder runs a split water jacket on the base. Power valve shape slightly changes to and oval shape. The coolant hose fitting on the head is located on the rear, next to where the bracket bolts onto from the frame .Expansion chamber changed to a female fitting with o- rings on the inside. Slid over the male exhaust port, which is the same to 2004.

2000 - Head: 4JY, Cylinder: 5HD.

Powervalve shape changes again, more to a rectangular shape with a circular stop. The coolant hose fitting on the head is located on the rear, next to where the bracket bolts onto from the frame. Runs a different base gasket, looks like with rubber lips for better sealing.

2001- Head : 5MV, Cylinder: 5MV

Pretty much the same as the 2000 model. The head design changed. The Coolant hose fitting moved from the rear next to the bracket to the front. Head stud pattern on the barrel had to change to suit the new head design for the coolant hose new routing.

This design was the same up to 2004.

2002 - Head : 5MV, Cylinder :5NY

2003 - Head : 5UN, Cylinder: 5UN

2004 - Head: 5UN, Cylinder : 5XE

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HOly molely. I Just bought a yz125 the other night and found out it has an athena cyl on it. After replacing the spark plug I noticed that the head on the bike had the motor mount facing the front>i.e not being used. This is a 2000 according to the engine case numbers. Im wondering if they used the orginal head but had to turn it around because of the water inlet. Im so confused now!!! Does it matter where the water inlet goes?

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