scape41

Member
Mar 17, 2009
1
0
I am redoing the top-end on a YZ85 for my sons buddy. Problem is there is no marking left on the cylinder to tell me if it is A, B, C, or D. So, I pulled the cylinder to look at the piston. It too has no letter designation, but it is not stock. The piston has numbers in the top of it (may be a Wiseco) anyway, it all looks to be in much better condition than what I expected (based on condition of the rest of the bike).

I have a re-build kit "B" from the dealer but the piston is way too small for the cylinder. I tried to mic both the piston and the cylinder but my meter shows them both being around 42.5 mm and the book says they should be 47.--....

So, my questions are

1. How do I find out what cylinder I have without the marking?
2. The B piston is way too loose in the cylinder to work..I don't think the thing has been bored. It should be pretty obvious right? The dealer says to put the B in and run it, but man it is a lot more play than the wiseco piston....what do you guys think?
3. The numbers on the top of the piston are 805M04750 and 85S7B ---update...I found that this is the standard wiseco piston size at 47.50 mm. Which is bigger than the D piston in the manual (47.472).

Any help or advice would be great. This kid wants to ride with us this weekend and I would like to take him. His mom is divorced and he doesn't get much "guy" time. After replacing the swing arm/linkage bearings, brakes, fork seals I am ready to be finished with this thing!! LOL
 
Last edited:

Pete Payne

MX-Tech Suspension Agent
Nov 3, 2000
933
38
Get a better dial caliper or take it to a machine shop and have them measure it . The piston to cylinder wall clearance should be .0015" - .002" (inch not metric)Generaly the wiseco's fit in this range in a stock cylinder.
 

IndyMX

Crash Test Dummy
~SPONSOR~
Jul 18, 2006
5,548
2
Amo, IN
scape41 said:
I am redoing the top-end on a YZ85 for my sons buddy. Problem is there is no marking left on the cylinder to tell me if it is A, B, C, or D. So, I pulled the cylinder to look at the piston. It too has no letter designation, but it is not stock. The piston has numbers in the top of it (may be a Wiseco) anyway, it all looks to be in much better condition than what I expected (based on condition of the rest of the bike).

I have a re-build kit "B" from the dealer but the piston is way too small for the cylinder. I tried to mic both the piston and the cylinder but my meter shows them both being around 42.5 mm and the book says they should be 47.--....

So, my questions are

1. How do I find out what cylinder I have without the marking?
2. The B piston is way too loose in the cylinder to work..I don't think the thing has been bored. It should be pretty obvious right? The dealer says to put the B in and run it, but man it is a lot more play than the wiseco piston....what do you guys think?
3. The numbers on the top of the piston are 805M04750 and 85S7B ---update...I found that this is the standard wiseco piston size at 47.50 mm. Which is bigger than the D piston in the manual (47.472).

Any help or advice would be great. This kid wants to ride with us this weekend and I would like to take him. His mom is divorced and he doesn't get much "guy" time. After replacing the swing arm/linkage bearings, brakes, fork seals I am ready to be finished with this thing!! LOL


On my YZ85 the cylinder size is stamped into the base of the cylinder on the bottom corner near the clutch release arm.

If your current piston is stamped with that part number, you know you need to replace with a standard 47.5 mm piston, and then measure the cylinder for roundness and clearance.
 
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