Piston / Cylinder matching/marking questions

[sgallaty]

So I think I have an understanding of this but I want to lay it out and talk about it to make sure I've got it -

I'm scavenging an engine from several other engines - top end from one, jugs from another, crankcase from a third, odd bits from all over.

one question I have is about fitting the pistons -

My understanding is that the pistons are marked with a number that represents their size - without the "74" prepending it. So, we will have (74).093.

Jugs are printed with a blued number on the outside of the jug which represents the bore size over 75... so that number is (75).xxx

All of the jugs I have are marked "010"

My understanding is that The clearance between pistons and jugs should be .050 at the min piston dia - max jug internal diameter.

So clearance = (75).<jug printed number> - <74>.(piston printed number).

My questions then are :

1) Do I have this right?
2) So long as they are in tolerance, does it matter if my pistons are the same dia. (and assuming I use the right pistons for the right rods etc.)
3) what gotchas do I look out for? I already understand to keep the valve cover with the head, and the pin with the piston. anything else?
4) any parts I should always replace? Rings? piston pin clips?

thanks in advance

 

[retiredgentleman]

Your understanding seems to be correct.

The piston/cylinder clearance is measured with the piston at the bottom of the cylinder. You should not be able to put a 0.004" feeler gauge between the piston and the cylinder, on the thrust side. 0.002" ( 0.05 mm) is good, 0.005" is bad

Allowable cylinder taper is 0.002"
Allowable cylinder wear is limit is 2.9567" (75.100 mm)

You should read the Yamaha service manual which is available at places such as www.biker.net. The service manual has all the specs (I just mentioned a few).

If you go to an engine machine shop, they can measure the cylinders, and determine if a rebore to a larger size is required, or maybe just honing or deglazing. With badly worn cylinders you would buy the first size larger piston. The machinist measures the piston and bore the cylinder to match the piston.