K321 Cylinder Bore/Piston Clearance

[J-Mech]

Checking piston to bore fit with a feeler (ribbon) gauge

Set the block on the bench. Take the piston and insert the wrist pin. You don't need to worry about putting the clips on it, but I would at least install one clip, that way you can push the wrist pin in and let it hit the clip. Get your ribbon gauge, (feeler gauge if you don't have ribbons). If you are using a feeler gauge, take the book of them apart, and pull out all the sizes between .003" and about .007". Lay them out in order. Turn the piston upside down, and hold it by the wrist pin. You can lay the block on it's side for this if you want, but I prefer to leave it upright. Advantage to laying it on it's side, is if you drop the piston on accident, you don't break it. I do mine on a work bench, so they only fall to the bench. No harm done. Ok... so you are holding the piston in opposite hand you prefer. (I hold it with my left hand because I'm right handed.) Hold the piston over the bore and slide it in the top (upside down) and in to the last ring groove. Leave the last groove sticking out. Now, grab the smallest gauge you have out. Hold it to the side of the piston, just slightly cocked away from the piston at the top, but the tip of the gauge at about the ring groove. Put the gauge at a right angle from the wrist pin. So if you are holding the wrist pin with the pin going from 9 to 3 o'clock, hold the gauge at either the 12 o'clock, or 6 o'clock position. Does not matter which. Now, slide the piston and gauge into the bore. Now, the ribbon gauge is flat, so you will have to help it get started as it bends to conform to the arc of the bore. Once it starts, it should go in. (I put a little engine oil on the gauge and cylinder wall. I mean, just a little bit.) You should be able to slide the piston and gauge together all the way to the bottom with no resistance at all. (Because a .003" ribbon should not be tight at all.) Do this procedure over using the next size up ribbon until you find one that makes the piston tight. Go back one ribbon in size. Put it back in with the piston. It should slide with some resistance. You should not have to force it hard, but if you let go, it should hold position in the bore. (Or at least try to hold position, mostly because you are bowing the ribbon in an arc to fit the bore.) Now, repeat the procedure in by rotating the piston in the bore. I will explain that better. Keep the block stationary on the bench. If the first time you did the procedure, the wrist pin was at 9/3 position, and the ribbon at 12. Then rotate the piston and ribbon so that the ribbon is now at 3 o'clock the wrist pin is a 12/6. Then do it again, 1/4 turn each time. Do it no less than 4 positions, but more if you like. MAKE SURE, that one of the positions you choose is the standard position for the piston when installed. Slide the piston and gauge together all the way to the bottom of the bore each time. This checks for any tight spots on the cylinder, or deformations. Usually, after boring, you won't find any. More over, this will confirm that the shop used a rigid hone to fit it, and not a flex hone. If you find a tight spot, the shop did not fit it using the proper tools, or the block got damaged between boring/ fit and you checking it.

When I build a Kohler, my target skirt clearance is .005" +/- .001". I don't like to see any tighter than .0045/.005". I will accept up to about .007" before I'm calling the shop. But I really like to see .005".

If you can find ribbon gauges in .0005" increments, buy those. I have about 10 or 15 ribbon gauges between about .002" and .010". I don't remember how many.... but I know where I keep them.

On how tight of fit it should be by feel.... it's just a "feel" you acquire over time and experience. You learn what a "slight drag" feels like after you do it a few hundred times.


I sell rebuilt engines..... Just saying. You guys can save yourselves the trouble and let me do all the hard work. Buy a short block and you can put the parts on the outside and get a warm and fuzzy feeling like you are a real mechanic. You will be happy. Your engine will last a long time, and I will be happy with money in my hand and another happy customer.