General machining question

[Alvy]

So I've had my latest k301 at the machine shop getting bored .030 over. I gave him the block, he measured, told me I needed .030 over because of the egged out cylinder (block was already .010 over) I ordered the rebuild kit from the bakt4kids seller on eBay and brought him the piston like normal.

Here's the problem. The machinist, who I've used for 3 engines now, calls today and says there's a problem with the piston. He's claiming the "thrust face bore clearance" is off after boring. He says the specs show this measurement needing to be .006 to .0065. It is to be measured "just below oil ring and at right angle to the piston pin". He said that is now .012" per all the specifications being at .030 over. At the bottom of the piston skirt, it is at .006" currently. He is concerned about this as its too much. He says it should be closer to .001" at that point. I called the bakt4kids guy today and he informed me that the .006" is to be measured at the bottom of the piston.

Who is right? These pistons are Taiwan but I've never had this issue. Machinist claims that Pistons are cam ground and fatter at the bottom than in the middle and I saw the kohler k series paper manual he has today and he's had it for 30 plus years. He's been in business a long time and always thorough and fair. He says if I get an oem kohler piston it will fit perfect.

I don't know what to do here. I'm assuming, if I can even find an oem kohler .030 over piston, that it's expensive. I'm now wondering if he didn't measure the piston first and went by the book but it makes no sense as he always waits for the customer to bring the piston in after he measures and before he bores. Am I overthinking this and will it be fine? He says piston ring gap is right on the money. It's a million times better than it was, egged out bad and could feel the recess in the cylinder, was smoking like crazy. My worry is the piston slap will be too much and noticeable maybe when cold? Any insite will be appreciated from all. Thanks.

[J-Mech]

I understand his concern, but it will be fine. .006" at the bottom is wider than I'd like to see it, but no where near as tight as .001". I never measure by the instructions sent with the pistons. I go by skirt clearance listed in the Kohler manual.
A Kohler piston would be big bucks, and it may not fit any better. At which point, he'll be sleeving it.

Run it.

[bocephus1991]

I had the same issue with my k301. I wouldn't worry about it like Jon said, run it! Mines been running fine since I overhauled it.

[Alvy]

Thanks guys, I appreciate the responses.

[J-Mech]

Kohler manual says that the thrust face to bore should be .007"-.010". I think that is a little tight for the piston's they make now days. I usually like to see the clearance between .003" to .006". I just assembled a K161 last week and I can't remember where it was.... I want to say it came in at like .005"

I use ribbon gauges to check the thrust face. For those not familiar with them, they are just very long feeler gauges. Makes it easier to put in the hole with the piston. I check it in several spots, pulling the piston out and reinserting it with the ribbon in a different spot in the bore. Always placing the ribbon on one side of the piston 90 deg from the pin. I keep the ribbon in the same spot on the piston. I just pull it out, hold the ribbon and reinsert in a different "clock" position. You should be able to slide it all the way down without much force. I graduate ribbon sizes until it gets tight, at which point the last gauge that let me slide the piston down the bore is used and I check the different positions. As long as there are no tight spots, it's good. Now, the bad thing is, this is what the machine shop should also do, so unless it's too tight.... not much you can do about it. Too loose is a very bad thing.

Hope this helps.

[bocephus1991]

Just remember to check end gap on your rings too! Refer to the engine service manual for the spec. I had to file mine a bit. I think the machinist was a little put off by the fact I wasn't using (genuine kohler) parts.