K321 Cylinder Bore/Piston Clearance
 

I just got my block back from the machine shop along with the new .030 piston. I measured the new piston with an outside micrometer (3.521) and and the cylinder bore with a telescope gauge and an inside micrometer (3.536) with a difference between the piston and bore of .015. I know the telescope gauge isn't the best way to check the bore but between that an the inside mic I think I'm close. I also checked the ring gap and found that to be .035. If I'm reading the service manual correctly for a A type piston, which I have, the clearance should be .007/.010. If my measurements are correct the cylinder is at least .005 to large. Should I assume the shop didn't bore the cylinder correctly and take it back for them to verify? If so should I be looking for another block or maybe a sleeve?

When you had the block bored, did you send along the Piston so they could use that as a "bench mark"...??

:bigthink:

You measure skirt clearance with a ribbon gauge. The way you did it will not be accurate. You can order a ribbon gauge, or you can use a feeler gauge. Only difference between the two is a ribbon gauge is about 12" long. Piston clearance should be .0035"-.007" on a new bore. I like mine about .005"-.006". .007"-.010" is too wide.

Where on the new piston did you measure? Pistons are tapered the top is smaller to allow for expansion. There is a specific height from up from the bottom of the skirt where you need to measure at.


I usually send the new piston in so they can match it properly for the correct clearance and then check it with a feeler gauge after the fact.

A good machine shop won't bore a cylinder without a piston. It's not really an option to get it bored without the shop having it.

You should always double check your machinist. (Just making that statement in general, not to anyone specific.)

Very true, I wouldn't trust a shop that didn't.


Mine will rough bore a damaged cylinder just to determine what piston size is required. Once that's done pistons are required and then they finish up boring and honing with a torque plate on the engine.

I'm usually doing Mopar V-8 builds. I have a 360 small block that's getting a 4" stroker crank build coming up for my car. A 408 stoker engine is a great driver, my last build was a 410 that had 488hp with 520# of torque that would run on 87 octane pump gas.

The shop that bored the block furnished the piston and the gasket kit. So I would think he didn't bore the block until he had the piston in hand.

The service manual indicates to measure "Thrust Face to Bore Clearance" an the Type A piston just below the oil ring groove. Is this not the correct location?

I'll try to check it with a feeler gauge and see what that shows. In the mean time I'll get a couple of ribbon gauges. How far down do you slide the gauge?

So are ribbon gauges the preferred method for checking clearance or would a bore gauge be and why are telescope gauges not accurate?

The caliper I use has 4 points of contact and a dial gauge. I've seen some telescoping ones that do not use a gauge and only 2 points of contact. Which style do you have?

It is just the 2 points with no gauge. Like a T.

I just checked the clearance with a feeler gauge and a .011 gauge seems to fit with drag and a .012 fits very tight. So it looks like it's about .005 to wide?

Ouch. I bet that cost you a pretty penny! :bigeyes:
Usually shops will only buy genuine Kohler parts.... Genuine Kohler is overpriced.


Just a quick question.... where are you getting that info from? How do you know you have a Type A piston?? It doesn't really matter to me. When you measure with a ribbon gauge, it makes no difference what "type" of piston you have. Stop measuring the piston and the bore and doing math. That's not how you measure piston clearance. Besides, with the tools you are using, there will be a pretty large margin of err. Possible .002" or more.


I need to make another tech article on how to measure piston clearance since so many people think they need to rebuild their own engines. This procedure is easy, but may be confusing to describe without pics. I will try, continue below and I will go over it.

Why is a telescoping gauge not accurate? Because to use it, you set it down in the bore. Then let it expand. Well, how do you know you have it completely square in the bore? I know how to use them.... rock them around, yadda, yadda.... but without a gauge on it you can look at, you could wedge it in a little too much, or not rock it enough and make the setting inaccurate. Then, you take it out, and use another tool to read that tool. So, there is another possibility for err. With a bore gauge with a dial on it, you can see what the tool is reading directly. But, keep in mind, with a bore gauge, it has to be calibrated to something... or it doesn't read anything. I had a set of gauge blocks made for each stock bore size of all the Kohlers. Then I can zero the gauge to what the gauge block is and read the actual size of the bore. (The gauge blocks are stamped with their actual size, which is close to the stock bore.)

Bore gauges are for measuring the bore. But I don't use them for checking piston fit. I want to feel how tight the clearance is. If you use two tools, and do math to get fit, what if a tool is out of calibration? What if a tool is cheap and inaccurate? Besides, those tools are expensive, and you have no need to own them for this job. A set of ribbon gauges are cheap, and completely accurate. I have all those tools, and still use the ribbon method. I can actually feel the fit and know if I like it or not.


Well, that depends on how you checked it. Where did you put the feeler gauge?



I'm going to make another post to explain the procedure for checking fit with a feeler gauge. This post is pretty long already.

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

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. :biggrin2:

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. :BlahBlah:

That’s not good Steve. If it is truly .011” it’s going to make a lot of noise if you go back together with that one.

Good info in this thread as this is one of the many important measurements that really matter when putting one back together.

Good write up Jon. If more guys would use ribbon gauges instead of snap gauges/micrometers, they'd see a lot better results from their engine "rebuild."

Great post Jonathan. :ThumbsUp:

Thanks for taking the time to do that.:beerchug:

Nice write up. :beerchug:
You are making too much work out of operating a telescoping gauge. You are close to doing it right but there is no monkeying around with it in the hole. Set the tension on it when it’s at an angle, rock it to the other side while hanging on to just the knob on the end. It will find the largest size on its own. If you hang on to just the end, you can’t make it be anywhere other than the largest part of the bore.
Not saying bore guage isn’t better. Just trying to make it easier.

Thanks for the great explanation Johnathon. I have ordered a few ribbon gauges and will put this to the test. I did use my feeler gauges and they seem a little clumsy being so short. Thanks again.

I got the new ribbon gauges and have verified the piston clearance to be .007. However the ring gap was way out there around .040. The shop ordered a new piston and ring set and now with the replacement rings the end gap is still a little wide @ .020.
I believe the guy bored it out a little larger than optimum but now that it's bored .030+ I guess I'll have to live with that and keep my eye out for another engine or block for the future.
I have the engine back together and in the 1450. It's sounding pretty good so we'll see how she goes.
Now on to the other issues with cracked sheet metal and air cleaner housing but that will be another post.

Well, I was going to make some suggestions, but since you already have the engine back together it doesn't really matter now. :angry:

I sure would appreciate any suggestions you have. I don't plan on getting rid of this anytime soon and if this begins smoking again I'll definitely be taking it out again. I've gotten pretty good at the R&R.

By then it will be too late to fix it. Probably is already anyway. This was a machine shop screw up, and you should have made them fix it, (by sleeving it since you are already at .030" over) or buy you another block. Your choices were to try a different brand of piston, in hopes it would fit tighter. You don't say what the ring gap was with a new set of rings, but I'm betting the gap was too far. A Kohler piston and ring set, if you used after market, may have tightened it up. Or, you could have made the shop that screwed up sleeve it. Like I said, too late now. If it starts smoking, only thing you can do is sleeve it, or junk the block. Next time, ask before putting it together and running it. :angry:

Thanks for your reply Jonathan, very good advise. I do agree that this was a machine shop error and I should have had them make it right. I will be looking for a new machine shop for any future work I need done. Up here in Northern Michigan they aren't very plentiful.