Piston-to-wall clearance. I'm confused
 

Hi guys,

I just dropped a 1.8l block & crank of at my local machine shop.

Cleaning + bore (if needed) + hone + decking (if needed) + polish crank. Check everything of course.

I was planning on going with Supertech pistons and that's where the confusion sets in: I will only know what size piston to order once I know what size bore it needed. But in order to specify the right bore size I need the actual size of the pistons + wall clearance. And that I can get only when I have those pistons in hand. So now I have a chicken and egg problem. I need to get the bore done to order the pistons, but I need the pistons to specify exactly what size to bore & hone to.
I'm overthinking this, but what should I do?

ST

biggest pistons you can get lolol

The shop will be able to tell you what bore options you have, then you make your choice.
Or you gamble and order the pistons you want, and if the block is unsuitable you hunt down a new block.
84mm is seldom a bad choice (unless you have regulations to follow).

I found some 84mm slugs and gave the machinist two motors to chose from. At least one of them should be ok. If not, sell both shortblocks and a get fresher one.

So just to get this clear in my mind:

It seems that there are tow different factors here that cause me additional confusion:

1. Ring gap. When I search online for piston/wall clearance, I often get references to an application factor (for example from Supertech: Moderate turbo / Nitrous 0.0050). This is used to calculate the ring-gap by multiplying it with the piston diameter. So for a 84mm piston with factor 0.0050 we would get 0.42mm.

2. Then there's the actual difference between the bore radius and the piston radius. So far I was unable to find any decent info on this.

What do I do from here?
- Let's assume the machine shop tells me: "We can bore it to 84mm and you're OK."
- Then I order the pistons, take them to the shop and he will measure the pistons and we'll discuss the application. Ring gap is easily determined as mentioned above.
- But we still don't know what actual Diameter to bore/hone the cylinders to.

I've read that 0.030" would be good for ST pistons. I've read 0.020" would be better. Do we measure the pistons individually and then bore each cylinder to a slightly different size?

0.030" seems quite a lot to me. I come from the metric world and 0.030" x 2 get's us a diameter of the bore that is 1.5mm larger than the piston. That's a lot of slop....

The machine shop shouldn't need to go too big in order to get a clean bore unless the cylinders are really out of round or galled up. Any over-sized piston should do, 84mm shgould be fine. Buy the pistons, give them to the shop, and they can match everything up. Yes they will measure the pistons and then bore the cylinder accordingly. They will actually be measuring the bore with a dial bore gauge during the process to get it perfect. The pistons should all be identical from the factory so each cylinder should therefore be the same. The ring gap is determined afterwards and the rings can be filed if they're too tight.

The piston manufacturer should give the exact measurement of the piston and then your machine shop can bore over that without even needing the pistons technically. I'm surprised they haven't given you more guidance on this.

How confident are you in your machinist, and his/her capability to hold the size you specify?

How confident are you in the piston manufacturer holding the diameter size?

I had the same question when I did my V6, the first engine I built myself. The gist I got is that, as was said earlier in this thread, the machinist takes the block as is and sees how f'ed up the bores are. He tells you how much he thinks he will need to go to make it all right again, typically machinists go up in standard size increments. You decide on a bore (your experience, the recommendation of your machinist, piston manufacturer, internet gurus, whatever) and get the corresponding pistons which would be made in the standard oversize. You set the bore (piston to wall) per your application/experience. Often the machinist will wait until he has the pistons to measure each and bore each cylinder per piston if needed. Because the machinist should be able to hold a tighter tolerance than the piston manufacturer, especially if you are buying off-the-shelf pistons.

Now for me, my machinist sucked and my piston manufacturer (custom wisecos) rocked awesome. I should have had the machinist just do the block and then measure the bores and have the pistons made to match each bore, which were done too big and didn't match one to the other diameter wise. The pistons on the other hand were all dead nuts what we spec'ed even with the skirt coating. Fortunately for me, wiseco said my large piston to wall was "what the big boost honda guys" run and I'd be fine, and sure enough I have no slap at all, no oil consumption, etc. Life is good, but theoretically not as good as it could have been.

you need to have the pistons on hand to bore out the block
order +1mm size. if your block is too shitty to be bored out to that size, get a new block. +1mm is good compromise between having cylinder wall thickness vs extra displacement.

make sure you tell your machinist exactly what specs you want. I asked for .004" piston-to-wall clearance on my 84mm wisecos

Depends on how much power you want. Loose clearances are good.

no need to multiply by 2. piston to wall clearance is measured by subtracting measured piston diameter from measured piston bore. 0.03" is a lot, yes. because you're off by a decimal point.

in metric piston-to-wall is measured in one-hundredth of a mm (e.g. 0.09mm is 9 hundredths. 0.1mm is 10 hundredths)
in imperial it's measured in thousands or ten-thousands of an inch (e.g. 0.004" is 4 thousands, 0.0035 is 35 tens)

A simple conversion is that 0.004 (4 thousands) is ~0.1mm (10 hundredths)

0.003" - 0.004" (0.07-0.1mm) is what i would recommend for a street engine that can run a lot of boost.

Thanks guys. Things slowly start making sense.

Tim: I don't trust this machinist. I never worked with him. The few basic questions I knew to ask, he answered to my satisfaction, but that doesn't mean much. He has a lot of work, has a few guys working in his shop and seems to deliver relatively fast. In addition he is only 5 minutes drive from home. I have access to some pretty decent measuring equipment, so I will be able to check his work before assembly.

0.004" makes MUCH more sense to me.

Edit: rest removed because it was clarified by later post.

Finally, (triple post for the win)

there's this thread which I found awesome when I was building my motor - Supertech p/wall clearance - Miata Turbo Forum - Turbo Kitten is watching you test compression.

Yes. If your machinist didn't know this already, find a new one ASAP.

.003", not .030" ;)

You answered it yourself in your first post. Only way to guarantee anything is if you physically have everything and measure yourself, give you specs to machinist, and verify their work after.

Try to work in thou's of an inch for this part. You'll notice a lot of info and specs are generally imperial and numbers are more intuitive to work with.

Figure out your ring gaps while the block's at the shop and then most likely you'll have to do some filing for the top rings at least if you go ST.

FWIW, I set my piston x wall clearance at 0.0033". Haven't run it yet but after talking with ST directly and referencing all the info on this forum I decided it was a safe # to run.