I'd take it +.060" and be done.

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Official Cookie and Mater Tormentor.

What Zilla said, and I said earlier... You won't hurt a thing.

Lou

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Home of Slant6-powered fun machines

though I have not had a /6 block thru a machine shop in years, (and the last one went to +0.030" at a machine shop that has been closed for at least 10 years now) the I have done a couple of each, 318s, 360s and 4.0 Jeeps. (at least those are Mopar-ish inline-6s, ha ha)
between 2 (different) machine shops. Asked (actually did a little begging and pleading on the last 360 and 4.0) that since I found 0.020-over pistons and rings readily available on each of these engines, to please only overbore to 0.020 if it would clean up there. nope. refused. If they bore at all oversize, it's automatically 0.030, minimum. I asked if 0.020 would have cleaned it up, and got a reply "didn't even try". I figure that (when they offered them) if Mopar Performance was only over boring their 360 Mag short blocks, that it would be plenty for me. And leave more "wiggle room" for future needed machine work. but no. they refused to even TRY to do as asked. They'd do 30-over, 40-over, or 60-over if I'd asked, or if the engine took that much to clean up but no less..... the last engine thru the machine shop I went somewhere else to see if THEY would do as asked (if the engine would "take" it) and nope. Picked up my machined block and again, 30-over, without even trying to see if it would have been "good" at what I asked them for.....

Did they say why they would not even try .020"?

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Dave

1977 d-200 crew cab ex-army pickup wants it's /6 back
1962 Valiant 2 door, 170, three on the tree
1972 d-100 parts truck
80 volare wagon now a parts car

just that "they don't do that, as much work to set up in the boring rig to do 20, as it is to go to 30, so they just automatically go to 30".... between the last 2 engines I have sent to machine shops, I asked about it on one of the forums, same question as you asked me. People there said "its my money, if machine shop A won't do as I instruct them, try a different machine shop". so I did. Same result at machine shop B.

Don't you want to individually match each piston to the bore?

So each bore could be +- a small amount of the overbore.

If you're doing +- a small amount for each bore, the overbore size shouldn't matter...…………


And what if you're going with metric pistons...…………..

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

That's done in the final honing if necessary.

Modern pistons are now made so accurately that a set will be so identical as not to matter.
That probably doesn't apply to pistons we might use for the Slant 6.

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--> Check out my FI Turbo build <--
--> And the race truck build project <--
--> The Diesel Corvette <--

I would say they don't want to bother with 0.020" on an old engine because it will often take more than 0.010" in radius to clean up any imperfections. This is sensible to me, and it saves them time (and thus you money and time) in checking or boring then having to re-bore at a larger size. If you rebore to 0.030" you can bore to 0.060" next and then 0.080" or 0.100" later as those are fine for most Slant blocks (at least the pre-80s ones).

Lou

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Home of Slant6-powered fun machines

I guess I have been lucky over the years in finding shops the will do what the customer wants, not what is convient for them. I used to have a great shop, I would ask him to do something, if he didn't agree with what I was asking, he would tell me, and why he didn't agree. Then it was my decision. Always work completed on time, to spec, and fairly priced.

I did have one shop that bored and honed a 440 engine .030 and I wanted .005 clearance, and he did .007. because he "thought" .005 was too tight. I made him replace the block. My money, my way.
The shop I use now treats me real good. gives me what I want, for a fair price, and good turn around time.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

Customer- I have a set of .020 pistons I want put in my engine and I need it bored.
Shop- Sorry, we don't want to do that for you, even though you are paying me to do it for you.
Customer- Good bye, it was nice talking to you.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

 it will often take more than 0.010" in radius to clean up any imperfections. 

have to agree with what Lou says,, If they only did .010 a side to get your .020 and some defect did not clean up, then where are you, and like he said there
is still plenty of wall to cut it again after another 200,000 mile down the road.
another thing to consider, when I say that I want a machine shop to do as I ask, that is in regards to clearances, chamfers and any other specific special machining that I want.
I always listen to and abide with the machines shop expertise, especially when it is on something as inconsequential as a .020 over bore vs a .030 overbore



I am not a mopar v8 expert but I believe that the v8's do not have the wall thickness to support a heavy over bore like a slant, hence a .020 may be appropriate for those.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

I agree with everything folks are saying. It is doubtful that I would go less that .060 on any new builds for myself. But my point is this. Just suppose I, no I meant someone , happens to have a set of .020 over pistons on a set of rods with a new set of rings. And I , I mean he wants to use them. If the machine shop says no we don't do .020, I would just move one and find another shop. That's just me. Your reactions may vary.

I am lucky I guess. Like Charlie I have access to machinists that do what I ask for at a fair price

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

It's pretty obvious why a shop would not want to do 0.020" overbores. They have probably found that half the time at least one cylinder doesn't clean up and they have to do the whole process over again. That costs them a lot more labor and they've already quoted a price to do the job. I'll bet if you told them you were willing to pay double if the 0.020" bore didn't work out they would be fine with it.

In a previous life I used to bore motorcycle cylinders for all the shops in town. Back then oversized pistons usually came in .25mm (0.010") oversize increments. I learned pretty quickly that unless the bore was in damn good shape to start with going only one size up was probably not going to do it.

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--> Check out my FI Turbo build <--
--> And the race truck build project <--
--> The Diesel Corvette <--

if you had a set of .020 pistons that you wanted tp use, but the bores needed a .030 to clean them up,,,

I had posted this info some time ago.

a piston skirt coating that allows pistons to be used in larger than normal bores

viewtopic.php?f=19&t=64548

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

wow, didn't expect to cause this thing to veer off so far....sorry. It may be possible that those engines would not have cleaned up where I wanted the machine shop to "stop".... (20-over) but its just the idea that I'm paying, and they weren't even willing to "try" to do as asked for any real reason besides "we don't feel like it".... I went so far as to tell than that IF it needed 30-over then so be it... but IF 20-over would suffice, then that was as far as I wanted to go.
as soon as I do some other "catch-up" around here, (ambition has to come back 1st, much of it's overdue "honey-do's like a bathroom remodel) I have a 225 slant that I want to play with, beef up, etc., but I have slightly different parameters than this OP, that I want my engine to be geared towards...
Dad Truck and Zilla know what I mean..... I've asked a few Qs on here, heading that direction, semi recently.