Are cylinders bored and honed differently depending whether the piston is forged or hypereutectic? I built my engine many years ago with .030 over hypereutectic pistons. Due to some unfortunate events I'd like to swap in some new rods and forged pistons. Any problems I might run into?

Hyper's are a much tighter bore than forged pistons. You should be able to hone for your new pistons. Tim

The clearances are usually made on the pistons to fit a certain size overbore, example the forged Icon piston from KB would be around 4.4440 to 4.4460 for a 4.350 bore while the same size hypereutectic might be from 4.4978 to 4.4980
They would both have the proper clearances that KB would want for that size bore
On your deal I would order a set of pistons and rods and measure them before deciding to keep and use them, if they clearances are to much send them back and go to the next size over bore
BTW, you can order a custom set of pistons in any size you want, IE, for 4.4530 to 4.4590 instead of going to 4.460

If it was built many years ago, then you will have a tapered cly wall from the rings wearing more at the top where they load more. I just repaired a motor that someone had done just what you are talking about. It had too much wear for the new pistons to compensate for with the skinny racing rings and had a bad ring seal. We had to buy .040 pistons and hone it to the next size. I have done what Cab suggests and ordered custom pistons for the exact size you need. I’d take the motor apart and measure it exactly before I ordered pistons.

think you meant 4.34xx

should be ok to hone for forged.
If you were going the other way ....that would be a problem.

I's this a fresh eng. that was never fired?

bottom line is:
generally
stock cast piston installed .001-.002
hypereutectic cast .0015-.0045
forged .0035-.007


You would have to have the known current bore size current and condition. then measure the forged pistons to know how much margin you have to play with.

Unfortunately its a late 70's 360 that's already .030 over. I was hoping to just buy a set of scat I beams and icon forged pistons and drop everything off at the machine shop.
BTW there was an article somewhere (can't find it anywhere) with using said components they were able to internally balance a stock360 crank with very little mallory metal.

In answer to your question: stock replacement, hypereutectic, forged 4032 and forged 2618 all require different clearances. I listed them in order from smallest to largest clearance.

As you can see, there is a wealth of information out there. You really need to measure your cylinders with a decent bore gauge. That will tell you what you need to do.
For example, suppose your pistons were installed with 0.001" clearance. You may be able to use a 2618 piston and hone the walls straight again leaving 0.004" clearance.
This kind of honing isn't easy and in the long run you'd probably be better off going to the next oversize with new pistons.
So you see the condition of your cylinder bores is very important to know before your next step.

R.

http://www.us.mahle.com/media/motorsports/2017-application-guide-final-compressed.pdf
https://www.uempistons.com/installation_instructions/kb_installation.pdf
http://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc4032.pdf
https://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc2618.pdf

You are correct I was trying to use the stock 340 bore size, 4.040
I need to proof read my posts better

I understood that the type of oil return under the oil ring was the biggest contributor to design/clearance.

I've never heard that one before and I've been into engines since the '60s.

Spend a couple of hours going through piston manufacturers' websites and you'll see they differentiate clearance by piston material(s).

Mahle has been doing a lot of work on steel pistons (not cast iron) because the coefficients of linear expansion are so close they can run tighter piston to wall clearances for better ring seal. Once again the piston material determines piston to wall clearance. Interestingly Smokey Yunick mentioned steel pistons as something worth looking into about 20 years ago. Steel has much higher hot strength which could be useful in some applications.

R.

from a 21 year old speed pro catalog. Not denying material is a factor also.

Read the first sentence. What is the first factor mentioned? Material.

Then they go into great detail about the piston skirt design, that is true. There must have been some controversy in the piston market at that time, to cause them to put so much emphasis on it. But compared to material IMHO it is a minor consideration. I have never seen piston skirt oil drainback holes mentioned in any other catalog or website. I hadn't seen this catalog either, so it's interesting to see it now. This factor isn't mentioned in their current catalogs, as far as I know.

Of course you are free to disagree.

R.

It's all good I believe for instance that the reason for running a factory cast piston so tight as opposed to a forged one set up loose is mainly in the design parameters. One of which is material. Some good reads.

http://blog.jepistons.com/understanding-piston-skirt-design

http://www.enginebuildermag.com/
from this article
"Some people think that the same thermal characteristics that allow forged pistons to run cooler also causes them to swell more as they heat up. Consequently, there’s a common misconception that forged pistons always require greater skirt-to-wall clearances. This is a notion that isn’t necessarily true because clearances depend on the type of alloy that’s used in a forged piston, the design of the piston itself and the application in which the piston will be used. [/b]Some forged alloys actually have a lower coefficient of thermal expansion than the alloys commonly used in conventional cast pistons!"[b][/b]

If a forged alloy expands less why would we run it looser. For performance reasons, which requires a skirt redesign.