Ford Mustang Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter #1
So I have been running my 4.1 6cyl on oversized piston rings for a while, but recently have had some issues with blowby. I am going to go for new oversized pistons.

Anyway, my problem is that I have no idea what size would be good. Should I go the smallest size next to what I've got in it right now? Will larger pistons give better performance (a friend seems to think so)? Should I buy cast, forged, or even racing pistons?

Any advice would be super helpful. Thanks!
 

·
Registered
Joined
·
2,747 Posts
You should measure the bores and see if they are round as well. Not likely you can just slap in over sized pistons without properly machining the block first.
 

·
Registered
Joined
·
8,022 Posts
...oversized piston rings...
Say what? Rings are selected based upon the piston grooves and cylinder bore size. There is only one size that will fit and work. I know of no such thing as an 'oversized' ring. The difference between a set of +20 rings and Std is that the +20s are about 0.060" longer (oversize x Pi) to match the increased cylinder circumference; in their other dimensions they are identical to Std rings. If you try to put even +10 rings in a standard bore...they won't fit since a normal ring gap is less than 0.030".

Its not just rings that wear, the piston grooves also wear and won't hold a ring properly. Since sloppy grooves will ruin new rings quickly you almost always use new pistons as well as rings.

Before changing rings and/or pistons you need to disassemble the engine and measure what you have as was suggested above. If the cylinders are worn in any dimension then you need a re-bore (not all that expensive) with new pistons. Only when you know what size the re-bore will have to be can you concern yourself with pistons or rings. Without any other changes from stock a standard cast piston would be what you need. I don't own a 6, but for a 289/302 the factory-style, cast pistons, in any normal oversize, are about $15 each from my machine shop.

No, a normal over-size re-bore will not give you any measurable increase in power. You will only add 1 or 2 cubic inches which isn't enough to make any appreciable difference. You will just be solving the problems of a worn engine.
 

·
Registered
Joined
·
4 Posts
Discussion Starter #4
Thanks. I am definitely getting the new pistons and a rebore, but I guess my question is this: What size should I do the rebore?

If I understood right the best would just be the least increase or bore possible right? right now I believe they are +0.020 so +0.040 would be my best option? (forgive me if those numbers aren't quite right... I just remember the bore size went up by something with a 2 in it.)

As for the ring/ piston confusion, I rebored it, but just kept the pistons and put larger rings on them. A friend told me that would work, but I have since learned otherwise. If I rebore it there will always be new pistons.
 

·
Registered
Joined
·
8,022 Posts
As for the ring/ piston confusion, I rebored it, but just kept the pistons and put larger rings on them. A friend told me that would work, but I have since learned otherwise.
Ouch! You are lucky the engine even ran. Blowby should have been horrendous and the sound frightening. You got some terribly dreadful 'advice' which is totally NOT true. You would have been better to put in new rings and NOT bore it. The piston to cylinder clearance on your 6 is 0.002" MAXIMUM. You can't bore it +.020" (10x times max clearance) and expect it to work without new pistons. You can never bore it at all without using new pistons. Pistons are cheap ($15), why even consider doing something that has no chance of working? A set of pistons probably costs less than the cost to bore the cylinders.

By trying such a stunt you likely screwed up the cylinder walls and may have to bore it again. There is only so much that can be bored out before you have to spend $100 a cylinder and sleeve the engine. At that point it would probably be cheaper to just throw away the block and get another one.

There is no way any of us can tell you what bore size to shoot for. The condition of the cylinder walls determine how much boring is required. You want the minimum to clean up the bores and only your machine shop can tell you how much that will be. Work with your machine shop, they will know what needs to be done.
 

·
Registered
Joined
·
4 Posts
Discussion Starter #6
Thanks!

Will do, and yeah, the rebore was .002". I'll just go get the smallest rebore possible and get the new pistons.

Although, the noise wasn't bad or anything, and the engine ran fine for a bit. Maybe he had me do something other than what I thought. Regardless, I'll need to fix it quick.
 
1 - 6 of 6 Posts
Top