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post #1 of (permalink) Old 08-23-2005, 10:15 PM Thread Starter
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best boring size

Im rebuilding my engine im going to run a 4.0 head and MPI kit i want to bore my block over but im not sure what size would be better for this set up i am trying to decide between .30 over or .60 over .90 over is to much with how my driving is i will crack the cylinder walls by the coolent passages
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post #2 of (permalink) Old 08-23-2005, 10:52 PM
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Re: best boring size

When I did alot of this, we alyays went with as little being removed from the cylinders as possible. Sometimes that was alot, but most of the time it just took the next size larger piston to clean things up. I do realize the potental HP gains by going bigger, but they are marginal compared to the reliability of a stock block.
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post #3 of (permalink) Old 08-24-2005, 12:19 AM
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Re: best boring size

If you plan on doing much driving around, off road and on I would NOT go any bigger than .60 over and that is IF it really needs it. .30 over is great and a safe over bore to keep it running in the stock cooling capacity.

Ive seen to many .60 and .90 over bored overheat at slow speeds. [img]/ubbthreads/images/graemlins/wall.gif[/img]
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post #4 of (permalink) Old 08-24-2005, 06:32 AM
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Re: best boring size

I think a good general rule is to only take as much as the block needs. If you can go .30 over do that. If you need to go more than .60, I think I would look for a different block.
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post #5 of (permalink) Old 08-24-2005, 08:00 AM
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Re: best boring size

Listen to the boys!
They are telling you true...

There are better ways of making (or freeing up) power than by weakening your cylinder walls!

First of all, YOU don't 'BORE' the cylinders, the block manufacturer does that.
You 'Over bore' cylinders.
'Standard' overbore (overbore sizes that most manufacturers make off the shelf parts for) are the way to go.
Custom made or fitted parts are a PITA.

Have someone clean and check your block for cracks, and have someone competent with a bore gauge check your cylinders for current size, and have them determine what the MINIMUM overbore you can get away with.
This is no time for an amateur to try and beat the learning curve, so leave it to the professionals.
You will have...
1. A 'Funnel' shape. The rings wear more at the top (farther away from oil, and the soft iron cylinder material wears more where it's hottest.) of the cylinder. Most of us are familiar with the 'Ring Ridge' at the top of a cylinder.
2. An 'Egg', 'Triangle' or 'Clover Leaf' shape. The cheap factory piston was ground cold to be round, but does not expand round when it gets hot. Connecting rod bosses have more material and expand larger than the rest of the piston body.
3. Cylinder 'Slotting'. With crankshaft rotation, the piston is forced up the 'Bottom' side of the cylinder (not under excessive force) and during the 'Power Stroke' the piston is forced down the 'Bottom' side of the cylinder under tremendous force causing more wear in that track.

If your cylinders only need 0.010" overbore to clean all of this up, then by all means, leave the extra material in the block (cylinder walls) where it can do some good!
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post #6 of (permalink) Old 08-24-2005, 08:00 AM
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Re: best boring size

There is the option of 0.040 over too. And btw, don't go 0.30, 0.60 or 0.90 over. Go 0.030, 0.040, 0.060 or 0.090 over.
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post #7 of (permalink) Old 08-24-2005, 06:18 PM Thread Starter
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Re: best boring size

thanks for the opinions im probible going to go 0.030 over maybe 20 if the machine shop can get a kit
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post #8 of (permalink) Old 08-24-2005, 07:39 PM
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Re: best boring size

There are two kinds of 'Machine Shops'...
There is a lot more to it than just making some holes bigger in a hunk of cast iron...

The cylinders have to be EXACTLY 90 degrees to the crank shaft, the deck of the block has to be EXACTLY parallel to the crankshaft alignment.
The cylinder walls have to be thick enough so you don't get hot spots, and to check cylinder wall thickness, you need an ultrasound machine.

Head bolt & Main Cap holes,
Threads have to be chased, and head bolts oiled or you will not get the correct torque reading.
The top of the hole must be chamfered, or the top of the thread will pull up and keep the head/head gasket from sealing correctly.
The same goes DOUBLE for main cap holes! This is metal on metal, and chamfering is a REQUIREMENT for proper main cap torque.

Make no mistake, if your machine shop doesn't or won't do these things, FIND ONE THAT WILL...

One kind will overbore your engine, hone the cylinders and tell you they overbored 0.010" or 0.030" or what ever...
And then you order pistons in a 'Kit' (usually with wrist pins and rings...)
There are a lot of these 'Custom Engine Machine Shops' around, and I don't trust any of them.
They have the capacity and the tools to do the job correctly, they either don't have the training or just plain don't want to do the job correctly.

The correct kind of machine shop will tell you in advance how much of an overbore it will take to clean up the cylinders, and allow you to decide what type of pistons, rings ect. you want to use...
After a proper cleaning and inspection, they will know if your block is sound, reusable, where & how the cylinders are worn and how much of an overbore will bring them up to specs.

Rods should have NEW rod bolts installed, resized in pairs, and then magnifluxed before the piston is installed.
Do NOT let them talk you into oversize rod bolts in cast iron rods! The steel rod bolts are already stronger than the cast iron rod, just use a good quality rod bolt and nut.
If you have forged steel rods, then by all means step up in rod bolt size.

When the pistons and rings come in,
THEN they will overbore the cylinders to fit individual pistons so the piston to cylinder wall clearance will be correct.
Ring gap should be specific to the bore it's going in, just like the pistons.
'Kits' or 'One Size Fits All Holes' will get you into trouble, or will leak like a sieve.
Make SURE your rings fit correctly.

Then comes time for the pistons to be mounted on the rods...
Make sure the pistons are mounted on the correct number rod, and mounted in the correct direction. If not, your rod bearings will suffer.

One way to tell if you have the correct place, look for a 'Build Board'...
This will usually be a piece of plywood with eight pegs across, and several pegs down.
Each peg across is for a specific cylinder. They may be numbered 1-8, or in the firing order of the last engine done.
The several pegs down are for the different rings, compression, oil control, piston, connecting rod, ect; for each cylinder.
This will tell you each cylinder is getting the attention it needs.

You should also look for the 'Sterilized Torque Wrench' approach in the assembly area.
There is no such thing as 'Too Clean' when assembling an engine.
If you see all of the rings thrown together, and the pistons rolling around on a dirty bench, with no cylinder numbers on them, you have the wrong shop!
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