Off-Road Forums & Discussion Groups - View Single Post - best boring size
View Single Post
post #8 of (permalink) Old 08-24-2005, 07:39 PM
Junk Yard Genius
Join Date: Sep 1999
Posts: 5,383
Thanks: 0
Thanked 1 Time in 1 Post
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!
Junk Yard Genius is offline  
For the best viewing experience please update your browser to Google Chrome