Techniques - Cleaning/Trueing mating faces

It's often necessary to clean up and flatten surfaces on engine parts, for good fitting or sealing purposes.
Exhaust flanges and thermostat housing faces both come to mimd, anything that takes a flat type of gasket
really needs to be as flat and true as possible first. There are parts on a Turbo's exhaust
that must form a pressurised gastight seal, yet without a gasket being available.

Introducton

There are times when mating surfaces have to be just that to high degree of accuracy. One example would be that of a valve seat face and the corresponding seat itself, attained quite easily using carborundum grinding paste. A harder one to achieve is a really flat face, on a part that is perhaps large, heavy or oddly shaped. In a proper engineering refurb shop, these would often be machined in one way or another e.g. a cylinder head - ground or milled flat; a flywheel face - usually ground on a special machine due to its hardness, or a round part trued-up on a lathe. Failing such facilities, what follows are a few tips on how small to medium sized parts can be surfaced or at least given improved flatness and finish.

Finding flatness

Often the best and quickest way is to start to clean and flatten the surface somehow, where quite quickly any high-spots or irregularities will become apparent, and usually, you won't be doing any harm. First though, if there's any thick rust, scale or gasket on the surface, use an engineering flat scraper to remove it [Tips tools and Techniques -Engineering Scrapers]

A good engineers straight edge, steel rule and a good eye in good light can often show up a few thou out-of-true or engineers blue can be used between two surfaces, either those that mate in practice, or each and a known dead flat one e.g. a surface plate, or good substitute. Any rust or pitting will show up immediately in outline, and wherever securing holes are present (particularly stud holes), high spots will usually exist around them.

Good substitute for an engineers surface plate... a foot square or more of thick plate glass. Whilst glass flows plastically, old plate glass is usually pretty rigid and true, particularly if kept horizontal on another good flat surface. It has the advantage in that it can double as a good surface for using carborundum grinding paste over a large area, or carborundum (emery) paper, should it be necessary.

Another useful surface is that of a thick, chopping block, of the very thick type where the wood grain runs in the direction of thickness. Made of stable wood, these are often of a good flatness, but check with your steel rule first (don't want to upset the missus for no reason!)

Then there's plate (not sheet) steel and ally, of 1/8" up to 1/2" being practical - always check it with a steel rule both ways. Deburr edges. Thinly spray glue, wait a mnute and apply Silcion Oxide, Alumnium Oxide or Emery paper. Trim off excess at edges with a hook bladed Stanley knife.

Steps to get a flat, clean surface

If you are skilled with a file, a bastard or 2nd cut can often be used to ensure a flat surface, but can do more harm than good in a few seconds in the wrong hands. A more reliable way, once most of the rust has been chipped away, is to use a dead flat surface such as a sheet of plate glass, and a new sheet of 80~120 grade emery paper (maybe lightly tacked with a thin but even layer of spray glue). Care must be taken to at no time 'rock' the mating surface, just hold it very firm and move it along and back a few times, and then both diagonal directions - clean paper with wire brush or just vacuum it every time you see a buildup within the grit, vacuum up job and repeat. It may take time (image below) but you'll have done as good as you can without a machine shop to take the high-spots off.

The outer two ports were originally about 0.060" below the two middle ones. Picture taken about halfway finished (a couple of hours here and there!) - its now about perfectly flat all over with burnt areas removed - that was another two or three hours - obviosuly a machine shop could get this level and flat with a grinder or fly cutter quite quickly - if one is nearby and tame - it wasn't... Cast iron is slow work over this sort of area with high distortion, but with a long thick flat steel using 40 grade, then 80 and 120, a good result can be had. Straight strokes then alternately at 45/45 or right angles are best. Vacuum de-dust frequently to extend life of flatting paper almost indefinitely.

Finally

Whenever using carborundum paste, emery or sandpaper, work as clean as possible and brush/vacuum up behind yourself - there is no place I know in an engine for it! Try to do this work in a separate area, maybe even outside on a bench.

Coarse Emery paper (120 and down) and files can be cleaned using a wire brush or file card. A good stainless wire brush is a handy thing to have. Steel rust itself is pretty abrasive, even the dust from it - stop and clean often and take care, especially with your eyes.

HarryMann