I find it always helps if one knows why an item exists and what it does/how it works.
Spacers: My memory from past reading and experience says these are used to fill in gaps that may exist:-
1. To prevent distortion of, for example the floor, when bracket and floor do not naturally meet but the nuts/bolts need to be tightened down. e.g. Big gap .....tighten down and floor is likely to split not too far into the future.
2. Spacers can be added to increase the distance between items so that other remote parts come together more easily eg when fitting doors it may be necessary to add spacers to jack up the body a little at front or rear of door area to get shut lines right or line up door latches. This is where it is sometimes helpful to assemble whole car loosely then gradually go round tightening up rather than tighten up one area then go on to fit more bits only to find out that the last bits have their holes about 3/4 inches out of alignment (I am now older and hopefully a bit wiser
The spacer material can vary depending on what is trying to be achieved. e.g for end of chassis cross-members to body floor where the brackets are riveted on and not adjustable I have used strip steel to fill the gap since it is just a metal to metal bracket to floor joint and it is just that you can't move the bracket to nicely fit against both surfaces. I would also make the spacer part that sits against the floor a larger area to spread the load from being just a bolt head size round the hole. I have often put a thin layer of rubber between the metal surfaces.
Bear in mind that the whole body and chassis are not precision built structures so there are occasions when some holes need to be encouraged to line up by putting a bit of packing somewhere to avoid local overstressing. Lining up the front body extension holes with those in the cow-horns ???
Material: Depends on what is trying to be achieved..e.g. engine mount rubber to absorb vibration while allowing a certain amount of movement to avoid stress on casting. Does there need to be a little bit of flecability or does the joint need to be rigid?
I feel it is good to ensure whatever material is used needs to be waterproof and maybe oil rot proof.
Chassis/floor strips need to be waterproof (and probably oil rot proof
) I use a selected thinish material..(can't remember if neoprene or similar derivative). This provides sound deadening and prevents steel to steel (or paint to paint) contact/chafing.
A very good source of material and technical information on which material in which situations is SEALS DIRECT
www.sealsplusdirect.co.uk.
Their catalogue gives technical info for which material to use for which situations as well as showing items at actual size with cross sections e.g. wing to body piping, piping for between body and cow-horns etc.
I have always found them very helpful.
Dennis
