Problems lapping the head and barrel in

No expert views have yet appeared - so here's my suggestion - George Cohen's site has info on this - item 23

http://www.norton.uk.com/expert.htm

Good luck!

Andy

The George Cohen site David mentioned is spot on for the principal and mechanics of lapping the head in, aiming for the wide barrel to head face to have around 1thou.gap when the narrow spigot faces are in contact. This puts extra pressure on the spigot joint when bolted down, forming the seal. Using coarse paste on the large surface, and fine (or none, once dull grey metal shows all around the spigot joint, showing a good fit and seal) gives that difference.

Where the lip on your head has come from is a bit of a mystery, it would have been machined flat from new. Your head and barrel may be from different engines. I assume Norton selected matched heads to barrels, as my very original '38 CS1 has matching numbers stamped on the two faces. I spent hours lapping to get the right clearance on the large surface.Grinding paste quality does vary, some makes seem to breakdown quickly with out doing much cutting.

The first thing to do is rest the head on the barrel, and check the gap of the wide mating surface. If you have a few thou, you can afford to try & lap the narrow spigot faces flat, although the lip you have may cut a slight corresponding dip in the top of the barrel spigot. Keep the grinding paste on the top of the barrel spigot only, you do not want any paste on the sides, which will increase the diameter of the spigot recess in the head.

If there is little or no gap on the wide faces, lapping to try and remove the lip on your spigot face will be futile, as you will never get a seal.It takes many hours of hand lapping togrind down the wide faces even a fraction.Engineers blue is very useful to see what istouching or not.

If everything seems to be miss-matched, you will need to get the head recessand barrel spigot and wide face machined back to give the correct fit.

SRM engineeringin Aberystwyth(01970 627771, www.srmclassicbikes.com)who are motorcycle only, have the knowledge & equipment to do this for you.

Roger.

Roger is exactly right (IMHO) but I doubt that first timers will get the coarse paste / fine paste job to work. Usually the joints have been messed around before you start.

1. Has the spigot to head joint any evidence of black combustion blow past to show it has been leaking? If not then leave it alone, or do a fine paste lap if No.2 is OK.

2. Is there a gap of 0.5 to 2 thou between head and barrel all the way around when put together? Look through with a lamp behind. It should be ideally one thou. If there is no gap then it needs the coarse paste lap on these faces only.

Lapping is not easy. Add only a tiny amount, use oil as a lubricant. Quarter turns, keep lifting, rotate a quarter after 15 seconds, stop after one minute, wash all thoroughly in paraffin, measure.

I looked at a Vincent 500 head and barrel for a friend that he had been lapping. There were absolute variations of seven thou between the lowest and highest points on the barrel outer and three thou on the spigot to head. It blew like a sieve. I re-machined both in a mill, but you need a whole set of jigs to hold both items. It took me two days to make the jigs and four hours to recut the faces.

You are doing the right thing taking it to an engineering shop!

Norm