The old head gaskets for B40s are 48 thou thick whereas the new ones are 36 thou with a raised lip around the cylinder. Does anyone have any views on why the gasket thickness has changed and pros and cons of the two types?
The raised lip improves the seal at the joint.
The thinner gasket is a problem if the head seats on the cylinder spigot before it has sealed on the gasket.
Only the maker or supplier can say why they have changed the thickness.
New old stock does ccome up on ebay.
https://www.ebay.co.uk/itm/BSA-WD-B40-B40-Genuine-NOS-Head-Gasket/253458871903?hash=item3b0353765f:g:9pAAAOSw345alyBs
Quote from: Greenfield on 09 Mar, 2018, 20:42
The old head gaskets for B40s are 48 thou thick whereas the new ones are 36 thou with a raised lip around the cylinder. Does anyone have any views on why the gasket thickness has changed and pros and cons of the two types?
It's best to measure the clearance between the head and cylinder without the gasket fitted. The head gasket then needs to be at least 5 thou thicker than the clearance otherwise it won't seal properly. The raised lip is not a substitute for the wrong thickness of gasket. In the past, you could get more than one thickness of gasket but it seems more limited nowadays.
Below is the reason BSA gave for the lipped gasket.
It fits B40, 4 and 6 stud heads, as well as B44.
To quote from Rupert Ratio: "the head gasket should be made from 18 gauge (48 thou) sheet but pattern gaskets are sometimes made from 20 gauge (36 thou) sheet". Therefore, it's best to measure the cylinder head clearance to work out what minimum thickness you require. It's likely a 36 thou gasket will be too thin in most cases.
Yes the gap on my B40 is around 38 thou so the thinner replacement gasket is no good for mine.
Nothing wrong with the clearance and just a question of whether you can get the proper gasket thickness. It may be a question of ringing the usual suppliers (BBB, Dragonfly, etc) and checking what thickness gasket they have got in stock. It might also be worth trying Kidderminster Motorcycles as they were quite good at one time with singles spares.