420 engine question

[sachsmx]

Is there an easy way to tell if the cylinder/cylinder head joint should have a head gasket? The 1979 and 1980 models are supposed to use a copper head gasket. The 1981 and later models do not use a head gasket. I know that my top end is a 1980 model by the frame number, so I assume that it should use a head gasket (the previous owner said the bike had its original engine). The problem is that I am just now reassembling the engine which was tore down years ago and I don't remember seeing a gasket. This combined with the fact that I kept all of the parts such as the old seals, bearings, and other components I planned to replace and there is no head gasket to be found. As we all know, these types of changes were incorporated at the factory at times other than model year changes.

[john durrill]

Sachs,
If the engine manual gives you a deck height you should be able to put the head on and measure what you have with no head gasket. That would let you know unless the head or cylinder is realieved for the copper gasket ?. I had a 79 400 that used a copper ring type gasket so i" think " i know what you are talking about.
John D.

[tofriedel]

Sachs,

I know that the only 1981 model that did not use a head gasket, was the mighty 495.  All others had a head gasket. To the best of may knowledge, all prior models to 1981 used a head gasket.

Tony

[sachsmx]

I'll try checking the deck height w/o a base gasket to see what I come up with. According to my 390/420/495 engine parts manual, the 1981 420 MC and 495 MC used no head gasket. I know that chances are that my 420 top end is a 1980 since I doubt many 420s were sold here in 1981 since the 495 was available. Still, there's an outside chance that the factory made a running change during the "1980" production year. Looking at the illustrations in the parts book doesn't help as it appears both the gasketed and non-gasketed cylinders and heads have an undercut around the top of the cylinder liner. There are different p/ns for both the head and the cylinder for the "1980" 420 (gasket) and the "1981" 420 (no gasket). There is also a 1mm thick head gasket for the 420 MC and a 2mm thick head gasket for a 420 GS. I will post any findings here for future reference.
The unfortunate thing is that I am swapped frames and no longer have the frame that the engine came out of. I wrote down the frame # somewhere but have moved twice since the tear-down and haven't located the number yet (I remember well it was a 1980 but can't remember if it was an early or late #). The same thing goes for the engine cases, the originals had bad mounts and were replaced for some nearly new ones, which is also why I replaced the frame.

[tofriedel]

Sachs,

My error, you are correct in that the 1981 420 MC did not use a head gasket.  Just so you know, the part number for the head is cast into the bottom fin of the head, at least it is on the 1981 495's.  I cannot remember for sure, but the part number may also be cast into the cylinder.

Tony

[sachsmx]

I will check the cast P/N tonight. I assume it will be a casting P/N and not a finished machined P/N. I also assume that both the gasket and non-gasket head types are made from the same casting. Maybe I am assuming wrong (I hope)

[ALB]

According to my parts lists, the 1980 - 81 420 (model 560) used a copper ring head gasket. There were 2 sizes. The model 80 used a 1.0mm thick gasket (#560-30-036-000), the model GS used a 2.0mm thick gasket (#560-30-036-026). The 1979 420 used a 1mm thick gasket (#560-30-036-000). Check the engine serial numbers for the year code and model codes to determine year and size of engine.

Alan Buehner

[sachsmx]

Thanks to all for their help. Indeed, the P/N is cast on the head and it is the actual machined cylinder head part number (not the raw casting number as I had thought). The top end is supposed to have a head gasket, either 1mm or 2mm thick depending on MC or GS model. I believe that most US spec 1979-1980 420s are MC models as the US had a 390 GS model that was based on the same bottom end. The 420 GS models didn't arrive in large numbers until the 495 MC came here in 1981.