Earlier Heads on a Later Model? Need info quick!

[Rapid Dog]

Hey guys,

I just acquired an '81 RS that's going to need unleaded sets and a valve job.
I just bought a set of already rebuilt/seated/dual plugged heads off a '77 RS.

Can you for see any mismatching to the '81 pistons/jugs? Drawbacks? Etc?

Please advise. I'll need to return the heads if there's a bad snafu.

[Garnet]

The 1977 RS came with squish band pistons and heads. You can use later heads that where ment for dome head pistons on squish band pistons (at a power loss) but not the other way around as there is a cleanace problem.

30 years on who knows what parts either bike has.

More piston info here: http://bmwmotorcycletech.info/pistons.htm

[moore84rs]

How many miles on it? Do you know it needs a valve job or are you just
assuming you need to do the changes for unleaded fuel?

My 84 rs has 50K on it, ( I know...just broken in ) still has original valves untouched with no
signs of recession problems.

If needs a valve job anyway, certainly do the "unleaded" up dates.

[Garnet]

I forgot the most important thing.

1977 RSs had 40 mm exhaust pipes. One year, one model only. So that is an easy way to tell if those are realy 77 RS heads.

BTW if they are 77 RS heads they where quite sought after at one time, maybe still.

[Rapid Dog]

It runs fairly strong. Smokes a bit on startup. The PO road it cross country and one side started to huff n puff, bad valve. He sourced a head and threw it on and rode back. I have to assume that the other side will crap out. It has about 100 lbs compression.
I figure winter is a good time to re-ring and do the heads.

The " '77 RS" heads look to be the real thing. Have '77' stamped on them. I don't see 'RS' though.
That said, the seller has told me "These heads have 38mm ext. I.D."... so I'm confused.

Here's a pic:

[Garnet]

The " '77 RS" heads look to be the real thing. Have '77' stamped on them. I don't see 'RS' though.
That said, the seller has told me "These heads have 38mm ext. I.D."... so I'm confused.

77 is when the heads where made. They would have stopped making parts for a 1977 bike in about March or April of 77 and began makeing parts for 1978.

Only 1977 model year RSs had 40mm exhaust pipes, so these heads are either from a 1978 or perhaps from a late 77 R100S or /7. In either case you must identify if they are "squish band" heads or not.

[Rapid Dog]

How do you identify that?

[Garnet]

You need to look at the combustion chambers on both sets of heads. They should look the same. The differance is very obvious when side by side.

[Rapid Dog]

Since I don't have the heads off yet, just for kicks....
This is the only pic I can find on the internets to compare with.
Pic from ebay which states:

This is the big valve 1981 type R100 1000cc head. Takes the 40mm carbs and is a 38mm exhaust port.

[Garnet]

You need to compare that outer flatish lip of the combustion chamber. On "squish band" heads, that area is a bit flater and much closer to the piston. That flater area will not have enough clearance with a (later high compersion) domed piston.