Vintage BMW Motorcycle Owners

Hello,

I purchased my motorcycle from a dealer a couple of years ago. This R60/6 had a modification done to add
a second spark plug/cylinder by the previous owner, as they typically have one each.

My question is, when I removed the plugs for the first time to inspect, the lower plug on each
cylinder had a spacer washer, about 3/32 to 1/8" thick, whereas the top two plugs did not. Is this necessary
or why is a spacer used here?

Any help is appreciated.

Motaman

Chances are the head does not have enough material to fit a full length plug. Spacer is used to make up the difference in material thickness. You can continue to use the spacer to fit the plugs or permanently mount the lower plug; delete the wires to it, and convert back to stock, single plug operation.

Dual plugs were popular around 1980. They did require different timing (less advance) because the flamefront in the cyclinder had less distance to travel.

My R90S was converted to dual plugs many years ago, but for the last 25 years the lower plug has not been used, just the stock single plug.

Mike

Thanks Mike. It appears that only the bottom plugs on each cylinder requires this spacer. I will simply continue
to fit the spacer on new replacement plugs, when needed.

Steve

Another aspect could be that this was a way to keep all the plugs the same...3/4" reach. Typically what is done is to use 1/2" plugs on the lower location and the regular 3/4" plug up top. If you mix things up, that can be a problem. So, by just drill and tapping the bottom hole, then using a spacer, it's 3/4" all the way around.

When high octane fuel started to become harder to find around 1980 pinging in BMW twins became a problem. Thick base gaskets to lower compression was one solution, but dual plugging was a much more elegant fix, especially with aftermarket solid state ignition. My 1973 R75/5 is dual plugged with Dyna III ignition and it runs great; I'd never change it back. If I remember correctly, the R60/5/6/7 was even more prone to pinging than the R75.

Bill

Bill remembers right...the R60 series from 1970 to 1977 had a mild cam profile which resulted in a tendency to ping. I'm not enough of an engine guy to understand how the cam profile causes that to happen. Must have something to do with the overlap when the pistons were moving up/down. Plus the R60 series was very high compression ratio, 9.2:1 according to the specs...I thought it was higher. The 600cc R69S that preceded it was 9.5:1 but must have had a different cam profile.