engine rebuild

[Bob_Roller]

The problem with the '81 model year bikes, wasn't unleaded fuel .
It was a different alloy of metal for the valve seats .
Unknown to BMW, it acted like an insulator and prevented the valve to transfer heat to the seat and then to the cylinder head .
The proper term for valve recession in this case, it valve face plastic deformation .
The metal of the valve, softens with the excess heat and flows towards the valve stem, when I had my top end redone at 45,000 miles, I was shocked at how much metal had flowed towards the valve stem .

Here's a link to a BMW tech site, kinda explains the problem at length .


http://bmwmotorcycletech.info/valves.htm

[Barry]

As far as I have got the question at hand (please, correct me if I am being wrong) is that the valve seat recession appears when unleaded gasoline has been run through the system. The lead in the gasoline was what kept the valve seats happy. When they switched to unleaded is what brought problems for older R65's. I am putting lead substitute to every tank fillup and hopefully that does the trick. But the substitute is not the cheapest so if you are riding high milages, at some point getting new valve seats + getting the specialist to do the job will pay off. But that pays off for about 400 fill-ups or? That would be my rough guess.

My guide in life is not to fix things which are not broken. So if the valve seats are fine, add lead subsitute and you'll be good.  

Depends on the year. On an 81 it's not the unleaded fuel that damages the valves but BMW's attempt to prepare for unleaded fuel. As Bob said they initially got the thermal properties of the valve seat material wrong.

The original valve seats were actually better in at least they don't have a thermal conductivity problem and also don't necessarily suffer from valve recession depending on use. Although it's still relatively low mileage my 79 doesn't show any valve recession at all between services and that's with no additives.

So I'm with you on not fixing what isn't broken. I'll leave it alone until I see some evidence of a problem. My take on it is this. The whole unleaded problem got a bit over blown and unless you routinely do sustained high revs the original valve seats may be fine without lead replacement additives.  On your 81 I don't see the additive doing any good as it's not going to help the thermal problem. If not replacing the seats it's best to do what you can to avoid high exhaust valve temperatures i.e. by not using sustained high revs or an excessively weak mixture or retarded ignition timing and remove the nasty air injection plumbing if you have it as that is said to increase exhaust valve temperature.  

[arvo92]

Very highly appreciated info about the valve seat recession!
I guess it is kind of a information which is easily mixed up if one reads it first time. I have digged into the valve matter before and it was not clear as you have explained me being wrong in this case. Thanks for that!

I have avoided taking the top ends down. This to me is a bit like a person who fears to have a cancer but does not go to doctors. Who knows what I might find? I hope it would be alright but my bikes previous owners seemed to have different riding style than me so it can be a bad case.

To close this matter for me I will face my demon and take the top ends down. Then I know for sure and if everything is fine, I will keep away from redline of rpm and ride happily into sunshine.