'68 R69s base gasket ??

[Grahamm]

Hi All,

so the '68 needs an aluminum base gasket.. but what thickness should I go for?

they come in .5mm, 1.0mm and 1.5mm.

I am not installing the vibration damper and the cylinders are fresh with 3rd over size.

It does not need to be a speed demon and want smooth versus top performance.

Does experience prove going with 1.5mm will reap the results or is 1.0mm sufficient?

any experience welcome...

[Slash2]

The thicker the gasket, the lower the compression.

[schrader7032]

On my R69S rebuild I used 0.5mm gaskets.

[Flx48]

so the '68 needs an aluminum base gasket.. but what thickness should I go for?
Does experience prove going with 1.5mm will reap the results or is 1.0mm sufficient?

Hi Graham-
That's a pretty subjective question.
For myself, I ran a number of R69S bikes before finally settling on the earlier R69 model, and I've been happy with that for the last four or so decades.
That's to take absolutely nothing away from the 69S, which is a terrific machine, the characteristics of the 69 are simply more my preference.

Back a bunch of years, Ed Korn had said many guys here in the states liked to "upgrade" their R69 to 69S spec, for more performance, which basically meant swapping the 7.5 CR pistons for the 9.5 CR pistons used in the S.

Around the same time Huggett had said that many guys in Europe liked to mellow out the performance of their 69S model bikes, for more longevity, by lowering the compression ratio.
Whether done by piston swap or just a thicker base gasket I don't recall.
That Huggett offers just the standard 0.5 base gasket and the 1.5 base gasket would lead me to guess the 1.5 was used.
Perhaps email him for verification/clarification.
Best-
George

[Grahamm]

Subjective.... of course...

but as I asked for experience on thickness based on balance of smoothing the bike versus buzz and fast.. I was hoping members would have some experience on the impact of 1.0mm versus 1.5mm.

I have never owned a R69s or ridden one, Have and ride a R50/2 and enjoy the smoothness but dislike the lack of grunt to maintain speed up hills without work.

The R69s has lot more ability..... but I don't need a race bike. and without installation of the vibration damper... lowering the compression should work to smooth ... but how far is far enough... 1.0 or 1.5mm.

I have also noticed that standard head gasket is .75mm and the one with integrated silicone oil passage seals and crush metallic is around 1.2mm... so there another opportunity for change in compression as well...

anyone experiment with these factors with results of nice balance of smooth versus performance without the crank balancer installed?

[schrader7032]

Something for consideration... On my /7, I replaced the original top end with a kit from Siebenrock. The stock compression went up from around 9.0:1 to about 9.5:1 with the kit. I was a little uncomfortable with that high of a compression ratio...I didn't want to consider dual plugging, etc. The supplier offered an R60/7 base gasket that was 0.67mm thick. His indication was that this would reduce the compression ratio down to about 8.8:1. That brought it back to close to stock. I was good with that change and have been happy with the bike. So just a point of reference...

[Grahamm]

Not much experience with this from the group...

So talking it over with myself I am going to start it an extra .75mm split between a thicker base gasket and head gasket.

will report back on the result.

[niall4473]

Do not omit the crankshaft damper, it is not a "vibration damper" even though some BMW literature calls it such, it is a Torsion Damper, and it is to absorb twisting moments which can build up in the crankshaft, not necessarily at high engine speed.

This was very, very expensive and happened at about 4000rpm. Afterwards, I fitted the damper (it was a 1961 and had never had it) and scrupulously check it.

[sherman980]

niall4473,
The pic you posted of the broken crankshaft certainly illustrates a catastrophic failure! That said, after 50 years of riding and wrenching on old BMWs (funny, many were new when I started down this rabbit hole, LOL!), I think your broken crankshaft is more of an anecdotal data point than a trend that should send the members here all running to buy and install harmonic dampers (or vibration dampeners - both are technically correct) on their 50's and 60's era motorcycles. All crankshafts "twist" under load (think combustion event) and "untwist" when that load is removed. This twisting and untwisting sets up a "vibration" that if not provided for will result in the kind of failure in your photo. The longer the crankshaft and the more "throws" it has (think cylinders here), the worse this problem is.

Virtually all large multicylinder car engines come from the factory with a damper that is "tuned" specifically to deal with this issue. But most smaller engines simply rely on the robustness and mass of the crankshaft design to deal with it. Do these crankshafts occasionally fail? Yes, but typically it is a rare event more likely the result of a metallurgical or machining flaw than a "normal" and predictable failure. For BMW to have gone to the trouble of adding a crankshaft vibration dampener along with the front engine cover mod to accommodate it, etc., after nearly 40 years of motorcycle engine experience, they must have had their reasons. But I just don't see the actual experience that says it is necessary. And interesting that the follow-on /5, /6, and /7 engines don't utilize a separate dampener either...

As a last note, any mechanic that worked in a shop when these first appeared likely saw way more of them with rubber damper rings that were so worn that the damper assembly did more harm than good. And the rubber "dust" they shed as they wore made a major mess under the front cover often causing problems with the generator and/or voltage regulator and occasionally with points contamination. Thinking I removed more of them back then than replaced the rubber damper...

Just one "old" guy's thoughts...

[schrader7032]

I seem to recall the issue with the crank vibration was that the main bearings were far enough apart that the unsupported portion of the crank could flex. But it seems that really only happened and created a problem when run at or near max RPM.