R80/7 Piston, rings and bore measurement

[Kurt in S.A.]

84.8 and 84.775 say stock bore to me...that would be stock rings, ie, no oversize.

How is the taper on the bore? Did you measure fore-aft and up-down? Those are important measurements too.

Kurt in S.A.

[elburro]

How is the taper on the bore? Did you measure fore-aft and up-down? 

I measured the bore at 5 points from top to bottom and the variance is 0.05mm.
Shall I worry?
Thanks

[Duane Ausherman]

In my head I figured that is .002" in 'Merican. That is nothing. Do you have a ring groove near the top? I doubt it.

How confident are you in the measuring technique that you are using? Does it repeat?

I have used cylinders that had .010" taper and they worked out ok. I once built a BMW engine with all junk parts. I mean that everything was out of spec. It ran just fine and burned very little oil. Compression was near the lower end of what I like to see. It ran a long time too.

[elburro]

To get an accurate measurements I have
put one of the rings back in
Then pressed down with the piston.
This would ensure the ring is sitting flat.
Use the ring as support to the gauge.
Transfer the measurements onto a digital caliper.
Et voila, job done.
This technique gave me a pretty accurate reading
(the variance between readings was 0.02mm at worst)

How do I upload pictures on here?

Regards
Elburro

[Jean]

That trick sounds like as good as it gets.
Your measurement of 0.05mm is pretty good...problem is not the cylinder wall, anyhow.

[Steve in Golden]

How do I upload pictures on here?

How Do I Insert a Photo in a PHPBB Post?

We need a FAQ section, this question comes up often, especially since photo attachments are not working due to reaching our total size limit on attachments. Linked to images look better anyway than attached ones.

[tsa]

Both cylinders are 84.8mm.
The pistons are marked 84.775.
What ring I need to get the rings end gap to spec?

According to <http://w6rec.com/duane/bmw/engine.htm> (don't be shy, Duane), those are standard measurements for an R80 with "B" -size pistons. Thus, as far as I can see, the standard BMW R80 set of rings should be what you want, the alternatives available are for 0.25 or 0.50 mm bored out cylinders.

[elburro]

Yessss!!!
Good news at last.
I have just received the delivery from motobins with the new rings standard size.
The gap is as per Clymer manual.
Now just pray for the weather to be kind.
It will probably be Monday.
El__burro