Forum bmwbike.com

I have original bores and pistons on my 56 R50 .Bores still within tolerances but just barely . A mechanic has told me I can go to next size rings on standard pistons , just filing piston ring ends to get the right end gap. Any comments Thanks Peter

Hi Peter,
Some points to consider:
Following the advice of your mechanic would mean working outside recommended procedures and specifications.
First oversize rings are designed to fit 1st oversize pistons (68.5mm) and cylinder bore.
Modified rings might eliminate excessive ring gap clearance but will not adequately compensate excessive clearance of pistons to cyl. wall and other signs of wear. If existing rings or a set of new standard size rings won't do, its time for a rebore to 1st oversize and a set of new 1st ov-sz pistons. Usually they come as a set including new rings, pin and snap clips.

I am very much an amateur myself but found it easy to decide since both my R26 and my R50/2 had a remarkable piston slap (shlupp, shlupp, shlupp, shlupp.....) once the engine was hot. That is because new pistons were not available locally for many years.

Have fun

regards

Thorsten

Next size rings, with the gaps filed, will help with oil control and compression, but not with piston slap which will develop soon and grow worse.

If the bores and pistons are that worn, it is time to also look at the slingers,rod bearings and main mearings.

Next size rings would only make sense if you could not afford complete proper repairs, and did not intend to keep the bike for long. Still, you would need to hone the cylinders and break in the new rings. Be sure the new rings have 3-piece oil rings, not 1-piece, or they may never break in.