1380 Piston Ring Gap?

Mini Technical Sections Problems, Questions and Technical
1

Hi all,

I did search the forum but couldn’t find the answer i was looking for hence this post so sorry if i missed it anywhere.

Ive got a 1380 engine with the 73.5mm “Powermax” pistons. (20659)

At the beginning during the first few hundred miles it was burning oil, which is normal during the running in but after 4000 miles surely it should have stopped and bedded in.

It got to the point where if i was only using it for short normal driving the end of the exhaust tip would actually have wet black oil on it.

Well i’ve now taken the engine apart and stripped it down again.

I started to think that maybe the oil control rings were not doing there job properly, I’ve taken the rings off the piston and put them in the bores and pushed the piston down the bore slightly to make sure there in there right.

The ring gaps are currently as follows:

Top compression ring - 0.022
2nd compression ring - 0.022
Oil control ring - 0.046"

Piston to bore clearance at skirt - 0.004

Are the piston ring gaps on a 1380 the same as a 1275GT engine?

If they are then im slightly above what it should be.

The haynes manual states the following for a 1275GT

Top compression ring - 0.011 to 0.016
2nd compression ring - 0.008 to 0.013
Oil control ring - 0.010 to 0.040

2

Any ideas?

3

Won’t be far off - there’s a formula to work it out if I could remember it

http://www.aa1car.com/library/ring\_end\_gap.htm

4

IMO, your rings are gone, and/or the bores are worn…

5

Cheers for the link.

Its a new 1380 half engine kit which has only done about 4000 miles in total. There is almost no wear on the bores just abit of shineyness as usual.

I really should have done a compression test on it before stripping it.

6

If it’s a newly bored block, then it may be that the block has been bored slightly too much oversize. I had a very similar thing last year with a 1380 which was burning oil and throwing it out of every breather. On stripping it the ring gaps averaged 0.025". Ring gaps should be between 0.003" and 0.005" per 1" of bore diameter (i.e. around 0.009" to 0.015". In my case, the block was about 0.003" too large on the bores and, as the block was already 1380 cc, it scrapped the block. When a new block was bored, this time to the correct diameter, the exact same rings gave gaps averaging 0.013" which was then fine. An expensive error on the part of the original machine shop though and as the engine had been re-built 2 years before, it was out of warranty. I now know to avoid that machine shop at all costs. The guy whose engine it is said they were cheap. You get what you pay for! They did not even know that a 1380 must be offset bored.