Engine rattle - is it caused by damaged piston?

[Mr2Mike]

Smart ForTwo, but with a Smart Roadster engine (identical to ForTwo Brabus engine) and running a ForTwo Brabus ECU. I gave this engine a refresh earlier this year, honed with new rings, all new bearings, oil pump, cam chain & tensioner etc. and it's been running very well with no oil useage.

A few weeks ago as I pulled to a stop I heard a rattle from the engine. Under load you can't hear it, at idle it happens irregularly, but of the engine is gently revved then the rattle is very prominent as it slows down. I made a video showing the noise.

A compression check showed the middle cylinder was down on compression, particularly when hot. My (cheap and nasty) compression tester was showing 150psi on both outer cylinders, and 125psi on the middle when cold, falling to about 100psi when hot.

It finally stopped raining today so ripped the engine out and stripped it down and found the following damage on the middle piston/bore:

Smart ForTwo Piston/Bore pics

The areas of damage are almost exactly opposite each other, and about 20 degrees away from the centre of the thrust faces. These areas will be close to the spark plugs, though not exactly aligned. The spark plugs themselves look fine, electrodes intact, insulator a reasonable colour etc.

Any thoughts on what caused this, and why it would make such a racket?

[Mr2Mike]

227bhp said:

In answer to the headline question I would say not caused by the small chamfer at the top of the piston.
I would be asking if the piston damage is caused by detonation and looking for other signs of it, I can't see it clearly enough.
I wonder if you have more than one issue now and whether the causes were from various different sources.
I would be checking: Bearings, clearances and mating faces at both ends of the rod and bore, piston dimensions and if detonation could have occurred.

I'm also wondering if the piston damage is simply coincidental, but I've not found anything else. Big end and main bearings all look fine, I couldn't feel any slop in the bearings when the bottom end was together, i.e. turning engine so the piston lowers and then pushing down on the piston didn't reveal anything. Little ends seem fine as well. Only thing I would say is that piston clearance looks to be on the loose side, so was wondering if it could be piston slap, but the middle cylinder is no worse than the end cylinders and why would it suddenly develop?

The cylinder head was previously rebuilt with new followers and hydraulic tappets. I already swapped another new set of tappets into the head whilst the engine was still in the car, hoping the noise was due to one of them sticking, but no such luck.

227bhp said:

Oh, one other point - there was a thread on here maybe less than a year ago where (IIRC) the failure modes of this engine were discussed, If you didn't see it it may be worth digging out.

I'll take a look thanks, though I'm aware of most of the common problems. My wife's Roadster is still going strong 3 years after I rebuilt the engine due to a broken cam chain guide.

[Mr2Mike]

227bhp said:

Sounds like you need to be investing in some accurate measuring equipment - or finding someone who has some, too much guessing.

A new set of bore gauges are on their way to me. I stupidly lent mine to someone and never saw them again

I haven't been able to find any specs for ring gap or bore clearance on this engine either. There are various rules of thumb, but it would be nice to have the correct figures from the manufacturer.

[Mr2Mike]

Took some close-ups, not the greatest quallity (using a camera phone down a microscope is quite tricky!) but shows how the alloy has been smeared and the crown has been rounded off at the edge.

https://www.flickr.com/photos/150776204@N08/shares...

Ignore the small white fibres, I had the piston and conrod wrapped in kitchen roll so as not to get oil in the lab at work

[Mr2Mike]

Boosted LS1 said:

I think the pistons have been rocking but can't see any scuffing on the bores. Are there any marks in the bores and how high up the pistons are the pins located? What are the lower skirts like?

Ask and ye shall receive

https://www.flickr.com/photos/150776204@N08/shares...

Skirts look pretty good for an old engine, a little bit of scoring from debris and the anti-scuff coating is worn. The lower edges of the skirts don't look bad with the naked eye but there looks to be some wear, so entirely possible it's been rocking.

There are some pics of the bore in the first link, there is some scoring coincident with the damage on the top ring land. It's not horrendous but I can just feel it with a fingernail.

I'd be quite happy to get it rebored and fit oversize pistons, but it appears they are unobtainable for any Suprex engine that uses a 17mm floating gudgeon pin (all 82bhp & 101bhp Roadsters and the Brabus ForTwos AFAIK), since Mahle, Kolbenschmidt, NPR and Nural all say these engines have 16mm pins. Searching the various Smart forums suggests I'm not the first to discover this!

Is it possible for a knock sensor to fail "unsafe", i.e. stop working without any ECU diagnostics being able to detect this?

Edited by Mr2Mike on Monday 31st July 15:34

[Mr2Mike]

Yes the scuff marks on the piston are diametrically opposed but offset from the centre of the thrust surfaces. This puts them closer to the spark plug locations in the head.

The little end is fine, the rod is not visibly bent, and there is no uneven wear on the big end bearing. However I don't have the kit to accurately measure bend/twist.