Tagless Big End Bearing Shells

[ukkid35]

I've ended up with some tagless shells, but the originals in my M160 Smart engine probably have tags.

It seems like there are a lot of tagless shells for sale, especially for smaller engines.

The brand I have is King, which seem to get rave reviews on some sites such as the US Scooby forum.

Should I just put them to one side, or perhaps just fit the lower shell if the upper looks ok?

Any experience of using King or tagless?

[ukkid35]

Thanks, good to know King are well respected.

One thing I didn't mention is that I'm not doing an engine rebuild, I'm replacing the timing chain and need to remove the sump, so it's more of a WhileYou'reAtIt.

[ukkid35]

HJG said:

In high RPM engines, the tags are sometimes relied upon to prevent shell rotation where the interference fit itself is not enough. This only works if the slots are designed such that the tags butt up against a 'stop' (i.e. one slot per half of the rod, diagonally opposed to one another)

Presumably the slots are machined before the big ends are fracture split - or is that manufacturing process reserved for smaller engines like the Smart?

Would that design also not prevent the bearing spinning, does it have to butt up against a stop?

[ukkid35]

Main failure mode for the M160 Smart is stuck oil control rings causing burnt out exhaust valves (probably due to oil change interval or quality). I've repaired four of these and always replace the big end shells while the pistons are out, the lower shell is often through the top layer with copper colour appearing. This particular example hasn't burnt a valve yet (unless someone's been there before) but it is the filthiest engine I've seen yet, more burnt on sludge than I've seen since my Capris many years ago. I fully expect the lower shells to be worn through.

[ukkid35]

GreenV8S said:

ukkid35 said:

stuck oil control rings causing burnt out exhaust valves

I'm intrigued - how does that cause that?

Oil means it runs hot, exhaust valves don't stand a chance.

What I don't understand is why the spark plugs don't break down first.

[ukkid35]

GreenV8S said:

ukkid35 said:

stuck oil control rings causing burnt out exhaust valves

I'm intrigued - how does that cause that?

[ukkid35]

Pic from a previous M160, I didn't take a photo of the lower shells, I guess they looked OK to an amateur like me

Edited by ukkid35 on Saturday 3rd December 10:19

[ukkid35]

227bhp said:

Twin plug head, I like that. What do the pistons look like, are they flat tops?

This is what I saw the last time I took the head off my Smart - I'm sure there's something missing...

[ukkid35]

227bhp said:

Oh dear, did you find it?

The other pistons are displaying signs of det at the bottom there (probably caused by the oil contamination), is that det' opposite the corresponding squish pad on the head?

No, other pistons are fine, that's just debris. Engine does have a knock sensor, the failure came completely without warning (no power loss or audible pinking) and was over within 3 or 4 seconds.

Bore scoring means the engine was scrapped. I'm in the process of replacing the timing and oil chain mechanisms of a replacement that lasted about 2 miles after fitting. I think I still have compression as it didn't die when one of the chains started flailing around, currently stopped in my tracks by a stubborn crank bolt - they're always stubborn aren't they?!?

[ukkid35]

Looking at the photos again (from the comfort of my sofa), perhaps I'm being unrealistic thinking I can replace the upper shells from underneath, access is pretty limited. And I'm 99% certain that it's the upper shells that will be suffering.

[ukkid35]

227bhp said:

Quite easy, poke a screwdriver in and spin the bearing round till it falls out, push new one in the same way but not with screwdriver.
You'll need small hands though and the bearings with tabs would make locating a bit safer.

I do have delicate hands, but the Smart is small (200cc combustion chamber vs typical 500cc). Also not sure how other shells are fitted, but these are sprung, so popping them out with a screwdriver is unlikely to work.

[ukkid35]

Mr2Mike said:

ukkid35 said:

Oil means it runs hot, exhaust valves don't stand a chance.

The motormouth that runs a popular Smart 'technical' website likes to tell people that, but in practice it's down to the oil burning causing carbon deposits on the valve seats. As soon as this happens the valve stops sealing properly and quickly burns out.

Perfectly happy to accept that explanation.

Also willing to accept that people ignore OCIs and use inferior grades, which is likely to be the ultimate cause.

[ukkid35]

Mr2Mike said:

Treat the PCV as a service part, change the oil regularly and ensure both spark plugs on each cylinder get replaced when they should and these little engines can last a good while. It seems many/most of them are ignored until they go wrong.

Thanks for the advice. The problem with the PCV is that access to the front of the engine is very restricted, so although the valve is in plain view few people seem to bother to change it. In comparison changing the 'lower' plugs is relatively easy, but as you know they don't get changed too often either.

I will also be using engine flush each change in the vain hope it helps free up sticky oil control rings. I already change the cheap cartridge filter every couple of months as that takes about three minutes and costs under £3.

[ukkid35]

I have a follow up question for anyone who can advise:

Having put it all back together and started it up, the rattling that I assumed was timing chain related, has instead become more obviously a knocking noise.

Fortunately I have very little experience of this (apart from my first ever car a V6 Capri that threw a rod at VMax plus as I was going downhill at the time - truly spectacular in the middle of the night), so what I don't know is whether I am likely to have suffered any damage to the crank. The engine has only been run for 30 seconds at most while obviously knocking.

Thanks in anticipation

[ukkid35]

99hjhm said:

How long has it run for total? Was it OK to start with?

My best effort to explain the engine history is as follows:

I bought an engine minus ancillaries with supposedly just 30k miles

I paid someone else to do the heavy lifting, but I did the induction and exhaust myself (mainly due to lack of working space).

On completing the job I took it for an MOT and on the way home after very short period of high revs a distressing noise developed such that I switched it off within 15 seconds and towed it home.

I assumed it was timing chain slap and replaced the entire mechanism including the oil pump chain, and then restarted the engine earlier this evening.

Sure enough the unpleasant sound has not done away, so now I'm assuming something fairly critical has failed.

[ukkid35]

99hjhm said:

I'm confused, did you touch the shells? Did you fit another engine and then take that apart!??

I have bought four 599cc Smarts with burnt out exhaust valves, trading up each time, it was the only way I could afford a Smart. I've rebuilt each one with new valves, rings, bing end shells.

I removed one of the big end caps on this engine (number 5 so far) to check the condition of the shell, it was tagless and appeared nearly new. I reckon this means someone has been in before and attempted a rebuild.