My engine ate a nut!

[Pooh_R]

I had an engine rattle for a few seconds which stopped as quickly as it started.

 

Somewhat concerned about what it was, I had a good look at lots of things, but I couldn't recognise where it came from. It drove fine. I then found out one of the nuts securing the carb backplate onto the carb was missing. This must have entered the engine. Compression tests were all good. I didn't know what damage the nut had done in the engine, so the head had to come off. This afternoon I did this, so see the photos here to see what I found.

 

The surface of the head looks well chewed and the piston is a mess too. I'm especially concerned about the edge of the piston in the third picture.

 

For those with Webber carbs, make sure you check your nuts!

 

For those with more experience of broken engines than me, I would appreciate some advice on what to do next.

 

Thanks in advance for the help

 

2.0 zetec on webber carbs

 

 

[Stationary M25 Traveller]

Andrew Ashby had the same happen .... With nasty results. Check your nuts !

[oilyhands]

You would do well to dress the areas in the combustion chambder to remove any protruding sharp edges, the damage although ugly is largely cosmetic, it will affect squish though. The damage to the piston crown looks fairly minimal and the crown is quite thick, the damage near the edge will have caused a degree of spread to the very top which will cause some contact down the bore but this will lessen in time. It is unlikely to be causing ring pinch but the only way to be sure is to remove the piston and check.

 

Oily

[6speedmanual]

Where are the remains of the nut?

 

 

Peter

 

BRAWNGP green SUPERLIGHT

FCITW 2009 😬

[philwaters]

If you can wait until the winter I'll have 4 std silvertop pistons spare.....

 

Phil Waters

[Pooh_R]

Thanks for the comments.

 

the nut must have passed through to the exhaust as there was nothing remaining in the chamber.

 

Phil, yes I might be interested in a set of used pistons. how are you upgrading?

 

oily, the spread to piston crown is visible - piston to bore clearance is low in that area. With that in mind, how likely is ring damage? Or should I get into the bottom end?

[oilyhands]

Ring damage is unlikely, although ring pinch is a possibility, some careful light running will probably cure the spread, but the correct way is to remove the piston and dress that area of the top land, check at that point to see if the top ring is pinched.

 

Oily

[Clousta]

I have weber carbs. But I am struggling to work out where the nut was ingested from. I have had a look at my 40 DCOE set up externally and unless I run the engine without the (K&N) air filters I can't see how the engine could ingest a nut.

one of the nuts securing the carb backplate onto the carb was missing. This must have entered the engine.

All my nuts securing the carb backplate on to the carb are locking nuts and are tight.

Please enlighten me. *confused*Or am I misunderstanding your description of carb back plate?

 

Gavin

1988 1700 Supersprint (LA, long cockpit)

[Roger King]

The standard fitting for the backplates are a spring washer and plain nut. They are NOTORIOUS for coming loose and damaging the engine just as described. I always changed them for nylocs which solves the problem 100%

[Pooh_R]

I've added another couple of photos to the link above showing the spring washer Roger describes one stud with a nut, and one without.

[Clousta]

Pooh_R many thanks for the photo. I thought you were referring to the lock nuts holding the cab to the inlet manifold, hence my confusion.

 

I see from the photos that you are talking about the split washer and nut holding the air horn retaining plate and of course the backplate of the (K&N in my case) air filter assembly.

 

I have checked mine and all are held on by a split washer and nut. Fortunately all are there and still tight. They are M6 threads/nuts. So I will get on my bike and get do to the Bricolagefor some fresh M6 nyloc nuts and matching plain washers (Roger, I am assuming that you fitted a plain washer under the nyloc - is that correct?).

 

I have looked through Des Hamill's 'Weber and Dellorto' book and had a quick look several photocopied extracts of manuals on Webers that I inherited from the previous owner and I can't find the recommended torque setting for the nuts?

 

Any recommendations?

 

Sorry about your damage - I hope it works out OK. But many, many thanks for the 'catch'.

 

Gavin

1988 1700 Supersprint (LA, long cockpit)

[elie boone]

Gavin, a 1/4 before they snap 😬

[Clousta]

Elie,

thanks for the technical advice!

 

Unless Roger chips in I will go with this general advice:

Torque settings

¼ UNF/UNC or M6 3-5 lb/ft

(7/16 AF or 10mm spanner)

---

It will have to be done by hand/feel as I don't have a torque wrench that covers that low range.

 

BTW Elie,

I have made good progress with my fuel fill pipe/elbow problem. I reversed the pipe to avoid the memory effect of the crimped hose (the raised section). I took almost 25mm of the hose's length. The hose is now an elbow with a greater than 90 degree angle rather than less than 90 degree angle. I can see the top of the tank's filler pipe now. The path is definitely all downhill from the entrance to the filler cap to the tank. I can probably get the tip of the petrol pump nozzle into the tank filler pipe now. I am looking forward to a test......I just have to change a few important little nuts before I drive anywhere!

 

Gavin

1988 1700 Supersprint (LA, long cockpit)

[elie boone]

Gavin, you are into a quick fill now , just do the nuts up tight i never use a torque key for that.

 

However there is a point to watch, if you have trumpets that slide into the carburetor and also use the cork gasket that sometimes comes with the filters beware

 

This cork gasket is to thick and prevent a correct tension of the trumpets, they may still be able to rotate.

I make a new paper gasket to solve this.

[Clousta]

Elie,

nylocs and plain washers fitted and tight. Everything looked clean and OK. The air trumpets do slide into the carburettor body but no gasket was present on any of the carburettors or any indication of one. The air trumpet retaining plates are firmly holding the air trumpets in place. They can't rotate.

Air filters look OK and are back in place.

Job done!

 

Gavin

1988 1700 Supersprint (LA, long cockpit)

[nickh7]

Poo let me know when you are taking it apart and I'll try and get over to give you a hand . Time for an upgrade i feel 😬

 

Fuel your Addiction here

[Pooh_R]

Head is back on now and I'll get it back on the road later today. I'll make a plan to change the pistons when I can make myself stop driving the car later in the year 😳 ☹️

[Graham Sewell]

Gavin,

 

Were you correct with your sizing of 6mm nylocs?

 

Cheers,

 

Graham

------

Low tech luddite - xflow and proud!

[Clousta]

Graham,

Yes.

 

I have a selection box (stainless steel, I think) of metric nuts and metric nyloc nuts.

I used this reference Dimensions of metric hexagon nuts (second table) and my micrometer to mark up the box with the correct M designator (I also marked up on the box what spanner size I needed for each for future ease of use). I now know that I have a selection of M8, M6, M5, M4 and M3 plain and nyloc nuts.

 

M6 nuts need a 10mm spanner/socket.

 

This table helped with the washer sizes metric washer sizes(third table). But once you know the right M size nut then getting the right M size washer is easy.

 

 

Gavin

1988 1700 Supersprint (LA, long cockpit)

[Pooh_R]

You're quite right that they're M6 studs on the carb. It is curious to note that Webber use non-standard nuts which need an 8mm spanner rather than the more normal 10mm as Gavin suggests.

[elie boone]

They are called K nuts and are used where space is rather limited

[Clousta]

Interesting. I have just rechecked the nuts that were fitted to the studs. They are normal M6 nuts and a 10mm spanner fits them. With K&N filters fitted there isn't a space issue and ordinary M6 nuts (or nylocs) fit.

 

My engine was built by Oselli Engine Services in 1986. K&N fIlters are specified on the invoice so were probably fitted by Oselli hence the difference in M6 nuts compared to a Caterham supplied engine.

 

Gavin

1988 1700 Supersprint (LA, long cockpit)

[Clousta]

Elie,

I have just looked up K nuts

K Nuts

and these were definitely _notfitted to my Seven/

 

Gavin

1988 1700 Supersprint (LA, long cockpit)