A very sad piston - why?

[Hallum]

Had some issues recently with the engine running hot, which I am pretty sure now is down to the radiator. While I had the radiator out I wanted to pull the engine and have a look over it (I received no service history with the car, nor have I ever spoken to a PO as purchase from a scrapyard).

After removing the head I found the gasket was in pretty good shape. Pistons were quite coked up. Sadly I did find this is what cylinder 4 looked like after removing the majority of the carbon (very carefully).



Now I haven't worked on an engine before, but even I can appreciate this is not good. So I am curious as to what has happened. It looks like a chunk of the piston has broken out and then been smashed back onto the surface of the piston by the valve. Strangely, the valve does not look to be at all damaged, nor the cylinder itself. What could have caused this? Overheating? Pre-ignition? Foreign material? I assume this can't have happened in my ownership as I would have thought it would have made a hell of a noise when it happened?

So, what are my options now? I assume the only one is a re-bore and new pistons?

Another view of #4


#4 before cleaning


A much happier looking #3


Engine out


Thanks,

Harry

[yorkshire_spam]

Maybe valve springs shattered or collets dropped out and it ended up with a valve in the cylinder - happened to us on the border raiders last year resulting in this:


Previous owner probably just replaced the valve after fishing the bits out instead of a full strip down and replace the piston?

[Nick Jones]

Looks like it's dropped an exhaust valve on #4 at some point and the piston has taken right beating.

Obviously it can still run in this condition and may have done so for some time.  If the bore looks good, it wasn't previously using much oil and the compressions were fairly even, then you could continue to run it.  Not especially recommended if you intend to use it hard.

The risks are that the piston damage has closed up the top ring gap and nipped the top ring.  Two of the gouges where the valve head has been caught sideways are really rather deep........ Bent rod not beyond the bounds of possibility either.

Probably nothing to do with your overheating problem except that if the car has a long history of running hot then that may have contributed to dropping the valve in the first place.  Have a good look/prod around in the block water jacket, especially at the back.  They are prone to silting up.

Nick

[Hallum]

Hi guys, thanks for the insights.

Oil consumption has always been, good, as is compression. I did a compression test, all be it cold as the engine was already out of the car, and #4 was the lowest. Just under 10% less than the highest 2 and 3 which I thought was acceptable. #1: 140 #2: 155 #3: 155 #4: 135.

How thick are the pistons? Is there a possibility of it blowing all the way through on the larger gouges? I hate to suggest it as I am not a fan of 'bodges', but could material be added to the piston in situ for strength? As much as I would like to do a rebuild, its not really possible at the moment, mainly due to financial reasons 🙁. As I said earlier, this is my first time on an engine so its all a bit unfamiliar to me.

Thanks!

Harry

[Nick Jones]

If you are going to run it on as is then don't mess with the piston beyond dressing off any sharp edges (if there are any).

Some slight possibility of it blowing through under hard use but if it hasn't done it yet then it probably won't.  Minimum thickness is in the centre and probably ~ 5mm.

Nick

[yorkshire_spam]

Well it's risk vs cost time!
1) Put it back together and keep fingers crossed. Might last years, might last minutes. Do ya feel luck? 😉
2) Second hand piston, but just change 1. Put it back together.
3) £££££££ full set of new pistons and rings, new bearings at the bottom end and piece of mind for the future.

[dusz]

If there is no lip at the top of the bores I would be tempted to take the pistons out, hone the bores and fit a new no. 4 piston with new rings all round. Possibly look at the condition of the big end and crank bearings and journals. If these look OK I.E. no scoring and the journals look clean with no scoring and they measure up OK I would put back as is. Check no. 4 con rod for straighness/damage.
Clean all the oil ways and use plenty of oil on the bearings when reassembling.
If you want to blind ut up a bit, then clean and paint the block before it goes back in.
Then enjoy running it in.

[Jonny-Jimbo]

My guess is it's suffered a broken valve on one side and the crescent shaped gauge is the side of the valve head, and possibly the other gauge is from the end of the broken valve stem. the circular marks are likely to be from as YS said, the other valve dropping into the engine.

How long have you been running this engine for Hallum? If it's more than a few years, you'll probably get away with it, even if it's not the 'right' thing to do.

[Hallum]

Interesting. Yorkshire_spam I am not sure I do feel to lucky after Nick saying the piston is only about 5mm thick. The biggest gouge is not far off the centre and probably about 3mm deep at its worst point. I think every time I went up a steep hill in the future I would begin to perspire uncontrollably

Dusz, I like your suggestion. I need to check the wear on the cylinders yet though. I think it is the most cost effective solution other than just leaving it.

Jonny, I have had the car since Sept 2013, but have only done about 500 miles since then (if that) due to working on it a fair bit.

Out of curiosity, if I were to go for a rebuilt, how much would I be looking at all in assuming a rebore is not required? I get that if the piston has not let go yet, it may last longer, but I don't think I would be able to drive it now without the fear of what if?

Thanks for the advise so far gents!

Harry

[Pete Lewis]

This is the old compromise or utopia problem

engines with this can run ok for years,  if you fit one odd piston it must be exactly the same weight
as the orig or the other 5 or serious balance problems will appear.

Depends how many miles you intend to use her or  diving style,
if you only do 5000 pa and dont use it for racing then with normal, spirited driving I would rebuild
sleep easy ,  keep the finding in the back of the mind

Pete

[Nick Jones]

Quoted from Hallum

Interesting. Yorkshire_spam I am not sure I do feel to lucky after Nick saying the piston is only about 5mm thick. The biggest gouge is not far off the centre and probably about 3mm deep at its worst point. I think every time I went up a steep hill in the future I would begin to perspire uncontrollably

Harry

Yes, but it's a dent rather than a gouge so although it will be thinned at that point, it won't be by as much as it looks from the top.

If you can find a decent used standard "F" size grade piston.......  But you will need to check the weights match and that the rod is straight.

I'd run as is I think

Nick

[cook1e]

Quoted from Pete Lewis

  if you fit one odd piston it must be exactly the same weight
as the orig or the other 5 or serious balance problems will appear.

Pete, it certainly will cause balance problems using 5 other pistons in a 4 pot!

[Nick Moore]

If you put it back together with the original distressed piston, we should hold a Club Triumph to guess how long it will last. My money is on it surviving a LONG time.

[Jonny-Jimbo]

If you do strip it out and put a rubbish County one in that won't put up with this, I suggest you keep hold of this piston as a keepsake. Cool little bit of garage art etc.

[Mark Hammond]

Pah!  That piston'll polish out.....  I've araldited them together before and they've lasted years!!!  

M.

[Hallum]

A little ode to the service of #4 so far.

In all seriousness, I am still undecided on which approach. I have asked round all the usual places for an original F grade piston with the same/similar weight. Waiting to hear back from most. No luck so far. If any do come up, what kind of tolerance is acceptable with the piston weight?

BTW I appriciate your optimism Nick 😉

Thanks,

Harry

[Nick Jones]

Love the pic 😀

I balance to within 1 gram.  You usually find that on an engine that still has the same parts it left the factory with the pistons and rods will be within a few grams. Engines that have been apart and had parts from other engines substituted can be 10s of grams.

What you really need is one within say +/-5 grams (preferably+) and if further off than that it's still ok provided it's heavier so you can lighten it.
Usually lighten by grinding material out of the ID of the ends of the gudgeon pin as steel is denser than aluminium.

Nick

PS.  I have two 1300s here that are potential donors (and a 3rd that isn't).  One is in bits so I can easily check, the other not but needs to come apart.  First one is standard size but the bores were well worn. The second one has at least one very rusty bore and I don't know whether it's std or not.

[JensH]

I'll take a look for what I have in the shed 😎

[Hallum]

Update: Yesterday I stripped the rest of the engine to check over everything else that I could (bagging and tagging along the way!).

Didn't find any more nasty surprises other than a very worn timing chain and tensioner (had about an inch of play either side of equilibrium). It looks like at least 1 tooth had slipped.

I have had a look at the journals of the crank and they look good to me. The do not have any scoring or excessive wear that I can see. Some of the bearings however have some medium/light scoring

I checked the #4 conrod for straightness against a metal rule, which showed that it is true - phew.

I have had a better look at the wear of the bores. I have no means of measuring the difference of the main cylinder from the lip, but its easily noticeable when moving your finger across.

So, the question of what to do still remains. I have not been able to find a piston to replace #4 so I think it is going to have to stay for now (unless one turns up in the near future). What is your guys thought on replacing the bearings, thrust washers, and piston rings along with honing each bore. Is that an acceptable approach?  

Nick, that's kind of you to mention about your surplus engines. Dependent on what you think of the above and the pictures it may be an option to consider.

Also, does the mark "010 U/S" mean +010 oversize on the bearings?

A better look at the #4 piston:




Timing chain:



Some more pictures:









Thanks,

Harry

[Mark Hammond]

Now you've gone that far, you may as well get the engine refurbished.  I used Sutton Rebore to do mine.

Mark

[nang]

In New Zealand, among my mates ,I would say the crankshaft is a 6 letter word beginning with f. (eek)
Tony.

[JensH]

I have a box of 4 standard pistons - 213719 - used, but undamaged. Don't know if they could replace yours. Free to a good home for postage from Copenhagen/Denmark.

From a 1300 small crank engine.

I would buy new pistons and get the bores done to these though, but depends on what you can afford

[RobPearce]

I don't think the crank is nearly as bad as nang would have you believe, but you probably want to get it reground. If it's 10 thou at the moment you should be OK with 20 thou.

[Jonny-Jimbo]

Yup, as Rob has indicated, on your bearings 010 U/S will mean 'ten thou UNDERsized'. Pistons are 'overbored', so the pistons are 'oversized' and bearings are ground smaller so are 'undersized'. Most people will know what you mean, but it depends how fussy you are.

As for the actual engine, although it may have been running well before, now everything has been disturbed you'll really want to at least get rid of the lip on the bore and the crank MIGHT polish up and not need grinding. Measure each journal in at least three places, which will take into account ovality of the journal. This will tell you the wear variations. Also, don't use a vernier, you'll really need a micrometer.

[nang]

Sorry, my comments may have been a bit harsh. What I meant was shaft needed a regrind.
Tony.