950cc engine knocking

[Giles]

Picking up the thread that veered off from '950cc valve clearences', I am still trying to work out what the noise is.

Not valve clearences. Not pinking. Not leaks from the exhaust (at least i can't find any), plugs all tight (and colour good)... I suspect small ends because of the engine's history (I was told that the big ends were recently renewed due to a dry sump issue), but i don't want to have to get the engine out unecessarily (but also i don't want to wreck the engine driving it about listenening to it self destruct!).

Here's a link to the engine running. Tappets a little noisy but the engine is cold (hence the hesitation when i pull the throttle: i have the choke in to reduce the revs and make the thing audible), but you can hear the clatter as the engine accelerates. It does the same pulling away or while cruising at speed. No oil pressure warning light, no lack of power through the rev range up to over 100kph. I can get light pinking foot to the floor in 4th gear at medium speed.

mojobaby tells me that pulling off the sump will be a bitch with the engine in the car... is this because the anti-roll bar and cross member get in the way? Can i just remove them first and put the front of the car on stands so that i can get at sump to remove it and get at the crankshaft? I'm still waiting for the Haynes manual to arrive ;-)

All suggestions received greatfully!

 

[Giles]

The reason i say it isn't pinking, by the way, is because it makes no difference whether the engine is hot or cold, I can just about hear what i do take to be pinking at other times (see above) and in the flesh it doesn't really sound like pinking (I used to have to tune the timing on the MG by ear, with an adjustable distributer, pretty much every time i filled the thing up) but listening to the vid is making me doubt myself!

 

[angel]

There are no small end bushings, the piston pins are an interference fit to the connecting rods, and run directly in the piston bore. The piston itself always gets damaged in other areas (skirt, ring lands) first long before the piston poin bore gets worn.
If the engine ran without oil, it's not a matter of just replacing main/big end bearing shells. If you are sure the noise comes from inside, it's way better to take the engine out. There is very little space at the rear end for the sump to pass, due to chassis getting in the way (the engine needs to be raised about 3cm anyway). Plus, you can't check components like camshaft, tappets or pistons and rings.

 

[Giles]

Trying to think my way through this noise issue before i get the Haynes through and start taking the thing to peices, could it be anything to do with the camshaft? I suppose i could pull the head off and look at the valve seats and cylinder bores? Or am i better off just taking the whole lump out and taking it to bits on the bench?

 

[harbourseal]

May I suggest that maybe the rockers are hitting the rocker cover because it is screwed on to tight?

 

[Giles]

May I suggest that maybe the rockers are hitting the rocker cover because it is screwed on to tight?

I did wonder about that because that is kind of what it sounds like, but you only hear the noise when the engine is under load and the rockers would rock at the same height whatever the engine was doing. Also, when i had the cover off to adjust the clearances there didn't seem to be any sign of damage on them. I didn't think to look inside the cover, because the thought hadn't occurred to me until i was tightening it up again afterwards. There are little rounded bulges in the cover where the rockers would be... but i don't know, they look manufactured?

 

[malcolm]

That's a good idea. I had a car with terrible knocking from the bottom end which turned out to be caused by a dent in the sump.

You might be able to narrow down the source of the noise using a large screwdriver. Hold the metal end to the engine and press your ear against the end of the handle. That allows you to hear the vibration, and the problem is most likely to be where the vibration is loudest.

 

[Giles]

That's a good idea. I had a car with terrible knocking from the bottom end which turned out to be caused by a dent in the sump.

What about the fact the in these cases, load should not have any effect?

 

[mojobaby]

Hello Giles, sorry about the misinformation about the bushing in the small end; Angel corrected me, thank you Angel.

I think that Harbourseal could be correct. That would save you a lot of work. Its a sound like hitting a tin can. Perhaps you only hear it at high revs is because the rocker is hitting the cover harder and more often.

The covers on both my cars have an inner cover plate, only visible when the cover is removed and turned upside down. It's probably there to stop oil flying out of the breather tube. Perhaps the rockers are hitting that?

Put your hand on the cover when you accelerate and see if you can feel a vibration. I suppose you could also run the engine without the cover, might be a bit messy though.

 

[Giles]

I'll have a look at that tomorrow.

I have also found out which garage did the post dry-out repairs so i am going to pop along and see if they'll tell me what they actually did and at what mileage (i am hoping that the last owner will send down some paperwork with the reg documents, but i'm not holding my breath). I hope they'll not be cagey and worried that i'm going to accuse them of dodgy workmanship... i bought the car knowing about the knock so I am prepared to take it to pieces, but i just want to know what i'm looking for when i do.

 

[mr-reno-139]

I once put a big end cap the wrong way round after fitting new pistons
That made a very strange tapping noise
Maybe lookout for a spare engine and just keep driving it
To strip and rebuild engine won't be cheap
I fit has been run out of oil and had big end failure it normally takes crankshaft with it
Do you know anything about what was replaced previously

 

[Giles]

I went to the garage and they were very cooperative... but, the rebuild was done more than 4 years ago and the place has changed hands since then and there are no computer records any further back than the handover (2012). The new owner was a mechanic under the old owner and can remember the car, but not exactly what they did. What they did find was a post shunt bodywork repair that i didn't even know had been done - good workmanship.

There is no mileage on the bodywork invoice, which is a shame because i can't make a guess how many miles are on the clock since the mech repair a few years earlier. I am hoping that the last owner is sending the old invoices down with the rest of the paperwork... there might even be a fair chance of this, he seems to be doing everything else by the book (the car belonged to his late mother and was sold as part of her estate).

So, it might be that there are enough miles since the repair that what i am hearing is entirely unrelated. Or maybe not. The waters are still pretty muddy. I have not had a chance to go and tinker for a couple of days... hopefully today.

What are the cost going to be for a stripdown? Not that great surely (unless i find something truly horrid)? Some gaskets? Some big end bearing shells (might as well do them anyway)?

 

[harbourseal]

Hi Giles,
I have rebuild my 845 engine because it had seized. I replaced many parts including head gasket sump gasket and many things more.
All the big end bearing shells were fine.
Some parts I did find online from people that had bought them and did not use them.
I got the cilinder liner gaskets from https://www.franzose.de/en/Home/
And the piston springs from a local engineshop.
With the planing of the head and balancing and planing of the flywheel I payed about €450,- in total.
I could have bought a spare engine. but it was good to get the experience of doing this myself.

But still it would be better for you if it is just a minor thing that causes the knocking.

 

[Giles]

Well I can't feel any tapping on the rocker cover and it is definitely not pinking because I retarded the spark bit by bit until it would barely reach 80kmh and there was no appreciable difference. I tried pulling the HT leads, and it is possibly quieter with no 4 unplugged but it's hard to tell. I am taking it to see the local 'car whisperer' on Monday (he has been away till now). If the noise is the same and the oil pressure is still good by the time I get there, it should rule out big end ;-) I would love it to be a minor issue, and I don't know how much better a spare engine would be (it still has to work AND be swapped out), so here's hoping. 450 euros isn't too bad though... I had budgeted for a bit of work needed - looks like the dents might have to wait.

 

[Giles]

Ok, so after much head-scratching and wandering around with a piece of pipe sticking out of his ear, the verdict was... early stage piston slap. This was helped by the fact that after the 30 minute drive to get there the noise had reduced to almost nothing, but returned after the car had sat for a few hours. Compression good on all cyls and nothing to see down the bores

Debating now about whether to strip down now, or pop in another engine (from somewhere) and strip down later when the weather warms up. I'm guessing it would need a set of new sleeves and pistons
... what is availability like on said items? And is it the same engine as in the twingo?

 

[Laurie]

that video sounds like a sticking valve...what were your compression readings? I would take the rocker cover off, start the engine again cold...the noise may be a lot more apparent and easy to locate..I have just stripped my head and the valves very dirty...lots of carbon build up...I have just dripped it off today to be skimmed

 

[Giles]

The compressions were 15 across the board... can't remember whether psi or kg (not my meter). I had a sticking valve on my old VW combi, and it sounded fairly different from how this sounds in the flesh. This deffo sounds like gently hitting a large lump of metal with a hammer, and the valve sounded like kicking the road with the sole of a hard shoe. Also, it runs smoothly up to over 100kph (on the clock) with no backfiring etc.

I don't think I will know for sure until I take it to pieces, and I'd be better off doing this with the engine out as I do suspect that it will turn out to be piston slap.

 

[mr-reno-139]

Piston slap wont do any harm to carry on running as is any as to fix it will involve major work as 8f you go to expense of pistons and liners you will end up with complete rebuild and car off the road
I know where there is a good running spare 956 engine here in UK but is not very practical for you to go and get
Maybe try and find a known good 956 or 1108 nearer to home that may be most economic way forward
Twingo engine won't fit nor any transverse engines

 

[mojobaby]

Just been researching "piston slap" as I knew nothing about it. The article that I read says that it probably won't do any harm to the engine, like Mr Reno says. It's the sound that's a bit annoying.
Someone also suggested in the article that it could be caused by a lack of the correct lubrication in the bore. So what grade oil have you or the previous owner put in? It should run on a thicker oil, 20W50.

 

[mr-reno-139]

Piston slap normally caused by overheating where the engine was driven too far with red light on as the rear piston gets hottest the piston expands and the resulting friction will wear some of the alloy skirt away
If this is diagnosed as just head gasket failure even skimming the head and new gaskets will result in the piston which is now too loose in the bore making a tapping noise un till engine is warm
Chances are the previous engine problems were related to gasket failure not bottom end as thought
My theory having done hundreds ? Of renault gaskets if it's driven until it's knocking forget just doing gaskets
Also my rule of thumb if head is distorted by 20 thousands of an inch or more give up and start with another engine
Have had cars run perfectly when I've explained to customers the risk of noise and skimmed head and just done gasket but doesn't make car easy to sell on