Light knocking noise in the engine.

Technical MGB discussion
Ian Fozzard
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Re: Light knocking noise in the engine.

Post by Ian Fozzard »

I think you said the engine had been rebuilt in 2016, and you have owned it about 6 months?
Has it always made this noise as far as you know, or did it start after any particular event or maintenance?
By the way, my 72 BGT has an 18v engine and has always had a duplex timing chain (at least from 1978 when I bought the car, and I'm pretty sure the engine and the rest of the car had no work on it since it left Abingdon).

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
Ian Fozzard
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Re: Light knocking noise in the engine.

Post by Ian Fozzard »

Bumpa wrote: Fri Apr 16, 2021 9:04 pm Clearances in engines are much closer than you would think. I woked on an engine where the crank webs were within a few thou of striking the block casting.

Dredging my memory, aren't the con rods designed to be only way round? Could yours have been installed the wrong way round, I mean with the front face at the back? Clutching at straws here.
Hi Mike, I had the same thought as you - rods possibly fitted the wrong way around. Some rods are markedly asymmetrical, but looking at the photos in the original post, these rods look to be symmetrical? May still need to be fitted in the correct orientation, but perhaps wouldn't make a difference to any clearances?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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Lakesailor
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Re: Light knocking noise in the engine.

Post by Lakesailor »

I am certain they are fitted as supplied as a gold seal. It seems to be symetrical. The caps and rods have been punched to allow replacement in the correct pairs. One punch mark on this cap/rod. Two on rod 2 etc.
I don't know if the noise was there as I bought the car the week before the last lockdown and haven't really done many miles in it until this past month. I do think it may have been there, but it only appears when the engine has warmed up.

I clamped the rod to an alloy tube and using a reamer with a snug fit in the gudgeon pin I measured at each side to see if there was a slight bend in the rod (this is the cylinder in which a valve and piston have been replaced).

Image

Image

Image

At each side ( I reversed the reamer) there was no discernible difference in the measure.

I notice that this piston has burn marks above the compression ring at one side (directly above the pin) and hardly at the other. The new piston was fitted in 2016 and has not done 1000 miles since (according to MOT records). This makes me think the piston is tending to be at one side of the cylinder during combustion, which is why I wondered if there was a bend in the rod. The piston could be moving to one end of the pin by default.
The piston moves very freely on the gudgeon pin, but not enough to have excessive play. I will pull another piston and rod (original) and compare.

I will take a bit of vid of the movement on the rod/piston.
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Bumpa
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Re: Light knocking noise in the engine.

Post by Bumpa »

What a fascinating puzzle!
Mike
1969 MGB GTV8 3.9 Conversion, 1977 Triumph Dolomite 1850HL, 1971 MGB roadster undergoing restoration.
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Lakesailor
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Re: Light knocking noise in the engine.

Post by Lakesailor »

I've taken the other pistons out and have done a little video to show what I think may be the answer. Or not. https://youtu.be/hYu_98XYFKc
The newer piston is looser on the gudgeon pin than the originals. It seems to slide along the pin more easily and may be making the knocking noise as it does so. The noise seemed to come from the front of the engine and was more audible from below the car. (Less tappet thrash confusing the issue).
Whilst the oil pressure is great, the big ends are a bit worn , The mains look untouched. The crank looks perfect. I think that the 4 years of hardly being used before I got the car has resulted in some start-ups after the oil has drained down. I have fitted a decent Mann 916/1 oil filter, with a proper anti-drain valve in it, now , but alas probably far too late. Some piston skirts show scuffing, possibly from dry starts.
I have ordered a set of new big ends (They are +0.020") and have a new cam and followers on its way.

But the question still arises, do I get a new piston (which means getting it pressed on). Some of the engineers are shut down as a result of Lockdown. Don't forget, many will not have done a B series engine for years. I am in the chilly wastes of Cumbria, near Penrith.
Ian Fozzard
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Re: Light knocking noise in the engine.

Post by Ian Fozzard »

Phil, this analysis seems to have moved on from the cam to big end knock? Are you now thinking that the witness marks on those items were just misleading you into thinking that was the source of the noise, or do you have two problems?
There should be quite a bit of sideways movement of the pin in the piston, but what you should have zero of is a vertical slop - I think that's what would cause little end knock?
I have a set of used pistons on rods if you want me to measure anything!!
Does that new piston have a dish in the town like the others?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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Lakesailor
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Re: Light knocking noise in the engine.

Post by Lakesailor »

Ah no. The cam marks are still suspect. :shock:
I don't think the big ends are anything to consider. No noise. Good oil pressure. But for twenty quid I would be mad not to replace the big end shells whilst I have it in bits. The ones there at the moment are scuffed, again I think from starting it up just a couple of times a year.

The 'new' piston
Image

The originals
Image

All are standard and the bores have no ridge. Just looking at the pics I realise the valve cut-outs in the block are the reason the pistons have burn marks above the compression rings. That may be why the piston moves along its pin. the explosive force pushing on that side of the piston.
Again, having just been started and never run properly, there is a lot of carbon in there from running on choke and never getting up to temp.
Similarly, it looks like the head gasket was never nipped down after a few heat cycles.
Ian Fozzard
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Re: Light knocking noise in the engine.

Post by Ian Fozzard »

Phil, a couple of things:. I see you have ordered a new cam, but if you want any measurements of cam/pistons/rods I can probably help.
Secondly, it is possible to fit the gudgeon pin yourself, but you have to be pretty confident in doing it. Getting the old piston and pin off the rod is the hard bit.
All of the engine builders I've seen doing this use a gas ring to heat up the little end until the gudgeon pin slides in and out easily. Then it has to be quickly assembled and centred (more or less!) before the rod end cools and grips the pin. Not recommending you do it yourself though, if you get it wrong it could be costly!
Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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Lakesailor
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Re: Light knocking noise in the engine.

Post by Lakesailor »

I was reading that Aldon prefer not to use heat. But the pin does need a push of between 3 and 5 tons to move it. I have a friend who is an independent engineer who has the gear. It's a matter of getting a piston/pin with enough interference not to slop about too much but not be too tight.
Quite how you manage that, I'm not sure.
Ian Fozzard
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Re: Light knocking noise in the engine.

Post by Ian Fozzard »

It seemed to me that the rod/gudgeon pin fit was fine? Any looseness may be in the piston fit?
Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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