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Tapping Noise At Idle

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#1 Sterling M

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Posted 27 April 2024 - 06:50 PM

I have just recently rebuilt my 1968 Australian BMC  Mini 998. Bored to 40 thou over-size pistons and rings, new bearings, with new followers, new lead free seats in the exhaust valves, and new rocker shaft, new duplex timing gear without the tensioner. It has developed a tapping noise and I just can't find the cause.

It is present when at idle and goes away when the rev's pickup from idle.

I've removed the rocker gear and disassembled, checked and everything looks good. Because it's a new shaft, it has a second feed hole at post number two, so I drilled and tapped this out to insert a blanking bung in the mounting post, same as post one to ensure it was not loosing oil pressure through the hole. No change to the noise.

Did a compression test (cold) and with the same amount of cranking, they were 180, 170, 170, 180 PSI.

The motor runs exceptionally well with great power and no missing what so ever.

I've checked with a stethoscope and still can't locate the source. Checked all auxiliaries, (generator, front pulley, fuel pump, water pump, distributor, carburettor, clutch/throw out bearing) and all ok. Front/back, each side with no louder at any point than the other. Clutch depressed makes no difference so not the primary gear.

I've reset the rocker clearances multiple times with no change.

I've ran the motor with the rocker cover off and ran a 2 thou feeler gauge under each rocker in turn with no change with the noise.

It has been doing it for about 2600km with no other changes in performance or noises.

 

It is running good oil pressure (60 PSI on the gauge and over 40 at idle when hot).

 

The tapping sometimes stops or changes in volume, but always returns.

Can anyone suggest anything else before I pull the head and check the valve seats as that's the only thing I can think of for the next step?

TIA.

 



#2 Quinlan minor

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Posted 27 April 2024 - 07:01 PM

Could it be?

"new duplex timing gear without the tensioner."

Chain slapping the timing cover at tickover.

Perhaps pull the timing cover and check for witness marks, before the head?

 

 



#3 PeterG

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Posted 27 April 2024 - 07:51 PM

I've had the same noise for years. I believe, but cannot prove, that its the exhaust valves seating on the hardened valve seats. Like you I've gone through timing chain and rocker clearances and also tried the stethoscope to no avail. If anyone else can give greater insight I'd like to know.



#4 sonscar

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Posted 27 April 2024 - 08:35 PM

One of the pistons wrong way round?Steve..

#5 Spider

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Posted 28 April 2024 - 12:53 AM

Check the Harmonic Daper (or front Pulley) Bolt is tight.



#6 Sterling M

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Posted 28 April 2024 - 02:37 AM

Thanks guys, I have checked the timing cover with it running with the stethoscope and there's definitely no tapping coming from there.

Pistons all correct, also the noise is slower than the piston travel so still leaning toward a rocker/valve/seat timing.

I have checked the front pulley, it is the earlier version without the harmonic balancer, but can't get any movement out of it at all.

The valve hitting the seat sounds quite plausible, thanks for the idea. I'm hoping to borrow an endoscope and check out the valves/seats area to see if I can determine if anything is loose.

I disconnected the fan belt today just to prove it wasn't the fan or water pump and it made no difference.



#7 PoolGuy

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Posted 28 April 2024 - 08:13 AM

Are the pushrods turning?



#8 Shooter63

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Posted 28 April 2024 - 10:37 AM

With the rocker cover removed and the engine running, try pushing down on the tappet adjuster end of the rocker with a wooden dowel or something similar, if the noise goes away its probably a slightly tight lifter in the lifter bore.

Shooter

#9 Sterling M

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Posted 28 April 2024 - 03:29 PM

Thanks for the suggestions, I'll check them out and report back, many thanks.



#10 alpder

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Posted 29 April 2024 - 02:39 PM

I had a bit of chain noise on my new rebuild (has duplex with tensioner). I'd not beaten-out the dimple in the cover (where the crank sensor would fit). Chain wore into the cover metal.



#11 johnR

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Posted 29 April 2024 - 03:35 PM

A duplex chain can catch the circular shroud around the breather outlet if you have one?



#12 Sterling M

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Posted 30 April 2024 - 02:51 AM

OK, running with the cover off, I could see most of the push rods turning and those that weren't were free enough to turn easily with a light push from a finger.

Pushed downward on all adjusting screws and that made no difference to the noise.

But, watching the rocker arms closely with a torch, I could see two of them appear to bounce with an extra gap for a while then it would take up, but when I check the gaps they are all good again....almost like "Shooter63" suggests a slightly tight lifter in the bore. 

Hopefully this rings a bell with someone?

Next step I'm thinking of is doing the gaps again with extra force on the push rods to ensure the follower is definitely sitting on the cam,,,,any thoughts please?

Thanks.



#13 PoolGuy

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Posted 30 April 2024 - 05:22 PM

OK, running with the cover off, I could see most of the push rods turning and those that weren't were free enough to turn easily with a light push from a finger.

 

The pushrods spin by virtue of the followers spinning, the pushrods spinning is an indication that the followers are spinning, if they don't you'll get premature wear on them and the cam, causing a noise like poorly adjusted valve clearances.



#14 bantu

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Posted 08 May 2024 - 11:42 AM

Any update on your findings?

#15 Sterling M

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Posted 08 May 2024 - 05:25 PM

Hi there, in the process of sorting out another head for a swap later when complete.

My latest theory is either a valve seat is on the move when at idle but not when at speed, or the valve guides were not reamed after install and is causing a valve to stick and not close fully.

It seems to be only happening at a certain rev figure. When I turn the motor by hand to set the tappets, all is fine. At idle the noise is present and when driving down the road at 40km or more it's fine.

As above, when I watch the rockers at idle with the cover off, I can see two valves periodically not return to their fully closed positions, giving a big gap between the valve and rocker causing the noise. Valve seat or sticking stem??

Compressions are really good at cranking speed which may support the sticking valve stem theory...

Can't get hold of a bore scope to have a look, so will swap the head when I have one ready.







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