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MG MGB Technical - Knocking Sound @ Threshold RPM

Greetings, I'm looking for some opinions on where to start with a problem I am having with my 1972 MGBGT with a 1978 engine.

Last year we put a good and very well maintained engine and transmission from a '78 into my '72...ran great until last week.

While at highway speed (pushing 70mph), I began to hear a knocking sound from the engine compartment (or so I believe) - this knocking sound can be described as something between a angry woodpecker and a jackhammer. Very rapid and consistent. Not a clunking or scraping sound.

Here is what I know...or what I 'think' I know...

When I start the car, I hear the sound for only a second and then it goes away. It then idles just fine...smooth...no knocking.

When I drive it, whenever I hit 2500 rpm, the sound begins again. Below 2500 it sounds fine. This same pattern holds true for gears 1-4.

The threshold for this sound seems to lower as the car warms up...I have been driving it to and from work very carefully and near the end of my trip, the knock kicks in at about 2000 rpm.

With the car out of gear or with the clutch pressed, I can rev the engine well above 2500 without the sound kicking in.

Other facts: Oil pressure perfect, fresh 20w50 oil change, tappets adjusted properly, all polluton control systems removed, heater removed, Weber sidedraft carbs.

Hopefully I have all the facts straight on this. I don't have a well trained ear or anything. Any advice as to where to start would be appreciated.

thank you...tk
tk Kunka

tk,

Noises are always pretty tough to diagnose by description, but a few thoughts do come to mind.

You said it does not knock in neutral with the clutch pressed. So, does this mean that it does knock in neutral if you don't press the clutch? If so, it leads me to believe there could be a problem with the thrust bearings. Check the crankshaft end play at the front pulley and see what you find. Also, this symptom could indicate a problem with the clutch.

Then, I might suggest loading it up with STP and see if the routine changes any. If it does change, it would lead me to believe it is some sort of internal engine problem.

Since you can produce the sound when the car is not moving, a stethoscope might help you locate the area of interest.

Charley
C R Huff

To clarify:

The sound does NOT typically occur when in neutral.

Also...

The sound does NOT typically occur when in gear but with the clutch pressed in.

I say 'typically' because of the initial 1-2 seconds of knocking when I start the engine.

Besides that inital 1-2 seconds when starting the engine, the car must be in gear and at the 2000-2500 rpm for the sound to occur. Thus, I should be moving.

If I am moving and either put it in neutral or fully press the clutch, it is the same as it is at a stop. I can rev the engine above the threshhold and the sound does NOT occur.

Hope this helps. Regardless...Your suggestions hold true though...a very good place to start! Thank you!

tk
tk Kunka

Sounds to me like bearing knocking. These are more likely to knock when first starting an engine i.e. before full oil pressure is reached, and when under load i.e. accelerating in gear rather than revving the engine with the clutch or gearbox disengaged.

STP is a double-edged sword. It may increase oil pressure on a gauge, but it does so by reducing the flow through the bearings so can be counter-productive.

What are the hot gauge pressures idling and running?
Paul Hunt

Does the knocking get quieter or stop under hard acceleration? Drive along in second or third gear at around 2000 rpm. lightly accelerate/decelerate, if it knocks between acceleration and deceleration but not under load I would suspect a rod bearing.
John H

Some great thoughts!

Paul...the temp gauge is normal. If you are refering to oil pressure, at idle it is at about 35 and when accelerating about 50.

John...I'll verify this on my way home but I've done similar to your test quite a bit since I have been driving to and from work and trying to keep it right under the 'knocking threshhold'

If I keep accelerating it gets louder and if I ease up a bit it gets softer. If I keep a tad under the threshhold you might not even know anything is wrong.

Regardless of how fast I accelerate, it doesn't seem to make a difference. When the tach hits between 2000 and 2500 it knocks the same regardless of how long I take to get to that rpm.

As I said, the threshhold does vary as I drive and after about 20 minutes it has gone from 2500 down to about 2000. When this first occured on the highway doing 70+ mph I was able to get off of the highway via a very well placed exit, a gas station, and onto an alternate route home.

While I did let the engine cool a bit at the station, it was still normal and for the 14 miles down the backroad the threshhold was 2000 all the way. So what I'm saying in a long-winded way is that colder seems to be 'better' -(higher threshhold) at least after the initial start and the oil pressure has a chance to increase. It gradually gets 'worse' (lower threshhold) and stays at about 2000 rpm when the engine is at normal temps.

You guys are a great help...so with the new information is the bearing idea still the best working hypothisis at this point?

thanx much...tk

tk Kunka

I agree with Paul and John, but would like to add another possibility that I ran into on our TD, and one that is relatively easy to check out. You may have excessive clearance between the outer thrust surfaces of center bearing journal and the thrust bearings in center bearing. This was the cause of a very similar symptom in the engine of our TD. You can check this out by getting a small pry bar (or large screwdriver) behind the crankshaft pulley and move the pulley as far forward as it will go, then transfer the bar between the pulley and the steering rack housing and moving the pulley all the way back while measuring the amount of movement you get. Compare the movement with the crankshaft end play specification in the shop manual. Excessive end play will cause a knocking like a rod bearing because it is in fact, causing one or more of the rod bearings to bind slightly. Cheers - Dave
David DuBois

David may be on to something. When the clutch is held down it holds the crank forward against the thrust bearing. If you haven't done it yet hold the clutch down when starting cold to see if that eliminates the noise on a cold start. If it goes away that may be your problem. As it appears related to oil/engine temp, other sources may be a problem with the cam chain tensioner as it uses oil pressure to push against the timing chain, a cracked piston skirt or loose piston to rod pin. Don't rule out anything that rotates such as the water pump, generator, clutch, etc. Sometimes noises travel and can be a bugger to find. you could find a shop to run it on a dyno, you can then search for the noise under real road conditions while not moving.
J Heisenfeldt

hi your problem is to do with the crankshaft ie bearings,if it knocks on start up when there is no oil pressure,in the 1 -2 seconds that you say,once the oil pressure gets up the knock stops,thats bearings, she's got to come out any way to find out ,
good luck with the knock,
all the best
dave
da wright

tk
Paul 'John and David x 2 all think bearings. Sounds like bigend bearings from here as well. It sounds like you have been driving it like it is for a bit- It will probably need the crank regrinding and get the connecting rod bores measured up for out of round, they will probably need machining as well. The two main causes of bigend bearing failure are lack of oil to the bearings (caused by any of several including. clogged filter-oil too thick either from being in there too long or wrong grade to start with -or oil surge if you are an enthusiastic driver-clogged up oil pickup in the sump maybee ) But in my opinion the most common cause of bigend bearing failure is overadvanced ignition timing. I'd get your dizzy checked out to make sure it's not going past 32-33 deg. at the top end. Sorry no good news today.
Gary
Gary

tk,

I guess I read your "or" as an "and" with respect to the noise/clutch relationship, but the thrust bearings are still a possibility.

To clarify, I didn't mean STP to be a fix, I meant it to be a diagnostic tool.

Also, to learn more, I would try running it alternatively on three cylinders. That is, pull one spark plug wire out, and go drive it. Then replace it and pull another.

If it is a single rod bearing causing the knock, you will probably locate which one it is this way as the knock will likely stop.

In fact, if you are in a jam and have to drive the car, you can prolong its life and reduce the damage by leaving the offending wire off. I did it once for several hundred miles in a 51 Ford 6-cyl pick up, and only had to replace the rod bearing at the end of the trip.

Charley
C R Huff

Slack off the fan belt and test the fan for rocking back & front - the water pump can make a similar knocking noise to the rod bearings.
Chris at Octarine Services

I pulled the engine on a 1098 midget once, it was knocking like it was about to put a rod through the side/explode. (This was 30 years ago, back in the days when I would get a lead light and work all night until I was done. Now I'm back in for Cocoa and bed at 9:00).

Stripped it down: perfect big ends, pulled the timing cover to remove the crank and the cam chain tensioner (a couple of rubber bands inside the cam pulley) had failed. I could have fixed that with the engine in the car.

IIRR the B series tensioner has a rubber foot that can come off and lose all the tension in the same way.
You can remove the timing cover with the engine in the car.

You can also remove the sump and check the big ends for wear, and see if the main bearing caps are all on. (Rover V8s tend to drop one or more caps, they run low oil pressure when this happens though)

You absolutely sure that a mount hasn't failed or gone soft allowing part of the transmission or exhaust to touch the body?


I also had a rattle once that came on at high speed. I looked everywhere. One day there was an enormous bang, a terrific vibration, and an upwards dent in the bonnet.
A fan blade had come off, must have been cracked for weeks.
Martin Layton

This thread was discussed between 22/08/2008 and 25/08/2008

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