68 Bentley T engine rattle under load.

Hi, yes I pulled the gearbox oil pump, took it to pieces, no damage, rebuilt, set the shaft end float correctly with new shims.

I've got a Shadow in the shop right now that I've been trying to find the source of an engine knock. I've posted a short video on Youtube, link below. This is the first time I've posted on YouTube, and when I view it the text doesn't show so I've copied and pasted the text that accompanies the video below the link. The difference between this thread and my video and ultimate diagnosis of the car in question is that the knock was always there, idling, accelerating, coasting, no plug lead on it etc. As a last resort you may want to try the same thing I did by removing all compression load on a cylinder.


This knock was initially thought to be a faulty brake pump, customer had previously rebuilt the pumps himself then with no change actually installed two brand new pumps but the same knock was still there. I pulled the pumps and checked pump to pushrod clearances, which were within limits. A stethoscope pointed to the knock coming from the back of the engine on the right (A) side. I eventually ran the engine with both brake pumps and associated pushrods removed and knock was still the same. Running the engine with a spark plug lead shorted out one by one made no difference to the knock. I next removed the A bank valve cover and while the engine was running pushed an .015 thou feeler gauge under the rocker arms one by one. When feeler was pushed under A4 exhaust rocker arm the knock stopped. Assumed it was a bad lifter then so stripped off the intake and valley cover and replaced the A4 exhaust lifter with a known good one. Got it all back together but knock was still there with no change. When lifter was replaced I did check cam lobe for damage and there was none and cam lift was in specs. So next I removed A bank rocker shaft and both A4 pushrods, put shaft back on and ran engine with no pushrods in A4. Knock is still the same. Then using a pry bar under the rocker shaft I open the A4 exhaust valve while engine is running, finally a result. With no compression load on the piston the knock disappears. So the result of diagnosis is that the A4 piston has likely broken or the wrist pin in that piston is badly worn. I'm leaning towards a broken piston as block corrosion issues in Shadows are known to squeeze the A4 cylinder liner until the piston seizes breaking the skirt or lands. Next step is head and sump off and pull out A4 piston and rod and inspect.

I opened it at the most an eighth of an inch, very carefully. At no time was there an indication that the piston was hitting the valve, I was very aware that it could hit if opened too much. It was definite that without compression in that cylinder there was no knocking. I got exactly the same result opening the intake valve too. The usual break is the skirt, but could also be a ring land broken. I checked inside with the borescope but could not make out any seize marks on the cylinder wall, and the top of the piston wasn't broken. However, there is excess oil getting into that cylinder. It shows as carbon on the plug, and when the engine was last shut off the day before yesterday the A4 piston stopped at the top of the stroke. When I went back to it yesterday with the borescope and cranked it over until the A4 stopped at the bottom of the bore, there were several drips of oil running down the cylinder wall from the top, much more than one would expect to see so I think rings are broken too. Haven't checked compression yet but that will be on the schedule today.

Thanks Jim and others for your valuable input.
Interestingly I did go through the process of removing push rods to try and identify the cylinder, but the noise was still present as in your example. All evidence points to my issue being engine load (and engine speed)related. So I am homing in on the piston, piston ring, wrist pin, little end bearing, big end bearing, as those are really the only load taking items.
I dont think I can do the feeler/pry bar test though. My noise is only present under engine load and not at idle. Although I can load the engine in drive with the brakes applied to induce the rattle, just need to be careful though and this only replicates the fault when all 8 cylinders are firing. I could do the feeler gauge test perhaps.
The key thing here, as with your example Jim, is diagnosis. If I am to pull the heads/engine that is a massive job for me especially if I can’t remove the head(s) in situ. This will be done on my drive at home.

So to get as much information as possible I repeated my compression test. Looks like my compression tester was sticking at 129psi. So I corrected that and got the following results. Warm engine, fuel pump disabled, throttle fully open, 5 cranks (which allowed max compression reading)
Psi readings
A1 160
A2 140
A3. 158
A4. 137
B1. 144
B2. 149
B3. 128
B4. 138

B3 was low at 128 so I did a wet test and got 147, which is a 15% increase. All others look pretty good.
So if I was to do the feeler gauge test that’s the cylinder I would target. I need to cover all and every test I can to diagnose this before diving into the engine!
thanks again.