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Knocking engine

Knocking Engine

I believe the engine in my 120 has about 170,000 miles on it. It is either that or 70,000 but when I had the transmission out last year, the bearings seemed loose enough to indicate the higher number and I replaced them. The first time I heard this car run was when I bought it two years ago. It had not been driven regularly for many years. As soon as it started, I was tempted to shut it off immediately due to an alarming knock (it was hard to talk over it when standing next to the car) but I decided to let it idle for a while and see if it got better or worse. The warmer the engine got, the quieter the knock got. When fully up to temperature, the knock was almost inaudible so I paid for the car and drove it 30 miles home. By the time I got home, the knock was inaudible. I stopped at Tivvy Shenton's place on the way home and he noticed that the distributor was loose and advanced to the point of risking damage to the engine. When I finally got around to setting the timing correctly, I discovered that the knock got worse as the timing was advanced. I now have the timing set just a bit on the retarded side. I didn't want to tear the engine down right away so I tried STP which seemed to help just a little. For a while, the knock sometimes came back pretty loud whenever the car sat overnight. Eventually, with regular use (about 1,000 miles per month), the knock was mild when starting up, only noticable when accelerating while the engine was warming up, and disappeared entirely when up to full temperature. After 13,000 miles, the knock is not getting any worse so I just warm it up gently and continue to drive it. I think I'll go at least one more season before I pull the engine for a complete rebuild. At this point, I would just like to know what is causing the noise. I can think of several possibilities. 1. worn wrist pin (or piston) complicated by restricted oil path to the pin which caused it to wear in the first place - over time with heat and fresh oil, the oil path has opened up enough that it is not as bad when cold and quiet when oil is thinner and able to flow in to fill the gap. 2. Worn piston which slaps when cold and expands enough when hot to fit the cylinder better 3. worn rod bearing - which I would expect to get worse when hot but I have heard rod bearing knocks before and that's what this sounds like - so the question is whether there is something about the XK engine that would cause a worn rod bearing to be quieter when hot rather than noisier. This inquiry is more than curiosity as I know I am taking a risk by continuing to drive it. At this point, I am continuing to indulge with the assumption that if I can't hear it when it is up to temperature, it is not going to go catastrophic on me somewhere in the middle of the Scottish Highlands next year. Does anyone have experience with a knock like this to add to my risk assessment? - Bruce Cunningham, '53 XK120 OTS

Bruce, Since you have put 13000 miles on the the car, and you didn't mention any degradation in your oil pressure, I would agree with the wrist pin or piston slap theory. Of those two I would guess piston slap;because the noise will completely go away when the engines warm, a loose wrist pin will still be detectible. If it was me I'd rebuild the engine over the winter. I don't think you would want your XK 50 memories filled with an engine failure fiasco. - Regards, Wray Schelin

I agree with Wray. Piston slap is probably the problem. As an addition help: A very loose wrist pin can give a double tick or knock and would not change as the car warms up. - Cleo Bay, XK120, XK140

I drove a tired 3.8 litre engine in a 150 which also had a knock when cold. You could notice it become quieter as it warmed and once it reached normal operating temperature it had disappeared. A mechanic friend accurately diagnosed it as a piston slap. When we tore it down last winter, the number 4 bore was out of specifications with a lot of ovality within that chamber. New oversize pistons, wrist pins, etc., were installed to give me a nice fresh engine. Another story about piston slap was in a Chevette that started with only 3,500 miles from new. I discussed the noise with my same mechanic friend who diagnosed it as excessive piston/bore clearance and suggested not to touch it. I took his advice and traded the car in with 94,000 miles and experienced no problems from the engine. Condsidering your 120 engine noise, I would also agree with Wray and Cleo in that it is probably a piston slap. If Tivvy Shenton is close, why not get his views? (I met him at Pittsburgh where he drove his 140 FHC with success at this past summers vintage racing event.) And, to go a bit further, if the noise is not a piston slap (which you can live with) it could be terminal and cost you dearly if disaster would strike when you are enjoying the forthcoming XK 50 event as Wray noted. As a final comment, I have an original 120 SE OTS with 33 K miles on the clock. Needless to say, I baby it. When I start it, I am always listening to the engine. The engine noise, with parts moving with each other, lessens as it warms. So, I accept the fact that the engine will be quieter when it is fully warm. After executing this drill for so many times, I would immediately shut it down if an unusual sound developed. Fortunately this has not happened to date. I do not drive my cars until the engines have reached the normal operating temperature. I will have checked the oil level before starting, looked for unusal leaks, and then start and monitor the oil and temperature gauges as the engine warms. Maybe this procedure is my little quirk but I like for the engine to be ready before I work it. And, I know this may be a little silly to some but I respect the overall mechanics, the finesse of the engine and the car in general to not abuse it. To work the car with a cold engine is to abuse it as I see it. - Bob Oates

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