Rethreaded spark plug hole

Hi all.

Need you advice.

I was in a search of 99MY 4.6l P38 in a mint condition for some time. Saw one recently and start to talk to the seller. We agreed on a compression and sniff tests. Test were done today by friend of him (independent garage), result is OK ( kind of).

145,130,130,130,...123,137,,130,130psi. It is low compare to some other results reported in here. Tests were performed with the device which looked like a pistol, it had an extended tube with rubber header. Header was pressed by HAND to the spark plug hole. Mechanic said the engine is perfect. (Spark plugs were all looking good, not washed out)

After that things started to go wrong. All plugs went back OK except one. It was on the left bank ( if you stand in front of the car, looking towards engine), third from you, second from firewall. Mechanic was trying to put it back like 20 times back and forth. He was doing like half a turn by hand, then it was getting tight and he continued with rachet for 1 or 2 turns ( 3-4 times changing positions after 90 degree turn). At this point it was clear that it was not in the thread, he backed up, repeated again and again.

His boss came up with threader and after some attempt they got it right. I think it was a couple of top thread lines which were affected. Then it was more. They didn't clear shavings from inside of the cylinder and put spark plug in, which went OK and was tightened with same torque as the others.

After that they did started the engine, it sounded very good. After the engine was warm, they performed sniff test, which was also OK.

I have a test drive scheduled for tomorrow. Should I worry about the engine?

Sorry for the long post, as I thought these details are relevant.

If they've left metal in the engine do you really need to ask that question ? On top of that its a mate of the seller……..

The Heads are known to strip if someone is careless in putting in the plugs and the threads take a battering.

Not clearing out the thread swarf would have been difficult with the heads fitted, which is why is is best to do it with the heads on a bench....but get it checked....

If you are mechanically able, pull the heads off yourself and clean the area up.

To get a measure on how good the compression results are - find out what the compression ratio of the engine is and multiple it by 14.7....

i.e. 9.1:1 Compression ratio - 9.1 x 14.7 = 133.77 etc etc

Obviously that is for a new engine, over time you could lose 5-7% due to wear, but aslong as the needle was in the green, you should be fine....

I have only used a screw in pressure tester, that way any human error is taken out from not holding the rubber bung in tight enough etc.

If this is a RR you are considering buying, and the previous owner uses this 'friend' to do his maintenance - I would be very wary - as it sounds like he could be a numpty....

then again, if you are handy with the spanners, and the car is sound alround - it could be a nice one - my first thing after purchase though would be to change all the fluids and check the bores after their re-threading attempt.

I've re-cut a thread in situ before, then I vacuumed out the cylinder with a piece of washer hose duct-taped to a wet'n'dry vacuum cleaner (small bore gives you the high air speed required to shift debris.) Of course I held a long match in the cylinder first to explode any unburned fuel - you wouldn't want an air/fuel mixture going through a vacuum cleaner!

I had a poke around in there at the various stages with my inspection camera, all visible swarf had been successfully vacuummed out. Most of the swarf stuck to the grease on the thread chaser anyway.

Thanks.
And, I remember that the swarf indeed stuck to the grease on the thread chaser.

The seller actually sells Jags and uses this garage for a long time. He got RR only 6 or so months ago from previous owner, who bought a Jag from him.

Car is 2002, 4.6l Vogue, black/black, very tidy. Tomorrow I will have a test drive.

StuckAgainSteve said:

I've re-cut a thread in situ before, then I vacuumed out the cylinder with a piece of washer hose duct-taped to a wet'n'dry vacuum cleaner (small bore gives you the high air speed required to shift debris.) Of course I held a long match in the cylinder first to explode any unburned fuel - you wouldn't want an air/fuel mixture going through a vacuum cleaner!

I had a poke around in there at the various stages with my inspection camera, all visible swarf had been successfully vacuummed out. Most of the swarf stuck to the grease on the thread chaser anyway.

Click to expand...

Hadn't thought of that - good tip for the future - cheers SaS

Saint.V8 said:

Hadn't thought of that - good tip for the future - cheers SaS

Click to expand...

I thought a talented spanner man like you would be calling me a right bodge artist for re-cutting a thread in situ!

PS next time I serviced the car I rechecked the cylinder with the inspection cam - no vertical scratches and the compression was still good.

StuckAgainSteve said:

I thought a talented spanner man like you would be calling me a right bodge artist for re-cutting a thread in situ!

PS next time I serviced the car I rechecked the cylinder with the inspection cam - no vertical scratches and the compression was still good.

Click to expand...

- Whilst not ideal as it would be best to take the heads off first - needs must at times, and your innovative solution to try and minimise the swarf, and also using a cam to inspect prior to engine start is indeed prudent and well thought of..!!

PS: Appreciate the compliment!! TVM