Compression Test Results and piston coke clean up

[Trever the rever]

This winters project is to finally undertake the engineering required for my 1972 TR6 to run on unleaded fuel. I've never been a worrier on this topic, as just 3k miles per annum whilst not really stressing the engine shouldn't be too much of a problem. However after last years bottom end rebuild, replacement camshaft, cam followers and pushrods with refaced rocker arms, I think something was telling me to finish the job. The engine runs smoothly enough however does breathe heavily at idle with the rich fume under acceleration with some oil consumption.

So my plan was to test the compression and remove the head for the hardened seat installation, new valve guide and general refurb. I guess I'm just hoping the bores are still serviceable at this stage. The compression data (using a Gunson gauge) over 3 rounds of tests are on the attached file. Other aspects to note are I started with a hot engine, and the exercise took about half an hour. No 3 plug had showed signs of oiling up, all the others were good. I had fitted reconditioned injectors two summers ago.

I would welcome comments on the data, my immediate thoughts are not too bad? the car has covered 133k miles since new and I've have had the car for 23 years and covered about 65k miles with only major engine work (bottom end) undertaken last spring. I don't know what was done to the engine before I had bought it. Oil consumption on my annual Le Mans trip is about 500 miles per pint, I do have drips on the floor too! There is some coke on the pistons, so my next question is what's the best method to clean this up. I haven't taken a head off an engine since 1981, so my experience is patchy.

The head is at Carlows in Benfleet,Essex for the work. I have a few weeks to tidy things up, hence the questions. I'll probably send the metering unit off for calibration too.

So in summary, thoughts on the compression data and piston clean up please. Guidance on what to look for on the bores will be useful too.

Many thanks Trevor Hughes, Essex TR Group

 

 

TR6 Compression Test 09 02 2019.pdf

[Andrew Smith]

With a 133k on the engine there's nothing wrong with those compression readings -  I've seen a lot worse on 6's with half that, so it seems previous owners have looked after it very well.

From your avatar it appears you have a CP, these are notoriously lumpy on idle due to the cam profile.

Cheers, Andrew 

[Trever the rever]

Thanks Andrew, that sounds positive. Yes its a CP model. Tick over particularly since last years engine out work has been a little tricky to settle down. When this planned work is sorted, I'll tackle it once more. I know its not a million miles off, primarily I get hunting at lower idle revs and have to set it around 1000-1200 to run smoother. Indeed it runs better on a little choke; is there a clue there?

Trevor

[Waldi]

Trevor,

the average values of the 6 cylinders are within 10%, so “good”.

I removed the carbon deposits from the pistons using a steel brush and I think also used a sharp knife, carefully.

If there is a ridge (step) in the bores, this indicates wear. Mine were fine,I just did a short hone myself.

I measured the pistons and bores, and only replaced the piston rings since they were still within tolerance.

My engine runs very sweet with no pressure in the engine from leaking rings/valves, but it has not been run in.

I installed new valve bronze guides with seals, because I liked the idea, using std double valve springs.

[Trever the rever]

Thanks Waldi, I haven't examined the bores closely yet. We've a TR meeting this week so I'll invite a friend or two around to undertake a visual inspection. If I did opt to renew the rings, can this be done by simply removing the sump, disconnecting the con rods and pushing up the piston from there?

Trevor

[Waldi]

Hi Trevor,

In theory: yes.

But is it really needed? Your compression is good and oil consumption modest.

If you have the head off,  I would first check the head (valve guide clearance). If your guides are worn, that may explain some of the oil consumption.

If still needed, you can remove the pistons, measure the bores from above and check the gaps of each ring, which is normally smallest at the bottom location.I bought Grant rings, had issues with cupped oil rings, but this was resolved by Grant, they sent me a new set, FOC. I did not need to correct any ring, but was able to have good clearances by swapping between cylinders (optimising).

If honing is needed, it is best to dismantle the engine further outside the car. The abrasive dirt may get on the crackshaft and damage the bearings.

For a good quality job I would take the engine out, this makes working more rewarding and will result in much a better quality, but You know better if this is needed. Just my opinion.

Assessing the condition together with friends is a good start.

Good luck,

Waldi

[Trever the rever]

Thanks once again Waldi, your comments make total sense. My hope is the bores are okay and I will not have to take this further, evidence to date supports this. I'll undertake further inspection as stated. The labour on taking the head off hasn't been too onerous and not too much regret.

 

[astontr6]

3 hours ago, Trever the rever said:

Thanks once again Waldi, your comments make total sense. My hope is the bores are okay and I will not have to take this further, evidence to date supports this. I'll undertake further inspection as stated. The labour on taking the head off hasn't been too onerous and not too much regret.

 

With an engine of that mileage, I would carefully check the bores for glazing and for the ridge at the top of each bore. When fitting new rings you have to gaze bust the bores, other wise the new rings will not bed in properly. Also the pistons have to be checked for wear including the small end bushes. If possible try to borrow a bore gauge to check the bores for roundness as they do wear oval.

Bruce.