Oil consumption

[peterg]

After getting my engine rebuilt four years ago by a major Bristol specialist and finally taking it for a long run I am absolutely gobsmacked to find that it is burning one pint of oil per forty miles.

After checking through my invoices I found that, although the engine was totally dismantled and checked the pistons were left in situ.

My question is could this level of oil consumption be caused by worn valves or am I looking at replacement pistons and another engine rebuild?

[Stefan]

The 2 main possibilities are damaged or worn out rings/pistons , or, more simply, worn valve guides. Compression testing and video inspection will help clarify. If the engine was well rebuilt -bottom end done properly with crank reground, and bores not scored or worn out - just fitting a new set of modern, forged pistons isn't too enormous a job. Annoying, though.

[405dh]

Hi Peter. If the engine has had very little use in the 4 years since being rebuilt and only just had a decent run, I would suspect sticking piston rings or possibly a broken ring(s) as a result of long period of inactivity.
I would advise the rebuilder of your problem and seek their advice as to how best to determine the cause. Only when you have found the cause can you decide if the rebuild has been at fault or something caused by long term inactivity.

A 4 year period since the rebuild puts you in a bit of a difficult position so tread carefully as I think you need some goodwill from the rebuilder to help in the diagnosis of the problem and its rectification. Good luck!

Chris.

[Geoff Dowdle]

peterg,
that is massive oil consumption regardless of the 4 years since the rebuild .
Did any of the spark plugs foul during the long run ??
I suggest you inspect all the plugs to see their condition and oil content .
Check if they are all the same . A compression test is also worthwhile but not an indication of oil consumption

I'm my experience even one pint in 200 miles is more than excessive during a run in period.

It will not be the valves !!
I agree you should talk with the rebuilder as the engine will most likely need a total strip down .

Geoff D

[peterg]

My specialist reccommends a rebore with Cosworth pistons at an estimated cost of £6,400 assuming no other work is required.

In between the car has been doing about 400 miles per year and has had 4 oil changes, hence I didn't notice the oil consumption although I thought it a bit low last time.

While I'm thinking of it on the trip to Germany the car was run at 3,200 - 3,500 rpm (65 - 75 mph aprox) at mid 20's c. and the water and oil temperature rose to 90 o F. (oil about 5 degrees lower) with no oil cooler. Fuel consumption was between 24 and 31 mpg.

The engine ran smoothly but was rather lumpy at tick over speeds (5 -900 rpm)

[peterg]

Further to this, is it feasible to replace the rings and hone the bore without taking the engine out?

I am faced with getting the work done at a low cost or disposing of the car, having poured tens of thousands of pounds into the car and still not having a reliable vehicle.

[Geoff Kingston]

It is possible to get the Pistons out to hone the bores and replace the rings with the engine still in the car but the head obviously has to come off.
£6400 for a set of Cosworth pistons and a rebore sounds absolutely crazy money to me, how much are the pistons and the rings?
If the pistons were left in place one would hope excessive wear was not an issues but if the bores were glazed that would not help especially if you have stuck worn or broken rings.

[peterg]

Works out about £1,000 for the pistons, £4,000 labour plus any other costs - boring, honing and materials.

Hence if if they can be replaced and honed without the head coming off and engine in situ there should be a big saving in labour and piston costs.

The engine was completely rebuilt with new liners and pistons (from Bristol's) 40 years or 20,000 miles ago, so there should be minimal bore wear. I do remember the specialist saying that he could still see the honing marks in the bore of the cylinder.

If there are any people in the west country who can do the job for less I would be very interested!

[Kevin H]

Peterg, that's a huge amount of oil for the engine to burn in 40 miles and continue running properly. Are you sure you haven't got an oil leak? It's possible for an engine to leak little or no oil when ticking over, but leak quite a bit under load when the oil pressure is higher, although even then, a pint every 40 miles is still a lot!

Have you driven it several times and each time experienced the same loss of oil?

As for replacing the pistons, £6400 seems very expensive. How many hours labour are included for £4000?

I think you need to do some proper diagnosis before getting too gloomy. As Stefan says, a compression test is essential and while you're at it you can check the state of the plugs. If it's really burning that much oil I would expect the plugs will be heavily oil fouled, to the point where the engine is missing and probably becoming hard to start.

Never had an engine "rebuild" where they leave the pistons in place, I hope you didn't pay much for it!

[peterg]

Sure the engine leaks, it's a Bristol! but no, not much. When we got to Dunkirk the plume of smoke on tick over was about 10 feet long (no wind) and other drivers had commented on the amount of smoke coming out of the car on the run over the weekend.

This was the first run over 40 miles as mostly it is only driven from home to where I keep it due to the constant problems I have had. It has also had numerous oil changes (aprox every 400 miles), so I didn't check the oil level very often.

I was told about 100 hours and this matched the costs from another repairer.

The engine runs rather well once off tick over with no misfiring. Performance wise this is the only Bristol I have driven. It seems a little slow off the mark but quite perky once it is rolling along. I red line it at 4,000 rpm.

[Geoff Kingston]

To do the job with the engine in place the cylinder head has to come off, if the car had new liners and pistons 20000 miles before you had the work done on the engine I can understand why the specialist left everything in place but that is taking a risk which in this case does not seem to have paid off, especially when everything else was presumably already apart.
Many years ago I had a 401 with new liners pistons and rings, it was not getting much use and after an accident it stood for about six months and the engine stuck, we eventually freed it off and following an oil and filter change it ran well but still had but a tendency to oil up the plugs on 5 and 6. I suspected broken rings so in the end we took the engine out, head and sump off, rods and pistons out and sure enough there were broken rings on pistons 5 and 6.
As the pistons and liners were perfect, the cylinders were lightly honed and all we had to replace were the rings and gaskets.
As I said it was oiling the plugs before it stuck so the problem may have occurred previously or the rings might have even been damaged on installation, before we finally took it apart it stuck slightly on two other occasions, it was a tight engine that did not like being stood.
As your car has covered a low annual mileage after the liners and pistons were replaced I would be inclined to suspect a ring problem and If it were mine I would look seriously at replacing the rings with the engine in place. Unless the car has suffered piston damage from broken rings or some other cause I would get the usual checks carried out and reuse them as well, just give the bores a very light hone and fit new high quality rings.
Obviously ensure the rods and pistons go back in the correct order i.e the cylinder they came out of.
I had a oil burning situation suddenly develop on a Bentley S2 a few years ago after a 200 mile trip, my mechanic discovered that two of the valve guides had worked loose allowing oil to leak into the combustion chambers, the guides had not been installed properly so I had to get them replaced and we also fitted improved valve seals at the same time and had no further problems. I have not come across this on a Bristol engine but it might be worth checking while the head is off.
One final thought I assume you are using the correct grade of oil to a classic formulation as modern oils can cause high oil consumption and low oil pressure problems on some older engines.

[Kevin H]

Have you done any investigation yet Peter, such as inspecting spark plugs or doing a compression test?

[peterg]

Got it checked today. All pots 100 to 110 psi, number 4 pot heavily oiled.

They also said the clutch was close to slipping, which is how my expert said it should be.

Think it is time to get the rings replaced.

[peterg]

So now the engine has been stripped and there is still no obvious reason for the high oil consumption. Three of the cylinders are showing carbon deposits but, apart from a ring that was cracked, no issues with the rings. The cylinders have little glazing and some of the original marks around the bore are still visible.

The oil supply to the head isn't enough for that level of consumption, so there is a lot of head scratching.

Apparently the pistons have spilts in the skirt which I know from motorcycle experience, but didn't know they were used by Bristol's.

Any sagely advice would be welcome!

[Geoff Dowdle]

petrg,
which ring was broken ?
What type of oil rings were fitted? A one piece or segmental ?
What is the clearance between pistons and bores ?
If they are one piece oil ring fit one in a bore and check the distance between the ends .
The cast split skirt pistons run tighter clearance than the original forged pistons .

Geoff

[peterg]

It wasn't no. 4 Geoff. He said it was 1/3 broken when it came out. Sorry to be vague but the car is a long way away and I haven't seen it.

Do you know what clearance the pistons run? my specialist hasn't seen one before. Also, do you know who manufactured them? although I'm sure he will go for a rebore.

Thanks for the reply.

[peterg]

Latest update.....

The liners are standard top and bottom but between 3 & 5 thou out in the middle with signs of the original honing along the surface. My guess is that they have done less than 10,000 miles since installation. Don't suppose I can return them to Bristol's now!

The recommendation is to replace but my worry is why should they have bowed. Is it due to a lack of support in the block?

Also, should the split skirt pistons be replaced with solid or slipper pistons or used with the new liner?

Things are not looking good...

[Geoff Dowdle]

peterg,
my guess is that if you measure the very top and bottom of the liners they will be standard as the rings don't come in contact with the liner .
If in the centre of the liners there is .003 to .005 the liners are well worn .

what is this centre measurement of the liner compared to the measurement at the top of the skirt at 90 degrees to the gudgeon pin .
It is only a guess but I expect it might be as high as .007 in compared to about .002 inch needed for those split skirt pistons.

If the pistons are in excellent condition you would then have to replace the liners & rings
or
buy a new set of .010 or .020 pistons and bore the liners to suit.
Either way it will need fixing correctly

Geoff

[peterg]

My man wants to replace the pistons because they are not suitable for use with modern petrol.

My anxiety is that the distortion is due to lack of support in the block, or are the liners rigid enough not to need it? My only experience is of wet liners that are much thicker.

[peterg]

Has anyone heard of JP pistons and are they a reputable brand?

Also, is it worth shelling out £200 extra to have 9.5 : 1 pistons in preference to 8.5?