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.

[peterg]

Yes, having checked

[Kevin H]

Okay. I assume you are in the UK, but if you are looking at piston manufacturers in Australia then Special Piston Services are recommended ahead of JP
Special Piston Services High performance Forged Pistons (ask for Colin!). However they are probably more expensive.

This came from a friend who rebuilds Bugatti engines, and is a bit of a perfectionist.

[Geoff Dowdle]

Peterg ,
you keep talking about distortion of the block .
I have been working on Bristol 6 cylinder engines for over 40 years and have never heard or experienced distortion in the block as you described .
My guess is that the liners are badly worn which would probably cause your excessive oil consumption.
Have you measured the pistons and compared it to the diameter of the bores as I suggested earlier.

JP pistons are cast copies of the original 8.5 compression forged Bristol pistons with three wide compression rings and segmental oil rings.
In my opinion the cast split skirt piston you have are a far better designed and made piston than the JP . Why does your engine man think that your pistons are not suitable for modern petrol ?? It sounds very strange .
Post a few photos of your current pistons .

But on the other hand I have fitted the JP pistons to a number of engines and they run quite well for road use. They are quite a bit heavier than the original forged pistons .
Why do you want 9.5 pistons ? Your block and head have probably been machined a number of times over the years and the 9.5 pistons could give you 10.5 to 1 compression . Unless you are racing it is not ideal for normal road use . What octane fuel is commonly available in UK .
I use 98 or 95 octane unleaded (with no Ethanol ) in my Bristols in Australia,
you would need that at anything above 8.5

There are probably a number a different good quality pistons available in UK far better than the JP pistons .

Ross Pistons in USA make a 8.5 piston for 2 litre Bristol , they are forged , two thin compression ring , flatter crown than high domed original 8.5 .
Overall better design and quality

Geoff

[peterg]

Geoff,
Working on a now very limited budget I'm throwing out worries, possibilities and options to gain opinions before throwing cash at them.

I couldn't work out how the wear occurred where it has. Opinion is, rather than engine block problems, that it was poor machining when the liners were originally installed.

On the options front I can replace the liners and reuse the split skirt pistons my specialist doesn't like, use JP pistons, 9.0:1 Hepolite or 9.5:1 Cosworth pistons at ascending cost; or rebore to plus 10 thou and install JP pistons or Cosworth (presumably). At a higher cost I can even get new pistons made. There may be other options but I haven't discovered them yet.

I have no idea why he doesn't like them. Possibly because they are not recommended for racing?

We have 95 & 98 octane here and I have no problems running my Gold Star (9.5:1) on 95, hence the consideration as the pistons are available.

If other pistons are available in the UK I would like to know and whether it is safe to rebore the liners as Bristol's didn't recommend it.

[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]

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?