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MG MGB Technical - MGB Cylinder Bores

I have a 1973 chrome-bumper MGB which I have temporarily taken off the road for investigation into abnormal oil usage. The first thing that I found was a leak in the oil cooler. This could in all probability be the culprit, but I decided to remove the head and have a look anyway. The piston tops and valves have a chocolate brown deposit on them which to me is an indication that the engine is not burning oil. However, on closer investigation I have seen two things that do not stack up:
The cylinder bores still show honing marks, (probably frm a previous re-build) but rather than the normal cross-hatch they are circumferential as though a honing tool had been inserted and rotated without moving up and down. I have owned this car for five years and done about 20000 miles in it, so I am surprised to see any marks whatsoever. I could remove the engine and have the bores honed properly, but I don’t really want to join the “might as well” club. (too expensive).
The other thing that I noticed was that the cylinder bores have been cut away below the exhaust valves. To my knowledge this is not standard for a 1973 motor?
I would appreciate any comments.
C.S. Barrow.

Firstly the clearance for the exhaust valves is necessary for heads up to 1974 i.e. the heads with part number 12H2709. Now the cylinder bores may just have been finished without moving the hone up and down but it really should have a 45 degree cross hatch pattern. Quite frankly I would live with it. It is certainly not unusual to see hone markings at this mileage in fact they have been seen up to 100,000 miles. With the quality of todays oils bore wear really is at a minimum.
Iain MacKintosh

Thanks Iain, I will take your advice!
C.S. Barrow.

I was surptised to see hone marks on my bores when the head was off after 20,000 miles. They were not all round the bores so I suppose just where there isnt much side thrust. I wouldnt worry about the angle of the hone marks, they are just there to ensure the rings dont glaze and bed into the bores, I dont see why parallel ones wouldnt work.
Stan Best

My V8 still had honing marks over most of the surface of all the bores after 100k if not more.
PaulH Solihull

Yes that's right, it is not uncommon these days to still see the hone marks after a very high mileage due to the vast improvement in oils and fuels
Iain MacKintosh

This engine which I am blue-printing certainly has honing marks also, not all around but there, and I do not even know its vintage!Ian MacKintosh is correct modern oils do their job, and bores don't wear much. I was even contemplating just a re-ring but contrary to CS Barrow, what the hell, in any case I would like a few more cc's. What was important is to inspect the journals. The main is knackered, and the number 3 big end must have knocked badly. Mike
J.M. Doust

Many thanks for all of the input. I am running at a hot oil pressure of 30psi idling and 60 psi at normal speed so I don't think that there is anything wrong with the Journals. The pistons also don't show much play in the pots, so it seems that the rings are in reasonable shape as well.
The only thing that does puzzle me: (and this has been there since the day I bought the car) When the engine is beyond cold, but not yet at operating temperature I see the oil pressure fairly rapidly swinging between about 55 and 70 psi at engine revs of about 2000RPM, almost as though the relief valve is "chattering" This goes away once full operating temperature has been reached.
C.S. Barrow.

Absolutely normal, and is a combination of characteristics of oil viscosity, temperature, relief valve spring tension, fit of the valve in the bore and all sorts unique to your car. It varies from car to car as well as the conditions, and changing any of the above is likely to change things, although it may just move it to a different combination of circumstances as remove it altogether. It used to be much more noticeable until Smiths fitted a sintered iron plug in the gauge to damp the the fluctuations, but it still happens.
PaulH Solihull

Thanks Paul, I needed a bit of good news!
C.S. Barrow.

The X pattern, in the cross hatching, is there to hold the oil in suspension on the cylinder walls while the piston moves up and down. Without a proper cross hatching, the oil tends to run off of the cylinder walls too quickly and fails to lubricate the pistons properly. Another reason for cross hatching being still apparent, after high mileage, is low piston ring tension. This is very common on Suzuki engines. They use piston rings that have extremely low ring tension and it is not unusual to see an engine with 200,000 miles on it that still has its factory cross hatching. Another reason for cross hatching to have lasted so long is that the engine may have been overheated and the rings themselves have lost considerable tension. RAY
rjm RAY

Some after-market release valves are not as good as the O E, i.e. they are not hard chromed. They do tend to "drag" a bit, plus builders seldom seat new valves to their seats. If a re-built engine is run-in on modern oils the rings and bores don't get properly bedded in. Oil consumption suffers as do cylinder pressures.
Allan Reeling

This thread was discussed between 24/01/2011 and 26/01/2011

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