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MG MGB Technical - Now this is technical!

Does anyone out there have drawings, or definitve data on the exact positions of the bores in the engine cylinder block?

I have had liners fitted to a block that had been linered before, and the bores are all over the place!

To the point that 2 of the inlet valves need eyebrows cutting in the block!! But not their respective exhaust valves.

Any help appreciated.

Colin
Colin Parkinson

Wow sorry can't help but sounds awful! Perhaps the 1950 engine builders can assist as sometimes the overbore results in the block being trash or needing liners and I think they offset bore to overcome some of the issues with the block.
A J Ogilvie

I have measurements from my 1622 block, original bores, but may not be the same as 1800.- Interested?
I would think that you would have to bore within the clearance limits of the crankpin and gudgeoon pin end floats. Otherwise it can't be assembled!
Art Pearse

AJ we could probably bore it again to get 1950, but we are only allowed 1840 for historic racing.

Art, I would think the centre of the bores would be the same as 1800, as it basically the same block. Do you have a drawing? Or very accurate measurements, maybe from an industrial scanner?

Colin
Colin Parkinson

Colin, I sent you a spreadsheet of my measurements. Hope it helps.
Art Pearse

When the prototype MGB proved to be slower than the MGA the B series block which started out in 40s as a 1400 was bored out to give 1798cc. This was the final stretch BL envisaged, and to get it the bores were slightly "siamesed" in the jargon. This means the centres zig zag bit along the block. A small cut away for valve clearence isnt uncommon in a modifoed engine, eg a Peter Burgess fast road head with its bigger valves on an MGA needs them. The results of course are gratifying.
Stan Best

THanks Stan. The cutouts are standard on the 18V engine but only on the exhaust valves.
We have the problem that we need them on the inlet on 2 cyls, as the liners have been put in on thee p*ss!

Colin
Colin Parkinson

Colin
That block sounds like it needs to be trashed along with whoever got it that far out of wack
Even on my monster valve heads with 1.740" inlet valves there has never been a need for pockets
It must be a long long way out
With your offset bores you should be aiming for about 3mm min. between 1-2 and 3-4
Cheers Willy
William Revit

Colin, they may have engineering drawings of the block in the archives at Gaydon. I have been planning to go there sometime soon to look for drawings of one of the banjo axle service tools. I could have a look for block drawings at the same time.

David
David Witham

David that would be very much appreciated, but I have to move quick on this. The racing season fast approaches.

Willy, I will not trash it, as too much has gone into it all ready. Will probably go to 1950 with that one, as long as they put the liners in the correct place.

Which brings me back to the original query - where is the correct place??

Colin
Colin Parkinson

This reminds me very much of my bo-bo with my cylinder head when I went through the pushrod tube. So the problem as Peter Burgess discribes is then getting a sleeve to follow straight into the original bore. I guess the same must be so if one is sleeving a block. to get the sleeve in the right place at the start. But lets assume the bore was a little out originally, then during re-bore one perforates at some point, then to place the sleeve must be tricky. I suppose one could sleeve lop-sidded so to speak then re- bore the sleeves themselves to the correct position? Or is this not done? Mike
J.M. Doust

Mike that can be done, but not on the normal boring machine. The normal machine picks up at the top of the bore and goes straight through. So it is wrong, it stays wrong!

CP
Colin Parkinson

Colin
you can shim the boring bar accross before locking it into place if it's an old single bar
The machine we use is a bit more advanced and the cutting head is on a rail The rail is centred up on the front and rear cylinders and then the head simply slides along front to rear We centre the head onto a bore and if we need to move the centre of the new bore to offset it say1/2 a mil. it's simply a matter of moving the head 1/2 a mil along the rail. The first cut is fairly noisy (chomp chomp chomp ) It's quite simple really, That's why I can't understand how yours got that far out

A simple check to see where it all went wrong, although not real accurate would be to sit a standard headgasket on the block and compare Sounds like it could be fixed ok with an extra thick wall liner to get it back to where it should be

Just read Mikes bit again - I wouldn't fit a sleeve and then offset the bore It would be better to offset to where you want it and then sleeve it, this way the sleeve will have a consistant thickness - better option I think

Good luck hope this helps somehow Willy
William Revit

Willy, I am not sure what machine my man has, but it is modern and state of the art.
Unfortunately it only follows the bore that is there.

He does have another machine for doing blocks up to 6ft long, which can be adjusted! Unfortunately too big for my little block.

Other than that he has a horizontal borer which will bore exactly where you want.

Which brings us back to my original post. I need to know the exact centres for the bores. Hasnt anyone, even in the US, got this info?

Willy I agree with you, the bore should not be offset in the liner.

CP
Colin Parkinson

Colin,

Perhaps grabbing at thin air but...I wonder, assuming there is a fixed relationship between the four cylinder bores in an unmolested block, would it be possible to deduce the centre of the 'bad' bore from the remaining three?

It would take some very accurate measuring and calculating, but perhaps a good machine shop would have that capability. It may then be possible to centre the machinery for the 'bad' cylinder bore.
Roger T

Colin, like I suggested, can you not calculate it based on the crankpin positions?
Art Pearse

Surely somebody has an 1800 block?
The only one I have apart is behind and under a very large pile of parts, about a day's work to dig out, and I'll never get it all back!
Art's 1622 should be the same spacing, since all B series use the same gaskets, at least after boring.
All you need to do is measure the bore size, then measure the distance across cylinder walls between each pair of holes. You can do this with a set of dial calipers. A bit of math and you have the relative bore positions. The block is symmetric and all 4 are in line.
I'd be surprised if some of the "big bore" types haven't posted this somewhere.

What exactly does "all over the place!" mean?

Here: http://www.mgbmga.com/tech/mga3.htm
there is buried info relating to overbored 3.070" MGA with big valve heads, stating that "We found, before we actually milled the relief valve notches, that #1 & #4 cylinder intake valves (1.625") made contact with the cylinder wall, the 1.565" size valve cleared but there was significant valve shrouding within the bore to warrant this valve relief notch."
So evidently there is some offset in relative chamber vs bore position.

FRM
FR Millmore

'All over the place' means out of line North south and East west. At this time we havnt measured accurately enough to determine if any of the bores are in the correct place. Hopefully we will do this monday.
Colin Parkinson

This thread was discussed between 09/03/2011 and 13/03/2011

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