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MG Midget and Sprite Technical - Engine rebuild planning - 1

I'm planning an engine rebuild this winter and I suspect this thread will be the first of many!

First couple of discussion points:

i) The engine is already +60 so I will almost certainly be getting liners installed. Not cheap but I'll end up with a stiffer block with better quality bores. Should I bore the liners out to +60? How much extra power & torque will +60 give me over the standard bore?

ii) I had been considering replacing my Kent 276 (which I'm hoping should still be in good condition) with a SW5. However a bit of reading suggests that the SW5 leads to quite a bit of valve train noise. Is it much noisier will it deliver torque lower down than the 276? Any thoughts please.

Many thanks
Jeremy Cogman

I have the Kent 276 and it is quite noisy. The recommended clearance is 0.4mm, wider than standard. I don't know about the SW5, but I'd expect most of the noise to be related to clearance.

I thought the main point of liners was to get back to standard bore sizes. To get liners in you would need to take quite a bit out of the block, and I'm not enough of an engineer to know what the effect of that is on stiffness. I do know that thin bore walls can expand and cause problems with ring sealing, but I don't know how much liners can mitigate that. There will be experts on here who can tell us.
Les Rose

What size is the engine?
David Billington

If it’s +060 then it’s 1330 ish so why not just go to 73.5 mm and 1380? To fit liners it will need to be bored to something like that (or more?) so seems a bit pointless to liner it until you need to. Most high performance A Series engines end up at 1380 without any strength problems.

I’ve also read somewhere that an in between size of piston is now available, something between 1330 and 1380. Might be Keith Calver? Worth a look anyway.
John Payne

John,

I was thinking more along the lines of big bore or small bore as I have no idea what car Jeremy has.
David Billington

David: 1275

John: Is 73.5 an offset bore?

Thanks
Jeremy Cogman

Jeremy, missed you at Goodwood this year. Saw your car but you were not around when I came by.

Trev
T Mason

Sorry to have missed you Trevor, the kids get fractious if we stay round the car for too long plus I actually prefer spectating over the other side between Fordwater and Lavant.
Jeremy Cogman

An update on the engine. I was hoping to limp on until I've done an Autosolo at Goodwood in three weeks time. However I decided to do a compression test last night and have zero compression on #1.

Head is now off and there are no catastrophes, valves appear fine and no piston holes. However that piston appears very easy to move in the bore so I suspect a broken ring.

On the original topic, having spoken to two machining companies (OCS in Winchester, Saunders to the West of Southampton, please feel free to post your experience of either or recommend others) and also had some very useful correspondence with Peter Burgess, I have learnt the following:

i) 73.5 is possible without offsetting though sometimes it might be necessary.

ii) Offset bored engines should not present reliability problems (I'd got it in my head that they might).

iii) If a block is already +60 then if it is lined you cannot go back to +60, +20 is all that is possible.

Including full balancing and assuming crank work is necessary then it looks like machining will come to around £900 incl. vat. Bet it's cheaper up North!

Jeremy Cogman

My recent rebuild was so successful (except for the drop in fuel economy). Here is the reference to the cam and pistons:

Cam: https://www.bpnorthwest.com/camshaft-performance-rifle-drilled-and-cross-drilled-a-series-1275.html

Pistons: https://www.bpnorthwest.com/piston-set-030-1275-9-75-1.html

Distributor: Professionally rebuilt and curved for the above by British Vacuum Unit in New Hampshire: https://www.britishvacuumunit.com/services.html

Carbs: Joe Curto, https://www.britishsuperior.com

The object was to obtain additional midrange torque to be able to pull when in overdrive (Datsun 5 Speed conversion).

Mission was definitely accomplished. I am the original owner of the car and if I had known better 52 years and 296,000 miles ago, I would have had it torn down and built to run like it does now when it had only 2 miles on it.
Glenn Mallory

It definitely isn’t cheaper up north, or easy to get done!

Re the offset boring reliability is the reason it’s done. The amount of material left between cyls 2 + 3 when boring to 73.5 without offsetting is tiny so head gasket reliability suffers.

I’m not so sure about the +020 on a linered block. True you can only go to +020 on the BMC linered blocks but if you were doing your own then maybe it’s possible to fit bigger liners that allow more. Fairly academic though as in reality if you were linering then you’d probably start off at 1275 to give another bore later on.


I had mine bored to 1380 last year. It’s difficult to find someone to do it now without letting them do a full rebuild, MED or Peter B wouldn’t do it. I think it’s because they don’t actually do the boring work themselves and get it done elsewhere. In the end I got Peter Burgess’s to do it as part of buying everything for a full rebuild. They have a machine shop they use. I’ve no complaints about the quality of the work from them.

Here is the link to the Calver ST pistons. They do 72.5 and 73.0mm.
If I were going for them I’d still try and get it offset bored as it would give you the reliability and also allow further boring up to 73.5.

At least you’ve got to the right time of the year to start the work!

John Payne

Contact AC Dodd (you will find him on FB). He will advise & probably do the boring and supply one of his (Newman ground) camshafts. His RT cam would probably suit auto solo.
Alan Anstead

Jeremy
IF you're going to 73.5mm bore it's super important to talk to the machinist doing the job and explain what the issues with headgasket sealing are-
Either way you need to stress the point about the area where the gasket sits-- Some machine shops get a bit heavy handed and overdo the chamfer on the top of the bore which drastically reduces the flat area for the gasket to seal--
Best deal is to get them to bore and hone but leave the top corner of the bore sharp/square and just take the edge off the corner with some fine paper---Doing it this way you will still have a nice area for the gasket.
You have to be a bit more carefull fitting the pistons with the sharper/squarer edge but the benefit far outweighs the little extra care needed during assembly

willy
William Revit

This thread was discussed between 19/09/2023 and 03/10/2023

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