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MG Midget and Sprite Technical - Valve guides and seat re-cutting

Hi, Is it possible to fit new valve guides in a home workshop(i dont have a fancy press) or should i pay an enginnering shop, will the seats need re-cutting or could i grind in the valves with coarse then fine paste, they are standard cast guides(AE) going into a 1275 12G940 head. Thanks Martin.
m a frame

Martin, you can do it yourself. You will need a suitable punch to remove the guides - it will have a step on it and the front end will slip inside the guide and the back end needs to be a bit less than the guide OD. You can use the same punch to install the new guides. Put them in the freezer first, and make sure you set them the right depth. As far as the valves go, its an independent question as to if the seats need cutting. Either they do or they don't. If they look good, you should still lap them in as described, because the old guides, being worn, would allow the valves to seat a bit off centre.
Art Pearse

I've done this before with no more than a light grind with paste but I did my bike last week and one was miles out so ended up having it re-cut.

I'd have a go at fitting the guides then you will at least save on that, you may be lucky and only need a quick grind. If any are out you could then get them done at a machine shop.

If you are drifting in the new guides make sure the head is on a nice flat surface to avoid putting any stress into it. You can get a mandrel to fit the guides but I just put a piece of soft ally on top and used a drift on that. If not you can end up burring them up.

John
John Payne

If drifting new guides in at home, do they naturally align accurately in the block or is there a special procedure for getting them properly centred and straight?
Guy

Forgot to ask, does anyone know the measurement to be left proud on the top of head. Martin.
m a frame

19/32" or 15.1mm according to BMC w/shop manual.
Dave O'Neill2

This is an interesting debate. I've never installed my own either, but often wondered about how easy it would be. They certainly come out easily enough. I've got a spare 1275 head in the garage that I stripped years ago, and never did anything with.

As regards alignment in the head Guy, I would think the lengh of the guide in the head, would be sufficient to ensure that it can only go in perpendicular to the head.

That being the case, the only real difficulty is getting them in.

A slightly modified upturned old valve could be employed as a drift.

Even without a suitable drift, couldn't they be pressed in?

And then assuming as previously said the seats are ok-ish, a coarse and fine grind and your done.

Think I'll take a look at this tomorrow, just for fun.
Lawrence Slater

Martin,

I've done this a number of times and it's not difficult although that is relative. I would suggest you make a depth stop to set the guide height in the head, it'll also help align the guide. A depth stop collar is easy to make if you have a lathe, as is the drift and tool for pressing them back in. Do you have a lathe or know anyone that does. More advanced lathe projects include the 3 angle valve seat cutter set which I've done years back but that's a bit extreme for one head unless you like projects.
David Billington

Wow!! So much help so quickly,Im definately going to have a go this w/e. This is a spare head that Id like to recon and just swap over, car is running ok, but i know this head to be better,one last thing i need a set of double valve springs, if anyone has a set for sale or knows the best place to buy a set i would be very grateful. Thanks again Martin.
m a frame

http://www.minispares.com/Product.aspx?ty=re&pid=33655&sku=0&tc=1#submenu
cheap and good quality
Onno K

Hi, All the guides now in head, was easier than i thought!!i made up a depth stop and a simple stepped punch and they went in nicely, had a go at grinding one seat without much success, but ive saved on the cost of having the shop fit the guides, anyone hazard a guess on the cost of recutting 8 seats? also do i fit valve stem oil seals on all 8 valves or just the inlets? Cheers Martin.
m a frame

My 'head' man used to just fit on the inlets. His argument being that with the lack of lead in fuel, the exhaust valves need all the lubrication they can get.
Dave O'Neill2

I did oil seals on inlet only on my 1380 - and wish I'd done all 8.
Dean Smith ('73 RWA)

I'll chip in that I did the seals on the inlets only on my 1215 and 1420 and never noticed any problem with that. Seems to be the case that for some inlets only is OK and for others it doesn't work. I had fully rebuilt the pressed steel rockers and used a hardened rocker shaft so there wasn't an excess of oil in the rocker cover.
David Billington

When the inlet valve opens, the oil can in theory, get sucked down the inlet stem, but not the exhausts for obvious reasons.

I've only used them on inlets and not had a problem.

But they are so cheap, I don't think it matters either way if used on the exhausts.

Unless there was a specific reason other than cost, not to include them at the facotry that is?
Lawrence Slater

With seals only on the inlets I get blue smoke, so I put then on all 8 valves.

I believe there are factors which have an impact, one way or another: The amount of oil "leakage" from the rocker shaft, the valve stem/guide clearance, and the type of camshaft in use. And the exhaust manifold. I think that although the exhaust valve stem should only experience a positive pressure, I suspect that a free flowing manifold can result in a momentary negative pressure in the valve chamber as a "slug" of exhaust heads off down the pipe just as the valve snaps shut, sucking oil down the valve guide.

But I know many others who get on just fine with seals just on the inlets, and I wouldn't argue with them!
Guy

Another for a mixed bag. On some engines burning oil, seals on the exhaust has stopped it. Others have had no seals on exhaust and no problem.
Paul Walbran

There's nothing straight forward about views on the these old engines. lol. It's a pity we don't have access to an old Austin/Morris A/B series engine designer, or at least his notes, so we can find out the original reasons for many things debated here.

It's my understanding, that oil is meant to leak out of the rocker shaft, and run down the push rod holes to lube the followers, and then return to the sump.

You need oil splashing about under the rocker cover, to lube the tops of the valve stems/tappets, otherwise running dry, they wouldn't last long at all.

If you've got smoke from exhaust valve guides, then I reckon the stems/guides are worn, but if seals can resolve that, then that's a good and cheap solution.

Still begs the qustion, that if they are needed (by some), why weren't they fitted originally, to both inlet and exhaust?
Lawrence Slater

As they weren't originally specified with exhaust stem seals I would always start with this arrangement. BUT, when I rebuilt my engine it smoked badly from day 1.

I tried all sorts of remedies following advice from others on the BBS and elsewhere. There were all sorts of theories about bore glaze, broken rings, damaged bores etc. The engine was stripped down and everything checked and still no solution. At the time no-one suggested putting seals on the exhaust valves. That was just me, following a whim and in desperation. It instantly cured the problem!

In retrospect I believe that the cause was related to the fitting of new (bronze?)valve guides for running lead free. I don't know if this is always the case, but I think mine were set up with more stem clearance than the originals.

Some people insist that it is impossible for oil to be sucked down the exhaust because when the valve is open the piston is always rising, providing positive pressure in the valve chamber. But for the reasons stated earlier, I think it is possible that there may be stages in the cycle when there is negative pressure in the exhaust tract.

That was over 10 years ago. The engine is due for a re-fresh, if not a re-build. Depending on what this involves I will probably revert to inlet only seals if I can do so without it smoking.

Guy

Do the guides have to be reamed to size?
Art Pearse

Hmm!!The proverbial can of worms,Im going to fit them on all 8, 35p each it wont break the bank, but obviously bmc thought otherwise, Art, no mine dont need reaming, they are a perfect sliding snug fit. Any thoughts on cost to recut valve seats. midget likely to have a stable mate soon, 1972 1 owner Mini with 29000 on the clock and full history from day one!!!even the original purchase reciept, I know its not a spridget, but at least it got an A series engine!!Cheers and have a good weekend Martin.
m a frame

3 years ago...

Unleaded conversion £111.00
Face inlet valves & seats £28.00

So unleaded was the exhaust valve seat replacement, the inlet was to recut the valves & seats to match.

So if you just want all 8 recut than at that time it was £56+VAT.
Dean Smith ('73 RWA)

I find that when you press standard guides into a head, you don't need to ream them, however usually with bronze valve guides they do require reaming for the valves to fit.

If you press the guides in square with a bench press you can usually get away without recutting the seats, but if you knock them in with a stepped drift you don't always get them quite square and they will therefore need cutting
James B

Martin

Perfect snug fit is too tight. Read your manual

James, how many heads have you done this way? Have you ever measued run out on guides before fitting? Also wera and tear means seats are not concentric even to original guides.

Peter
Peter Burgess Tuning

Peter, Meant to say they slide in under there own weight nicely, not tight anywhere!! Martin.
m a frame

Hi Martin

How do you know the clearance is correct if you do not measure it in some quantifiable manner? The 3 or so thou clearance for CI guides in A series heads tends to feel like disasterous wear until measured properly. Don't forget ex stems tend to be half to one thou less than inlet stems to allow for thermal expansion.

The dynamic clearances will not be the same as the static on the bench clearance.

Peter
Peter Burgess Tuning

Hi Peter

To be fair I haven't put guides in a head for some years - it is possible that in the intervening time the machining tolerances on bronze guides have improved. I do remember doing a series of heads for Bill Richards where we couldn't get the (new) valve stem into the new guide more that a few mm until they were reamed.... Quality 'bronze' guides from Mini Sp#res of course.
James B

Hi James

Is that not as it should be? If a guide is such a loose fit it doesnot close down the ID at all I tend to fear the guide will not be a very secure fit in the guide hole. I have seen many many TR guides a loose fit. V6 conversions can be very suspect and a lot of Bs and As seem loose. To be honest we tend to add a half thou to standard sizes when we make guides. Most heads have been fitted with guides quite a few times in their life and the original 'virgin' holes are not the same. The heads have also heated and relaxed in many cycles. We tell all customers of our guides whether trade or retail they must ream after fitting.

Peter
Peter Burgess Tuning

This thread was discussed between 11/01/2012 and 15/01/2012

MG Midget and Sprite Technical index

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