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MG Midget and Sprite Technical - Head cracked. Repair ?

Hi,

In 1994 I restored my HAN7 Sprite. The car was found in Belgium but spent his life in Phoenix AZ. Since then, all the interior was burned by the sun. Only the interior ?

When It came to the head, I bought new guide, valves and send all this to a company to fit the guides, skim the head. They did fit the guides and after that called me back : "Sir, your head is cracked..."
Since then, I bought a rebuilt one in UK and the restoration continued.
The head was a 12G206 http://farm3.static.flickr.com/2068/2442802054_db3c2d9d01_b.jpg and I later understood that this was a magical number

Later on that year, the wonderfull book of Terry Horler came out but it didn't explain well the head specificity.

YEars later, while checking the valves gaps, I noticed that the head number was reading 12G202. So a heart shaped combustion chamber.
Since then, I wondered if the old head was really beyond repairs. I already did weldings to cast iron but in a head ...
So, could it be that by fitting valves seats, I'd be able to overcome this. µ
I believe that this head should give a couple of additional donkeys

Some pictures :
Both center cylinder exhaust valves are displaying the same crack that I underlined in green.
http://farm3.static.flickr.com/2184/2442802088_1fbfa129b5_o.jpg
µ
Here a detail to understand the potential extent of the cracks.(visible)
http://farm3.static.flickr.com/2276/2441972891_f84311c405_o.jpg

So, what do you think ?

I don't mind fitting slightly larger valves since during the rebuilt, the crankshaft was regrounded, heat threated and balanced.
S Mourelatos

Thats a wierd place for a crack,,,they (IN my limited experiance) always crack between the valves....Im wondering if they cant (machine hop) install a hardened valve seat, for running on unleaded fuel...Id certianly ask before I wrote it off


prop
Prop

Looks like it may have hardened seats in already that have cracked. Sometimes it's really hard to see the joins.
Cheers John
JOHN HALL & JULIE ROBERTS

I wish you were right. But I don't think the car had seen a mechanic earlier than 1980 (before me) and I doubt that it was then usual to fit hardened seats.

So, not an usual fracture place ?
S Mourelatos

and even if you would not fit hardened valve seats this can perfectly be welded by a skilled mechanic

pleas do be carful with the head they are more rare than hens theeth!
o Könemann

It is common for A series valve seats to crack in this way when a valve has burnt badly.

They may be cracked old inserts as John notes. Inserts, if fitted, may be hardened but would more likley be of the cast iron variety used in the past to effect repairs or reclaim worn seats (given the 1980 comment above).

I have yet to see a crack occur in hardened seats, however - in one case, even though the valve was severely burnt there was no damage to the hard valve seat insert, which instantly blued up as a perfect fit against a new valve without remachining.

From what I can see from the jpg's, these cracks look as though a valve seat insert (preferably hardened) would provide a suitable repair wheter or no there are existing inserts.

Note that it is often difficult to determine whether seat inserts have been fitted before, when viewed only from the chamber, - the original machining usually left the type of pattern shown, which is not to dissimilar to how it looks with seat inserts.

The usual give-away as to whether inserts have been fitted is to feel round the short radius with your finger. (short radius - the tight side of the 90 deg bend between valve seat and port. Alternatively described as where the bottom [when the head is on the engine] of the port turns the corner to the valve seat.) If an insert has been fitted to an unmodified head there is invariably a bit of a ledge where the seat finishes.
Paul Walbran

I checked the head.
No inserts.
µ
Welding ? If I use welding on aluminium head, it's already far from easy. I had this done and it requiered the head to be heaten a long time before starting , then TIG with alternative current ...
To my knowledge welding cast iron requieres to heat the head during a very long time, to get rid of any tension inside, the after the weld , a 1 or 2 days cooling process.
So I'll try with hardened inserts. No risk to have the crack developping further with the inserts ?
S Mourelatos

I really think thats the best option, is to take it to a good machine shop and ask them about having hardened valve seats installed....

good luck let us know what happens

prop
Prop

If crack testing shows the crack does extend a bit beyond where an insert would cover, ask about getting the end of the crack drilled & plugged
Paul Walbran

S,

I have no experience of using a seat in this situation so would ask the advice of a machine shop but cracks propagate in tension stress regions and placing an insert in the head will lead to some tensile stress in the area as the insert is either a press or shrink fit resulting in the insert being under compression and the material around it being in tension. Whether it will be significant or not I don't know, just a thought.
David Billington

You must find the right person to do the work. Ask around, there's an expert somewhere, hopefully near you. I'd make the rounds of local machine shops and ask them all. Maybe even talk to truck and tractor mechanics. You should find a consensus this way.
Tom

The head I am running on my GT had consideable flow work done to it when I got it. Unfortunately it was craked. The machine shop did not blink and just said, we can fix that no problem. The nothod is called "pining" and is behond me why it works! 12 years later, still going strong and I like to drive it like I stole the car.

Pete
Pete

Hey pete,

that is what ""Paul Walbran" is describing, we all so call it "pinning" here in the states

prop
Prop

check it out, your head is on ebay staerting bid is $40....consider the fact with the worthless dollar...you can get it for half what we would pay here in the USA....

http://cgi.ebay.com/ebaymotors/MG-midget-Austin-Healey-Sprite-12G295-Cylinder-Head_W0QQitemZ320246490393QQcmdZViewItem


prop
Prop

Well, a 12G295, not much difference from a 12G206.
But do consider the postage cost (UPS, not the cheapest choice) and the fact that it might be cracked also (impossible to see on pictures).

I'm more proud when I'm able to reseruct an old part than by simply replacing it. So, most of times, I will spent more time and money but ... I like it.

You really convinced me that there might be possible to do something with this head, so I'll try. Not today or this weekend : everything is closed until monday.

So, I'm currently considering what else I'll do to this head.
Skim it a little to get again the same compression ratio I've currently with the 202.µ
Standard valves ? Or these nice 1.25" rimflo valves.µ
With the new standard guide already in since 1994, or bronze ones, to deal better with the extra heat from the unleaded fuel ? µ
Gaz flowing : no, not by myself. If I'm doing it on a 4 ports aluminium it, that's a much more stronger cast iron one, it has already cracked and it's a 3 port, something difficult to understand. '
But then ... I should consider getting rid of the original exhaust manifold for a better one. One that would stop to develop a leak at the joint , what a innaccessible place ...µ
But then... I might have to consider a slightly more performant camshaft and then worrying about "will it take the oil pump drive ?"...

Currently the car is really looking original and I don't want to destroy this
Spyros

This thread was discussed between 26/04/2008 and 01/05/2008

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