Replacement Cylinder head

Technical MGB discussion
ChargedAutoGT
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Forename: Graham
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Re: Replacement Cylinder head

Post by ChargedAutoGT » Sat Jul 18, 2020 11:30 pm

It could do with a repaint. it has single valve springs. i will check the valves for leaks but have no reason to suspect they are leaking. no signs of such.

I do not know if it has unleaded seats and anyone who tells you that they can tell....well let Mr Burgess know as he said to me that's it's pretty impossible without scratching them to find out....BUT....I have used unleaded no additive for last 3.5 years. I had to with Lambda sensor as additive destroys them. I'll take the valves out and see what they are like and if there are any signs of burning. photos will be posted, not hiding owt.

i have a spare set of doubles and could lap all the valves in if needed.

so price depends on 'as is' or with work. i'll strip it and have a look in a week or so - like i said, ...for sale soon...

i will also be disposing of 2x front wings (chrome GT) , 1 x tailgate, 1 x bonnet and 1 or 2 x doors. all needing work but worth doing. I am driving whilst restoring so bought others to prep whilst i could use the car.

G

Hazza1190
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Surname: Bukin
Location: Devon

Re: Replacement Cylinder head

Post by Hazza1190 » Sun Jul 19, 2020 11:28 am

Hi,

That's great thank you for the info! Please do let me know as I'll be interested when you come to sell it! Thank you. :)

Cheers,
Harry

Vic Butler
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Re: Replacement Cylinder head

Post by Vic Butler » Sun Jul 19, 2020 1:11 pm

I'd be willing to take as is since I will get Peter Burgess to stage 2 it and unlead it.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Peter Cresswell
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Re: Replacement Cylinder head

Post by Peter Cresswell » Thu Jul 23, 2020 5:00 pm

ChargedAutoGT wrote:
Thu Jul 16, 2020 10:41 pm
I will shortly be selling my 12H2709 if interested.

Got my modded head back (for my supercharged engine) from Peter Burgess and just removed my 12H2709 which I have been running for the last 3.5 years.

I have had no issues at all and though I will be buretting it tomorrow it looks to have plenty of skimmable left to up your CR - and that is where you notice performance increases.

G
Buretting won't necessarily indicate how much can still be skimmed from a cylinder head. Better is to check by measuring the thickness of the cylinder head using a vernier gauge. A standard (BMC) head is 3.172in thick so if 1/16in (0.0625) has been skimmed from the face it will be 3.109in thick. The maximum amount that can be skimmed off depends on the casting - some are better than others. The 12H2709seems to be one of the better casting and mine for example has had nearly an 1/8in (0.125) removed. This gives a compression ratio of around 10.5:1 with a +060 overbore.

As a rule of thumb, for every 10thou skimmed from the face, the combustion chamber capacity decreases by 1cc, so skimming it by 1/16in the capacity of the combustion chamber capacity reduces by 6cc. Then the bigger the bore size the larger the cylinder capacity and the higher the compression ratio, so there are a lot of variables that make up the compression ratio for a particular engine.
Pete
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1969 MGB Roadster
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Plus 34 other cars since 1965

ChargedAutoGT
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Re: Replacement Cylinder head

Post by ChargedAutoGT » Fri Jul 24, 2020 11:53 pm

Thanks Pete - 3 11/64 +.015 -0 i have in my notes. but not sure from where I got it. i had forgotten about that.
...or 3.172 as you wrote.

with a skim of that size, any issues with valve/piston clearances? i am guessing you have to be careful with rockers & cam. what squish are you running?
G

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Peter Cresswell
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Re: Replacement Cylinder head

Post by Peter Cresswell » Sat Jul 25, 2020 1:38 pm

The 3 11/16 in is quoted in the MGB Special Tuning booklet.

When using an 18V cylinder head the valves are spaced slightly further apart and the block needs small cut outs for the exhaust valve to open into. The 18V blocks have these but the earlier blocks don't and as part of the build they need to be cut into the cylinder wall. The 12H2709 is an 18V head, and with a high lift cam they are more needed. My 18GG block had these cut outs already so all I had to do was check they were sufficient for the lift of a Piper 285 cam - which they were.

Initially I thought I would need 120thou spacers under the rocker shaft pillars to get the correct rocker angle so I ordered some from Cambridge Motorsport. When I assembled the engine with these spacers the rocker angles were crazy and at first I couldn't work out why. I am using Iskenderian Mini Cooper S cam followers, and eventually it dawn on me that these were the only non MGB specific parts I had used and when I measured the depth of the internal bore I discover they the 1/8in deep than the MGB followers. So I took the spacers out and lo and behold correct rocker angles!
I also had the top face of the block skimmed by 10thou to make sure the face was clean and flat, and to reduce the bore height above the pistons. I can't remember how far down the pistons are now but they were all equal.

I modified the cylinder head myself using the information in Peter Burgess' book and also the Huffaker MGB Competition Manual (See https://www.mgexperience.net/article/mg ... Manual.pdf). I had bronze guides and 3 angle seats cut as part of the machining. Valves are 1.625in inlets and standard exhausts and are the race valves (AFD1 and AFD3 from Brown and Gammons).

When I set out to build the engine a personal challenge was to see if I could still build a reliable and powerful engine (I used to race an MG TC which ran 125bhp reliably). So far I seem to have achieved this and although I haven't had the car on a rolling road yet it is substantially quicker and the engine revs very quickly to 6,500rpm.
Pete
1969 MGB Roadster
2020 MG HS Exclusive
2007 Mercedes SLK
Plus 34 other cars since 1965

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Peter Cresswell
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Re: Replacement Cylinder head

Post by Peter Cresswell » Sat Jul 25, 2020 4:07 pm

I tried earlier to attach pictures of my rebuilt engine, but the was a problem. This is now fixed (Thanks webman Paul) so here is the engine just about ready to be attached to the gearbox and installed in the car.
New enigine 2.jpg
Pete
1969 MGB Roadster
2020 MG HS Exclusive
2007 Mercedes SLK
Plus 34 other cars since 1965

ChargedAutoGT
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Re: Replacement Cylinder head

Post by ChargedAutoGT » Sat Jul 25, 2020 11:15 pm

and mine...just finished painting engine bay .

One thing i have not done is check my rocker alignment and I should do as I have skimmed, decked, renewed rockers (stock), added a cometic for s/c and changed cam.

Now some of these items cancel each other but I should check net effect.

I have read the measure is that valve top and rocker shaft axis should be at same horizontal level sometime between 1/2 & 3/3 lift.

Was that your measurement?
Attachments
done-c.jpg
engine-c.jpg

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Peter Cresswell
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Re: Replacement Cylinder head

Post by Peter Cresswell » Sun Jul 26, 2020 11:27 am

Rather strangely, Peter Burgess doesn't mention too much about this in his book, and you have to read it quite carefully to find the brief mention. Even then it refers to blueprinting standard spec engines, so can be missed by readers who have a modified engine.

I used this as a guide to the geometry requirements:
http://www.flowspeed.com/rocker-tech.htm
I then set them up by eye so can't offer any measurements.

If the pedestals need raising to achieve the horizonal alignment between 1/2 and 2/3 of lift, then Cambridge Motorsport do a range of thickness for the shims, see: http://www.cambridgemotorsport.com/historic/73837
Note these are for the inline oil feed - check with them if you have the offset oil feed. The standard thickness for the spacer fitted under pedestals 1 & 4 is 0.005 in.

Pete
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Ian Fozzard
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Re: Replacement Cylinder head

Post by Ian Fozzard » Sun Jul 26, 2020 1:19 pm

Peter,
thanks for these links - interesting reading. On the subject of rocker geometry I believe the Huffacker guide does not mention shims being employed (unless I missed it in a quick read through)? Not emphasised by Peter Burgess either.
I confess it's something I've never bothered with.
Do you regard it as an essential adjunct, or is it only required for very heavily skimmed heads? I usually aim for a CR of circa 9.5, which does not require removal of a great deal of skimming of the head.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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