MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Early HRG Derrington Crossflow Head

Hi,

Is an Early HRG Derrington Crossflow Head usable on a 1500 b serian mga engine when the combusion volume on HRG head is 33cc.

I guess I need to work out the volumes but I want to know if its worth buying because its not cheap.

Its been crack tested etc and look in good shape but my only concern is the 33cc head combustion volume.

I guess I can play around with the volume taken up by the piston and head gasket but I wonder if 33cc is really too small to invest in this head.

I guess futher skims will make the head unusable pretty quickly.

Any thoughts welcome.

Chris
CJT Thorley

Original cylinder head had 38-cc chamber volume. Total chamber volume, including space above piston, piston dish, and head gasket, was about 51.0-cc (for the 1489-cc engine). Reducing chamber volume by 5-cc increases Compression Ratio from 8.3:1 to 9.1:1.
barneymg

Wait till Bill Spohn answers, he knows lots about these heads
Dominic Clancy

Just cleaned out my cupboard - thanks, looking for info
I have the original paperwork that came with my Derrington years ago
Unfortunately it doesn't quote the standard combustion space
It says on a 1500 (MGA)-8.3:1 or 9.3 with the Derrington flat top pistons and can be raised to 10:1 by removing .030" from the head
At least my cupboard looks tidy now
Cheers
willy
William Revit

Chris,
I'm running a HRG head with 39 cc combustion chamber capacity on a 1950cc B series (11:1 compression) so reckon 33 cc on 1500 is in the right proportions, that said you can adjust with piston selection and chamber modifications. Depends what you want from your engine - be careful that it isn't soft, a problem with these aged heads, they say can be heat treated but requires removal of all steel bits and success is not guaranteed. (Mine is deemed soft but have had no problems so far - re torque every now and then). While they look good, are historical and develop good mid range torque engine tuners like Peter Burgess (UK) get more power out of conventional CI heads.
Mike


Mike Ellsmore

Hi Guys,

Thanks so much for all information really really useful and I have now purchased the head with a pair of 40DCOE2s and the original HRG manifolds.

All I need now is the HRG original aluminium rocker box and wonder if anyway know where I can find one shiny or crackle black?

I am also looking for any copies of original documentation to help with the project and keep with the car as do the conversion.

Please do email me cjthorley@hotmail.com or call me on +44 (0) 7827938879.

Thanks
Chris
CJT Thorley

Chris,

Have a look at the website of Bill Spohn in Canada.
Follow all the links on the page.

http://www.rhodo.citymax.com/HRG.html

Mick
M F Anderson

Chris

I have a period HRG catalogue that gives performance data for their heads on various cars. If you would like I could scan some.

Paul
Paul Dean

I would like a copy. I would post it on the MGAguru.com web site.
barneymg

I've got an HRG rocker box on my A, they do appear on Ebay occasionally - check that the markings are correct as some are not quite what they seem... A guy on the mini forums does the correct plates and new fillers are available. Don't have an HRG head just an alloy version of the cast iron head made by some bloke called Burgess ;-)
David k Brenchley

Years ago I measured mine I think it was at 33. I will have to check and see if I have the figures somewhere. I do know Derrington would skim the head if you wanted when new.
gary starr

Chris
Are you after original documentation or copies of

willy
William Revit

Hi,

Thanks for all the help.

I would willing to make good offers on any original documentation.

I would also be very keen to have copies.

I am basically interested in anything relevant to HRG alloy crossflow heads.

Please email me on cjthorley@hotmail.com

Thanks
Chris
CJT Thorley

Biggest thing to watch on buying used HRG heads is that they haven't been shaved too much. Too much = > 030" or so.

The alloy does not have the strength of a cast iron head.

Chamber sizes correspond to the small (pre MGA Mk 2) and large (Mk 2 and later) of the stock cast iron heads.

Barney, help yourself to the material on my site or just link to it as you please.

http://www.rhodo.citymax.com/f/HRG_Information.pdf
Bill Spohn

Bill - How do you know how much is taken off? Is there a spec listing say thickness between head surface and vlave cover rail?
gary starr

The thickness of the stock and HRG heads are the same - they start at 3 11/64" IIRC.

That is 3.1719 or 80.56 mm.
Bill Spohn

Stock iron head is originally 3-3/16-inch thick (3.187"), except the MGB "L" head which may be slightly thinner (like it was already shaved from new). I would only guess that after market heads should be same thickness to avoid screwing up the valve train geometry.
barneymg

This thread was discussed between 09/06/2017 and 03/07/2017

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.