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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 - Twin Cam engine drawings

I'm still messing about with my Derrington cylinder head and manifold trying to get it all to fit properly. With the manifold that came with it and 1 3/4" SU's with twin cam filters the front filter just clips the underside of the bonnet shroud. I will have to change the angle of the manifold mounting face on the head to get clearance but without the rear filter fouling the starter switch!

As a reference I'd really like to know the dimensions for the twin cam arrangement and where the filters and carburettors are in relation to the bodywork. Its all very tight. The top of the front carburettor has to be in the right place to go into the cut out in the bonnet stiffener.

Does anyone have a source of a dimensioned sectional drawing for the twin cam showing it in the bodywork or can anyone produce a drawing or several photos for me from their twin cam? Its a long term project for me so I'm not in hurry.

Also does anyone have a view on how important it is that the float chambers are exactly vertical. If I change the flange angle by say 5 degrees will that be a significant problem for the floats.
John Francis

John the early 1500's had a flater bonnet.When the Twin Cam came they went to a more curved one to clear the engine and then used them on all cars. Is your car an early one? Post some pictures when your done please. This is on my list of things to do to my MG someday.
gary starr

John,

I can probably supply some sketches and photos.
But before I do that could you supply some information?
Do you have the later curved bonnet? (Hood for US people).
Do you have a copy of Clausager's book "Original MGA"?
Have you read the section on the changes to the body for the Twin Cam, there are several changes?

What is your body number?

The SU's could tolerate a slight tilt of the bowls, but not much.
On the competition Twin Cams the factory often used 2" SU's. They have a different face angle from the 1.75" SU's.
The factory fitted a washer at the top edge of the bowl to straighten it.
I will also send photos of that washer.

Mick
M F Anderson

Hi Mick

My car is a 1600 with the curved hood (bonnet). Chassis number GHN 86339. I do have Clausager's book. Thanks for reminding me to look at it again. I know the starter switch was moved but I'm not sure if the engine mounts were moved/lowered.

I am going to have a new inlet manifold produced at some stage because the one that I have has slightly smaller bores than the head inlet apertures and the existing manifold can't be opened out becuase its thin wall steel. I will want to draw up the manifold to get it right.

Interesting comment about the 2" SU's and the special washer.

Thanks for your assistance. John
John Francis

John-
" am going to have a new inlet manifold produced at some stage because the one that I have has slightly smaller bores than the head inlet apertures and the existing manifold can't be opened out becuase its thin wall steel."

How much smaller - actual numbers? It is arguable that a smaller manifold dumping into a larger port gives better low end at little or no loss on top - and where do you drive it? Largely depends on the cam. Maybe Derrington knew what they were doing?
Further, if it is a steel tube manifold, it could be bent a bit by a good man with a torch.

Finally, might it make more sense to alter the filters?

FRM
FR Millmore

John is your manifold like mine?
gary starr

oop's forgot pic-


gary starr

John,

On the Twin Cam a packing plate was fitted to the left hand engine mount only.
Other than that the engine mounts and their position is the same as the pushrod car.
This has the effect of rotating the engine slightly, in a clockwise direction viewed from the driver's seat.
This would lower the SU's very slightly.
This would bring them closer to the body.
It is difficult to compare cars as the rubber of the engine mounts can vary in condition and the bracket on the engine block can tend to be bent.

Mick
M F Anderson

The right side engine mount is different on the Twin Cam.
Barney Gaylord

That different engine mounting, the rubber bonded part, is different on the Twin Cam from the 1600 but it is not a Twin Cam specific item.
The 1600 Deluxe with a pushrod engine also has the same RH mounting as the Twin Cam.

Mick
M F Anderson

Gary My manifold is exactly like yours. Have you installed yours in a car?

Mick. Interesting point about the packing plate on the engine mount. That might help a lot in just making it all fit without fiddling with the manifold. The guy I used for adjusting the head, ex Mclaren, was a racing guy so I guess the matching of bores in the head and the manifold would matter for ultimate air flow and power in his mind. I would rather have the mid range torque hence sticking with SUs so thanks for that information. I need to check the angles on the manifold to see how they relate to the cant of the SU bowls.

The manifold is away from me, so is the car, at the moment so I don't have the numbers FRM.

Barney, when you say the right side mounting on the twin cam is different do you mean right side looking from driver's viewpoint or from the front of the car.

John

John Francis

No I haven't John.I have had it many years and it needs work. It had frozen and been cracked,thats repaired but it needs all the seats replaced. It is on my list and I am interested how it works out for you. Would have never thought it would not fit though.It's also interesting, to me anyway,that your manifold is the same.I have never seen pictures of another,the others being aluminium.


gary starr

Right side, always as sitting in the driver's seat.
The right side engine mount pedestal on the frame and the rubber mount is different for the Twin Cam. This is carried over to the "Deluxe" cars, using the same frame.

Barney Gaylord

John,

For interest I have attached an image of the RH engine mounts for a MGA 1600 and Twin Cam.
I am thinking that any reference to a Twin Cam is meaningless for fitting your Derrington head.
You mention measurements for the inlet manifold. But what would you measure?
You cannot compare the distances from the cylinder head to the body as the Twin Cam head is much wider.
I think you should forget the Twin Cam and talk to other users of the Derrington head.

Mick


M F Anderson

John,

If you use a manifold as shown by Gary Starr or one like those in the attached image you have plenty of room in a MGA for the manifold and air cleaners.
In the image attached one is of the four bolt type SU and one for a two bolt type.

Mick


M F Anderson

Mick

The reason I am interested in the twin cam design is purely to identify what clearances and nominal installation arrangement the factory designed. As it is it doesn't fit with the bits I have in my car and I don't want to bodge anything, like the air cleaners. I am sure it will fit but only just and I need to work out what to adjust. The twin cam is a useful reference point as it was a standard design. I expect all the Derrington installations are different.
John Francis

PS If anyone has another 1 3/4" manifold like the ones in Mick's pictures I'd like to buy it or borrow it just to see if it would fit better. I could get it copied if it was on loan.

John
John Francis

John,

I think I at last understand your problem.
The bottom edge of your front air cleaner touches the inner panel of the wheel arch, not the panel at the side of the bonnet (hood).
Is that correct?
If so, the Twin Cam setup is of no use to you.
The Twin cam manifold slopes upwards (see image).
Note also that with a sloped manifold and sloped SU's the bowls are still vertical.
I assume that your SU's are vertical and the bowl is vertical.
Correct?

I would not cut the manifold face at an angle and then have the bowls at an angle.
The solution is to have the panel reshaped to clear the air cleaner, or fit different air cleaners, or cut the manifold face but at 90 degrees.

See image.

Mick


M F Anderson

Hi Mick

Not quite right. The top of the front air cleaner hits the underside of the shroud body panel where there is a stiffening rib. The floats and carburettors are in the same orientation as in your diagram but I think the carb and filter are just a bit higher vertically than on the twin cam. The Derrington manifold slopes upwards as well but is longer than the TC. I did try to fit short body SUs so they would be lower but the damper pot hit the bonnet stiffener. It only works with TC type long body carbs. In spite of the extra length the damper pot fits neatly in the cut away recess in the bonnet stiffener. Presumably that cut away was made for the TC.

I bet the designers had a nightmare trying to get the TC engine to fit!

John
John Francis

This thread was discussed between 10/10/2012 and 13/10/2012

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