Technical Section – MGB Engine Replacement

Please note: Questions and answers and Upkeep and Performance Hints are provided for information and advice purposes. No liability either express or implied is assumed by reliance on the information presented either by the writers or the AMGBA.

Some or all of the below is from our message board at http://board.amgba.com, Facebook group at www.facebook.com/groups/americanmgbassociation. This is just a part of what appears there and in the member magazine.

Also be sure to see our message board on our website and Facebook group for immediate help from fellow members.

Q:     I’m hoping you or another member can help. I recently bought an 18GH engine to replace my worn out 18V in my ’78 MGB. The problem is we can’t seem to get the drivers side engine mount to fit correctly. We have both the old style and new style mounts, but neither lines up to both the block and the frame. Is there any modification or kit we can get to accomplish this. Please let me know as the summer is here and I’d like to get back on the road.

Brian Turner, Beachwood, New Jersey

A:     The 18V and 18GH engines are certainly interchangeably replaceable as compete units, but there are differences in the engines and bodies.

While it is said the B was unchanged for it’s entire run, the rubber bumper body looks the same, but was heavily altered to meet later crash standards and there are differences that make the swap a bit more than removing one and sliding the other into place.

In your case, the major difference you see is in those minor differences in the body sheet metal and require that you swap the front engine plate with your old engine’s to assure the mounts line-up and to use the later motor mounts.

In addition to changing the front engine plate and mounts, you may also encounter some interference between the left front motor mount and the bulge of the oil gallery that runs just inside the engine block in that area. So you may have to use a grinder or some judicial use of a BFM (mallet as opposed to a hammer) with a block of wood to move (or remove) the sheet metal flanges and panel enough give you the clearance for it to fit.  You should be able to do this without doing damage to the car.

I hope that helps you.  Take photos to share and let me know what you find when you get into it.

Safety Fast!
Art Isaacs

Q (part 2): We got the “new” engine in and running, but it heats up and over very quickly and the headers became cherry red also. We blocked the hose that would have gone to the choke outlet as described earlier, but it doesn’t seem the coolant is circulating. Is there something we missed about this outlet that is preventing circulation?

Brian Turner, Beachwood, New Jersey

A (part 2): There are any number of reasons the coolant wouldn’t flow or the engine would overheat.  Blocking the choke water outlets is not one of them.  Fast questions:

The cheap stuff to check first:
– Did you check or change the thermostat?  A stuck stat will stop or restrict coolant flow.  Just remove it to see if anything improves.  You can replace it afterward.
– Did you flush the engine before installing it?  You can still do that as you remove the stat.  Take the lower hose off  as well and flush both the engine and the radiator from the top to see what comes out.  Run it with just water now (no fear of freezing) and replace the proper coolant when you are satisfied with its operation.
– Have you removed and inspected, cleaned or changed the water pump? Hoses? Have you checked the heater control valve to see if it’s blocked?  If it is, that often means gunk or corrosion in the engine.
– Have you checked the timing?  That should be set per the engine spec, not as the car would be. Too far off and they do overheat.
– Is the carb running too lean?  Tends to run hot if it is, but not to the extent you seem to describe.
– Have you tried running the heater with the fan on to see if it runs cooler? If it does run cooler, that indicates other possible problems than flow.

The big stuff:
– Do you know the history of this engine?  Why was it available?
– Have you done a pressure check on the cooling system?  Head gaskets leaking internally are a fairly common issue on these engines, as are cracked heads.  A pressure test generally shows this up, even if there is no external leakage.  Often the only external leakage seen might be a moist line of coolant at the head gasket below the spark plugs.
– Having run it hot and had it overheat, is there coolant in the oil? Or white smoke (water vapor) exiting the exhaust?  Or coolant backing out of the overflow pipe? If there is, see above.
– Was the radiator checked for blockages or the cap replaced or tested?

Check these out and then let’s talk further.  Talk to you later.  Good luck.
Art Isaacs

Q (part 3): Here are some pics of a question we have.

What do we do about this?

Brian Turner, Beachwood, New Jersey

 

 

 

 

 

 

A(part 3): Not a major issue.  Of course the difference is the outlet fitting for your water heated choke from the Stromberg 175CD carb on your old engine.

Older engines with twin SU carbs never had them because they all used a manual choke.

If you are continuing to use the single Stromberg, then you would either have to change the heads between engines or covert your carb to a manual or electric choke and blank or plug the other end of the choke heater hose.  The manual choke conversion kit is available from Moss (part number 386-325, about $125), which is more than I can say for the replacement choke heater hoses for the Stromberg.  I have not see the electric kit of late.  These conversions are easier to do than changing the heads.

It is also much cheaper than swapping to an HF44 carb (Victoria British part number 3-740, about $750), which includes a new air cleaner and the manual choke kit, but is otherwise a bolt-in conversion.  But the performance difference may be worth it.

There is also the Weber downdraft conversion, which requires new air cleaner and intake manifold (both included) as well as an exhaust manifold and engine pipe (both not included) and linkage modifications, but you could get the electric choke version of that carb (Victoria British part number 3-450, about $700), which does not require the manual or water choke connections.  Again the performance will be much better than the original 175CD carb.

If you have the twin SU carbs, manifolds and linkages from the donor car, and your local emissions laws allow the swap, that might be the best way to go.  The head on the engine you are swapping-in does not have the outlet, so it’s just the front that needs to be blanked and you need the choke cable, which can be the actual MG part or one available from the universal parts rack at any Advance, NAPA or Pep Boys shop.

I hope this helps you, but write or call any time if you have any other questions.  Good luck.

Safety Fast!
Art Isaacs

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