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MG MGB Technical - Is it a 18GB or a 18V?

Need some help in identifying what kind of engine I have. Recently bought a restored body shell for a 1966 B roadster. The shell came with an engine and a (rebuilt?) 3-synchro overdrive transmission. Both were stated to be original to this particular car. The engine number is 18GB-U-H 27237. According to a supplied BRITISH DOCUMENTATION SERVICE document, this is the correct engine number for the tagged body shell (chassis # G-HN3-L/74874).

The first thing I noticed about the engine was that the head had air injector ports (66's didn't have these). The PO said that the head had been replaced. I accepted that. Head replacement is common on older engines. Took engine down to a local machine shop to have it checked out. Wasn't expecting major problems. According to PO, engine was running fine before removal for body restoration. Crank was fine, but piston rings had rusted to and pitted cylinders. Engine had been out of car for nearly ten years (on Florida coast). Shop guy said that cylinders need to be bored out. He's guessing 0.030" over, but may get by with only 0.020" over. I asked to see the other parts. He showed me what came out of it. I was surprised. Con-rods and the camshaft sprocket/chain are for a 18V engine. What other things should I look for and how can I determine if I have a 18V engine with a 18GB engine plate? Where are the casting numbers for the engine block located and what should they read for a 18GB and/or 18V engine?

Have already bought a NOS replacement head (no air injector ports). How can I determine the casting date for this head. It's at the machine shop now having hardened valve seats installed. Didn't know what numbers to read or how to interpret them. One number that I did write down was 12-H2708. Does this number have any meaning?
Steve Buchina

The 12h2708 number is a cylinder head number from a 1971 18v MGB engine. Alyn
Alyn

http://www.mgbmga.com/tech/index.html
http://www.sterlingbritishmotoringsociety.org/files/mgbtunings.PDF
http://www.mgb.bc.ca/service/tips/tips-engine
http://www.mgbmga.com/tech/index.html
Some good links about MGB

Alyn Murray
1973 MGB
Alyn

Steve, The Doug Jackson website has some information on cylinder heads.
http://www.mgbmga.com/tech/mgb25.htm
18V blocks have provisions for a mechanical fuel pump under the the manifolds near the front of the block.
The opening on MGB engines is covered by a plate and gasket, see item #30 on page 6 of the current Moss catalog. The 18V engine block should should have cutouts at the top of the cylinder for exhaust valves. Some older blocks have been modified when a late type head was installed so the cutouts are not a definite identifier.

Chris Betson can identify different versions of MGB parts by casting numbers. Go to his website and email him with all the casting numbers you can find on your engine parts. Chris has always been very helpful with information.

Clifton
Clifton Gordon

Steve, Chris Betson's website is;
http://www.octarine-services.co.uk/

Clifton
Clifton Gordon

Thanks to all for the info. Will be checking out these web sites tonight. Alyn - the #12-H2708 on the head does suggest what you said. However, there are no air injector ports. I'm guessing that this head represents a later replacement casting for a pre 1968 head.
Steve Buchina

Steve what you have is a non smog head see this link for further clarification.

http://www.mgbmga.com/tech/mgb25.htm

FWIW Alyn
Alyn

<The 12h2708 number is a cylinder head number from a 1971 18v MGB engine.>

Not quite. 1971 MGBs for North America used an 18GK engine (one year only), and the correct head casting was a 12H 2389.

Paul K

See this link for information about what engine had what head.

http://www.autochart.com/MGB_Head.htm

FWIW Alyn
Alyn

That list is not complete - it doesn't list the 12H4376 casting numbers that go with the 12H4375 part numbers and completely misses the 12H906 heads fitted to the very early cars.

Akk blocks & heads have dates cast into a circle - check these, they should be no more that a month or two earlier than the build datte of the car.
Chris at Octarine Services

Thanks to all for the comments and links. Especially enjoyed reading the article posted at:
www.sterlingbritishmotoringsociety.org/files/mgbtunings.PDF

although most of it was over my head. There is a lot in it that one can use and I would recommend it to everyone.

Went down to the machine shop yesterday to I.D. the block. It is definitely a 18GB block. The casting date was 10/19/65 and it had the following casting numbers:

12H1365
12H1442
12H1777

The pistons and rods are definitely from a 18GG or later (18V) design (i.e., 3-ring piston & horizontal split big-end rod). Lifters (tappets) and push rods are of the newer 18V design, as are the timing chain components. The head, as I mentioned, has air ports and from its casting number (12H2923), appears to be a '72-74.5 model. Crank and camshaft appear almost new and only need to be cleaned and polished. So, it appears that I have a 1965 block stuffed with 1970's components.

Pistons were not rusted to cylinder walls. Pistons #2 and #4 were just stuck. Couldn't really discern the compression rings. Their grooves were packed with a bunch of junk that almost made them invisible. Not sure what it is, but there was a lot of it. What happens when coolant gets into a firing cylinder? This engine had a head gasket with a solid copper top and fiber-like bottom side ("Made in England").

While inspecting everything at the machine shop, I noted that none of the oil galley plugs or core plugs (freeze plugs) had been removed prior to vating the engine block. Neither had any of the other oil fittings been removed (e.g., cap nut for oil release valve. Bad sign - decided to pull everything out of this shop. Told shop supervisor that I'm not going to spend any more on this non original engine and paid him for what he had already done - which wasn't much. Just couldn't trust his or their work.

I'm going to hand scrub and power wash the engine block tomorrow. I'm sure I will find that it still has a lot of junk in it. Will also lightly hone the cylinder walls. Just want to get a better look at them. Right now they look pretty smooth. There is, however, a protruding ridge that can be felt at the very top of all four cylinders. I'm guessing that the block will need to be bored out. I just hoping that it's 0.020" or less.

Three of the pistons appear old and well used. The forth matches the others and appears relatively new and has the number 19491 stamped on the top. I'm thinking that these piston are the deep dish, low compression pistons found in the 18V engines.

So where do I go from here? Not worried about the head anymore. In a search of my garage, I found that I have three other heads that will work, in addition to the one I just bought (12H2709). Age - I guess. Really need to do an inventory one of these days. I just forgot that I had them. Anyway, two of them (12H906 & 12H1326), that I found boxed on a top shelf, had already been totally rebuilt (double springs and all with bronze guides). The third one (12H1670) is still on an engine and is of unknown condition, but looks good and clean on the topside. Well, back to this engine. Wonder what I should put in it? How fancy do I want to get on the machine work? I'm just wanting to build a dependable, near stock 1966 version motor. How flat and shiny do I want the engine deck and head face? How about dressing up the edges for a better gasket seal? Don't really want any more of those messy oil and coolant leaks. What kind of valves/guides do I want to use? Can't remember what I put in those rebuilt heads. Have a friend who had his engine rebuilt - twice. Valves kept breaking. Course, he likes to rev near 5K before shifting. How about some of those extra oil ports on the crank? Do I really need one of those adjustable timing gears? Should I bump the cam up a bit? What kind of pistons (3 or 4 ring, flat or stock tops)? Don't really want the compression any higher than 9 to 1. How about the rods (stock or the lighter 18V style)? Have an aluminum flywheel and crank pulley. Should I use them? Should I have the engine balanced? Question? Questions? Questions?

Any comments and/or suggestions would be welcomed.

THANKS!
Steve Buchina

I am at the same spot as you are. I have a 2923 big valve head and a block (18v) that has the eyebrows cut into the block. It had a very small ridge on the cylinder walls at the top. The block has the dished low compression pistons. I have decided to: bore to .20 oversize, new pistons (high compression 6.22 cc)small lifters/long pushrods,Stock 1968 cam (have a new one on hand) The clyinder head has just been refurbished - as well as new valves/guides.From what I have figured out I shouild be at 9.2 - 9.5 compression ratio. I can live with that. I do not redline the engine everytime I shift gears. I want good midrange torque.I have O/D. This is not my main everyday driver. Premium gas prices are not a concern. I am using stock HIF4 and matching distributor #41491. Single valve springs. Double link chain and sprockets.Crane electronic ignition. I have read all of the stuff on the above links and have read Peter Burgess "How to power tune the MGB" All good stuff. The trick is :what type of car do you want to have? Everyday good in traffic? Has some power when needed? (such as passing another car on the highway as quickly as possible)or flat out racer that idles at 1150 rpm and will not pass smog test (if your locale requires one) I tried to balance the originality of the car with reliability and reasonable performance /smooth idle, not too many $$$ but it has cost a bit more than I had planned. Oh well... Alyn
Alyn

Alyn, I don't think you will have a lot of mid range power with a stock 41491. It has a slow lazy advance curve and a total mechanical advance of around 40* at 4900 rpm. I think with the higher compression you will have to retard the timing to prevent detonation. You can fix the total advance by having the arm on the points cam welded to give around 32* total advance and change the springs to get a quicker advance curve. My car had a 41491 and I wasn't happy after rebuilding the engine so I now use the 41491 as a spare.

FWIW, Clifton
Clifton Gordon

Have been up in Colorado the last couple of weeks and am still playing "catch up" on the board.

The early 18V heads required the "eyebrow" cut on the block if they were to work properly. Thus, not a good choice for an earlier block unless it is modified.

My daughter's 77B, which has a cylinder head very similar to the early cars, has been bored .020" over, has a CB cam and a Peter Burgess "Econo-Tune" cylinder head. There is also a Weber DGV carb on a TMW manifold. This engine will "take" my 68GT with its 18GF engine. There is no comparison with my own 79B having .040" over pistons, CB cam and a Mike Brown (Seven Shop) cylinder head. I cannot keep up with Theresa's car when accelerating.

Thus, I would suggest you bore the block to the minumum necessary. Intall a CB camshaft or a Piper 270, have Peter modify your unmodified cylinder head and put the engine together using a Fel-Pro upper gasket set (the Rover brand gasket set does fine for the lower portions and is less expensive). You should have, at that point, an excellent engine which is fully reliable.

As to distributors, you should, if possible, source the early model Lucas 25D4 distributor. This was used up until 1968 and represents the best curve then available for power. Mine (rebuilt) seems to provide excellent power and performance. There is, probably, a better power curve (mechanical advance) for the current "blended gasoline", but, to my knowledge, no one has bothered to note what that "better curve" is.

Les
Les Bengtson

Clifton, Thanks for the heads up on the distributor 41491. As I not gotten the engine assembled yet, so I have not run into this "lazy curve" problem. I have been looking at the cam arms inside of various distributors and I see that there is a number stamped on the end of the arm. Do you think I should put the cam from a spare 41155 distributor I have on hand that has a number "10" stamped on the cam arm??
Here is a webpage that I have been looking at. I think I should study it a bit more. Any input from you who has done this is much appreciated. Thanks Alyn

http://www.jcna.com/library/tech/tech0015.html
Alyn

Alyn, The Marcel Chichak article is good, I printed it for reference. I don't have his website link but he also has an article about using a Nissan distributor with electronic ignition modified to fit the A or B engines.

The 41155 distributor cam should be a better choice than the 19.5 degree cam in the 41491. You will need to use some springs that give a faster advance. I used the Chevy spring kit Summit Racing sells. If you need a little more mechanical advance you can file or grind the contact arm as needed.

The early distributor Les mentions should be better but you would need to add a nipple to your HIF for ported advance, otherwise you will have lots of vacuum advance at idle and may not be able to lower the idle speed. Adding the nipple can be done, it's somewhere in the archives but you need to be sure you do it right, a mistake can ruin the carb.

I used what I had in my spare parts to make a distributor that works better than the the 41491 for my car. I do not use ported vacuum. You have to spend some time with a tachometer and a timing light with a dial back setting to get what you need. It works OK for the street driving I do.

Clifton Gordon

Thanks Again Clifton,
Last evening I took the cam arm (10 degrees) from the 41155 dizzy body I have in my old parts box, and put it into the 41491 unit. I did this because the 414 dist has alot less miles on it. Like new. I will get the springs that you have mentioned within the next few days. I have thought about drilling my hif4 and installing a port nipple as I also have an old SU HS4 body in that parts box. Or should I just use the manifold vacuum port? Also what cannister are you using/suggest? The 41491 or 41155 unit? I would like to have it idle at 800-850 rpm, don't mind if it has a slight lope to it. Maybe I should drill/install the nipple and get a vac cannister for the 41155? As an aside, how do the cannisters differ from one another? I am using a stock 1968 cam and stock 3 inch air filters in the stock funky filter boxes. Thanks Again for all of your input. Alyn
Alyn

Alyn, The canister/capsules are marked near the hose end with 3 figures, the 41491 should be marked 10 15 5. Begins advancing at 10 in. vacuum, total advance at 15 in. vacuum. Distributor vacuum advance is 5 degrees=10 crankshaft degrees. I'm using a 41339 canister/capsule 7 13 5. It gives the same advance as the one from the 41491 at slightly lower vacuum readings. Either of these should work OK using manifold vacuum and you can idle the engine down as low as you like. This setup has worked well for me for the past three years and about 10k miles.

Clifton
Clifton Gordon

Hi Clifton,

I checked the distributors that I have on hand. The 41155 that I have does inded have the numbers 5-13-10 stamped on the vac can however the vac unit does not oass the "suck" test. Now the other dizzy the 41491 has the numbers 4-7-7 stamped on the vac can. This is the dist. that now has the 10 degree cam arm I installed the other night. What do you think ?? Thanks Alyn
Alyn

41491 should have a 10-15-5 vacuum can. It also should have a maximum centrifugal advance of 39 crankshaft degrees i.e. 20 distributor degrees. Whilst any can, stop plate, weights and springs can be fitted to any distributor, and that distributor adjusted so as not to pink at any combination of throttle opening, load and revs it is unlikely to be a decent match for the original curve and so cause a loss of performance and economy in all but one part of the range. Even if you can get a good match for the original, with the selections for North American engines being emissions biased and not performance, the original is probably not the best, and with todays fuels and the condition of your engine the selections should almost certainly be changed anyway even to meet the original emissions requirements.
Paul Hunt 2

Paul,
The point was that the engine is not stock. It has a 9.5 compression, big valve heads, no smog equipment, bored .020 over. I was going to use a new stock cam I have, or maybe a new 292 cam that I also have on hand. As Clifton says, the original distributor would not be feasible for my application. As it has a lazy advance curve. With the higher compression, the charge will burn faster and the stock distributor will prtobably cause preignition because of its slowness. I want 30 - 34 degrees advance all in at 2500/2800 rpm. I have found light springs from GM distributor to replace the heavy large springs that are stock with the 41491 distributor. I am trying to set the car up with a new (rebuilt)18v engine with 39 cc heads and 6 cc pistons hotter cam and reworked HIF4 carbs loosely around 1969 GH specs. The vacuum can that I have on hand (spare) is marked 4 7 7 . Inside the 41491, I have installed a new cam arm with the # 10 stamped on it (20 degrees at the crank)With 12-15 degrees initial advance, and this cam arm I should be close to where I want to be. 12-15 + 20 = 32-35 degrees total. If the vacuumcan starts advancing the timing at 4hg and reaches a total of 7 degrees vacuum advance at 7hg. I should have good low rpm response. The timing should be Ok I have port vacuum available on the HIF as well as the regular manifold vacuum take off. I will try both of them to see what works best. Poppet valves discs have been discarded in favor of solid ones from an old set of SUs that I have. I installed new butterflies in these carbs and have not been use since. Alyn
Alyn

This thread was discussed between 29/10/2005 and 06/11/2005

MG MGB Technical index

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