Building a 1950cc MGB Engine
- Gerry Masterman

As of:  18 April  2006 


This is in response to questions about building a 1950cc MGB engine.  Since I have built an engine like this it was suggested that I share the experience with other potential big bore builders.  Well here it is.

First I believe that a bit of background info would be helpful.  I own a 1972 MGB which at the time had a weak engine.  It ran fine but drank the oil nearly as fast as it consumed gas.  I was getting about 18MPG on fuel and about 150 miles to a quart of oil.  Engine was stock in every way.  Well as much fun as a B is to drive I hated to set it up for an extended time while the engine was pulled and reworked as needed.  Being a student of Murphy's law, I knew that it would take a couple of months in my spare time to attend to everything, with removing the engine, disassembling it and cleaning the parts up, doing the needed measurements, ordering the proper parts, machine work and then reassembly.  In an attempt to avoid this lengthy downtime I decided to get another engine to rebuild as needed so I could just swap things out over the course of a weekend. Logical

I set out looking for another 18V engine to start with.  I found one within 60 miles from my home so I bought it.  It had set up without a head in place inside a shop for a few years and was frozen but there was no ridge that I could find at the top of the bore so I bought it.  Upon disassembly I found some rust pits in one of the bores that were small but since I was planning to rebore anyway I figured no problem.  After cleaning the old pistons up I found that they were +20.  Well, OK, I can live with that, I thought.  

I figured that +40 would be enough to clean the bores up to a usable state so I ordered my parts.  Crank was +10 and in good shape so I decided to go back with +10 mains and rod bearings.  BTW, I ordered my parts from Brit-Tek and found them to be super support in the upcoming trying times.  Well, I cleaned my block up and everything looked OK so I sent it to a machine shop finish the job while I waited for parts to come in.  They burned the old crap off the block then shot blasted it.  I also had them pull the oil gallery plugs and thread the holes for screwed plugs.  I wanted that block clean! 

The block came out looking like brand new so I put a coat of paint on it to keep things from rusting.  In the meantime my parts came in so I took the block back to have to bored to match the pistons.  The shop bored the cylinder with the rust pit first and found out that +40 did not remove it.  He said that I might be able to put things together and have it work but he wouldn't and neither would I.  I'd already spent too much money to do a halfway job.  I got on the phone with Bob at Brit-Tek looking for a set of +60 pistons.  He found me a set and would hold on to them.  I had the machinist bore the bad cylinder out to a rough +60 to look and see if the pit went away.  

No luck.  He suggested installing a liner and going back to stock at $125 a hole! 4 x $125= no way!  I will not put a liner in just one cylinder and have one standard bore and three +40's although I have seen it done!  Well at that point I had already sunk a bunch of money in what turned out to be a bad block and could not force myself to walk away from it.  After all I could make a 1950 out of it, right?  I might add that at this time I had installed my new pistons on the rod after matching and equalizing the weights.  And remember this is a 18V with press on rods/pistons. 

Well I bit the bullet and ordered a set of 1950 pistons and figured that would be the end of my problems.  The 1950 pistons are Lotus TwinCam pistons in +20 as I found out.  In the back of my mind I am thinking of the usual disclaimers on 1950 engines.  I took the pistons to my machinist and had him bore the block to fit.  After a discussion on how much cylinder wall would be left he said that it would probably work out OK and he had built engines with thinner cylinder walls before.  He was an old ex-racer and engine builder from way back so I believed what he told me.  

When I got home my phone was ringing.  He had bored one cylinder and found something strange-a perfect little 3/8" square of different metal showing up between #1 and #2 cylinder about an inch down from the top of the bore.  I went over there and looked at it and decided to go ahead and finish that hole.  By the time the cylinder was honed the oddity had disappeared so I decided to go for broke.  At this point the alternative was to reline the bore and go back to a stock size.  The problem here was that we had already gone past the OD of the liners so in the cylinder in question he would have to put in TWO liners at $125 a shot!  Now it's up to 5X$125'no way! He finished the bore job.

In the meantime I had removed the old/new +60 pistons from the rods, breaking one in the process.  I start to put a new 1950 piston on the rod and the piston pin will not fit the rod.  MG has used several piston/rod setups through the years.  They have used press fit pistons where the pin is held in place by a friction fit in the piston and they have used circlips to locate the pin in the piston.  And guess what - they are not interchangeable!  To use the Lotus pistons you have to use earlier than 18V rods.  BTW, the 18V rods can be used but they must be reamed and honed to fit the Lotus piston pins.  The Lotus pistons use wire circlips to locate the pins, too.  In the past I have lost several engines due to circlip failure and will not use wire circlips in an engine if I can help it.  In this case, after I got the right rods, I made up some Teflon buttons to fit in the pin cavities to locate the pins in place. This has been done for years in racing engines.

 Aluminum and bronze have been used but I like to machine the Teflon and it does not expand as much as metal plus it is softer.  The first piston I saw set up like this was a Ducati motorcycle piston made by Venolia in the early 70's.  This piston also had three buttons on each side of the skirt to keep the piston from ever touching the cylinder wall!  Super neat!  Anyhow, I made up a set of these Teflon pin buttons and they worked out great.  If anyone is interested in these I can provide more info on what I did for the asking.

OK, so now I have a block bored, oil galleries threaded but still open and ready to do a trial assembly.  I put the crank in place with three mains, installed one piston and rod combo, torqued everything to speck and checked piston height.  Remember that when using Lotus pistons the block must be decked to raise the compression because the height of the pistons is shorter than the B pistons.  The compression has to be calculated in order to find out how much the block has to be decked.  To do this you must measure down from the gasket surface, figure the volume, add that to the cc volume of the head and arrive at a compression ratio.  You also have to decide what comp ratio you want in order to decide how much to remove.  You also have to take in account the dish area of the pistons if there is any.  Thankfully mine were flat top pistons. 

After doing the math I decided that I needed to remove 0.100" from my block in order to get a CR of 9.5 to 1.  I would not go any higher than this unless you like to by race gas or additives by the case.  I run Premium in my engine and have not had any pinging problems.  Another thing to remember - on the top of the bore on 18V engines there is a cut out on the top edge because the exhaust valves overlap the edge of the cylinder.  If you deck the block this must be replaced or your valve might hit the edge of the block.  When my block was decked it completely removed this cutout.  To replace it place a head gasket on the block and mark the location about 1/16" inside the gasket.  With a Dremel tool cut a 45-degree angle along this mark no more than 0.200" deep.  Look at your block before it is decked and you will see what is needed.  Go too deep and you will create a lip to hang the rings up and break plus allow flame from the combustion chamber to get at the side of your piston.

OK, now I am ready to put things together for a trial fit.  In doing so I found that after all the mains are installed and torqued the crank will no longer turn.  Shit, what now?  Pull the mains off and redo and find the #4 main was scuffing.  Plasti-guage shows things are within spec but the crank is still too tight.  Frustrated, I hauled the block and crank off to the shop to have it checked.  It needed line boring!  Now, the block was fine before all the machine work but now it isn't.  I took a lot of meat off the block but I didn't think it was enough to allow it to twist!  Had the block line bored and everything was fine.  Go figure!  Do another test assembly and everything was as it should be.  Took it apart and did my last cleaning of the block.  I used about four cans of brake cleaner through the oil galleries along with a rifle cleaning brush to scrub everything.  Then used soap and water in the driveway to finish things off.  I blew air through every passage and nook and cranny.  Left it out in the sun to dry then put the oil plugs in with permetex. 

When I got to the rods I broke two rod bolts at about 20#.  These were reconditioned rods and apparently the bolts were not replaced.  Watch out for this!  The next engine I build will use all new hardware!  The rest of the story was pretty uneventful.  I used 285 cam from Brit-Tek.  The stock head gasket worked fine.  It has been reported that the gasket overhangs into the chamber but mine did not.  

I installed this engine along with a Merkur five speed transmission from John Black at Waterloo Drivetrain.  Love that transmission!  It was a direct bolt in swap.  A bit expensive at $1850 but after weighing both sides I think it was worth it.  By both sides, I mean the possibility of buying a standard O/D transmission, installing it then finding out that it need rebuilding and having to remove it again. 

Several people sell this conversion kit although the price has gone up a bit.  My engine developed an oil leak in the back somewhere.  I suspect the cork gasket at the rear main is leaking but have had the time to pull thing apart to fix it.  I ran the car with the stock HIF's till I needed them on my other car, a 71 BGT.  It was and still is my intention to replace the head with a ported and polished head and to put a DCOE and Malloy dual point on it but time just has not been there to do it.  The head that is on the car now, although rebuilt with new everything, is a large combustion chamber head so my CR is not where I want it.  I have an 18V head to P&P and raise the CR up to where I want it.  There is plenty of info on heads and CR's on the web.  The performance is a marked improvement over a stock 1800 but not what I was expecting, but then again, I am not finished.  I agree with others that the 1950 needs to be done as a part of a complete package, including a warm cam and more fuel, or better yet, Fuel Injection.  I have been told that the FI from a Chevy Cavalier will bolt on the intake for a down draft Weber.  One of these days I will find out

Would I do it again?  

That is a big NO, at least not the way I did this one.  There is just too much info out there that is incomplete or full of speculation.  I never intended to build a 1950 at the start, I just kinda got carried away because I didn't want to give up and start over with another block.  In hindsight I would have been smarter to stop when the rust pit showed up an found another block.  For what I spent I could have bought a 2.0 engine from Proper MG.  I considered that option in the beginning but the delivery time put me off. 

This whole ordeal took nearly four months from the day I started.  There were other complications after the engine was installed that were not directly related to the big bore but they added to the disenchantment.  The first thing that happen when I cranked the engine was the line to the oil press gauge blew so things had to come to a stop while another was ordered.  When it came and was installed one of the hoses to the oil cooler blew.  Once again, another order, another second day air charge, another delay.

In retrospect, if I was going to build another 1950 engine I would not use the Lotus pistons.  There are pistons available that can be used with the regular MGB rods and have a higher top so that the block would not have to be decked.  This is the way to go!  Had I known what I was getting into this would have been my choice.

Well I hope this info is of some use to someone.  If it keeps you from making just one of the mistakes that I made it will be worth it to me.  I would like to hear from anyone who reads this.  I am also more than willing to talk to anyone about any of the details that I have left out.  Below, I will include some miscellaneous info that may be helpful.  


        Correct rods for use with Lotus pistons from MGB engines 8GG thru 1971 18V engines after this use Apress fit@ pistons.  Moss part number #437-000

        British Automotive, .  These people sell the pistons that I would use in the future.  They require NO block decking and can be used with MGB rods.  They make a 1924 CC engine so that leaves room for another rebuild job on your block which makes more sense to me that the Lotus pistons.  Lots and lots good reading in his tech pages.  Maybe the most info of it's kind in one location! You definetly want to bookmark this site!

        Stan Weiss-  This fellow has a neat Compression Ratio calculator.  This is what you need if you decide to use the Lotus pistons and have to deck your block to suit.