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MG MGB Technical - Rebore an engine

Hi.

Yesterday My '73 BGT just blew a hole in the 3rd piston.
I don't know the cause yet, but I must investigate.
The engine is stock but the head was skimmed for a higher compression ratio. 0,7mm material was taken.
I might as well rebore it for 1950cc. I've got also installed a piper cams 272.
What do you think?
The centre of the cylinder bores are the same, right?
I don't need to offset them, do I?
Should I put new liners in the block?
Any special advice?


Thanks a lot.

Valter.
Valter >´99 VVC & '73 BGT<

Valter-
Due to the variances in cylinder wall thickness that are the result of a less-than-optimum casting process, it's necessary to torque the block to a reinforcing plate prior to overboring to prevent the finished bore from being distorted. Fitting 1950cc Big Bore pistons requires boring the cylinders out so far that the side thrust loading of the piston against the thin cylinder walls of some blocks can in some cases cause the bore to distort, the consequent loss of compression and high oil consumption becoming a headache. Either sonic testing or X-raying of the block in order to determine cylinder wall thickness prior to boring becomes a necessity at this point. Even so, a certain amount of cylinder wall flexure is to be expected, so state-of-the-art pistons with very thin rings are likely to become a necessity. When boring to such an extreme diameter in a B Series block, it is not uncommon to encounter porosity, in which case the installation of sleeves will become a necessity. Such sleeves (liners) are available from County (County Part # CL1950). These have a wall thickness of .130”, are 6.060” long, and have an external diameter of 3.380”. Sleeves have the additional advantage of being made of spun cast iron which is of better quality than the 'block-type' cast iron. If the sleeves are shrink-fitted and silver-soldered into place, then the heat distribution should be as good as that of a normal cylinder of equal wall thickness, although the ultimate rigidity at the cylinder/block interface will be less. In addition, the larger the bore becomes, the less sealing area that remains between the cylinders for the head gasket, so the torque settings of the head will have to be scrupulously maintained in order to avoid pressure leakage between the cylinders or a blown head gasket. Another downside is that the future reboring and fitting of oversize pistons can't be done as the cylinder walls will be too thin. However, both of these drawbacks can be overcome by offset boring of the block and fitting oversize sleeves with adequate wall thickness. This involves offset-boring the cylinders toward their respective ends of the block in order to maintain sufficient clearance between the cylinders, prevent the blowing of head gaskets, and the development of 'hot spots" that can cause cylinder distortion. Offset connecting rods would consequently become necessary as well, although they would cause uneven loading of the connecting rod big end bearings and consequent accelerated wear. A higher-pressure oiling system with modified feed passages can assist in protecting the bearings from the additional pounding of the increased power output. Of course, this implies that the engine would have to be built as oil-tight as possible, but all of these have been done before, so dealing with these issues would hardly involve blazing new trails into uncharted territory. All this, of course, is not to mention the problems of the excessive heat that would be produced with such an uprated power output, which in turn will require modifications to the cooling system. For anything other than use on a racetrack, a fully developed Big Bore engine is likely to prove to be financially impractical. A compromise displacement of 1900cc-1926cc is probably the practical limit for a fully developed street engine. No matter what you do, the ignition timing and the carburetion have to be scrupulously maintained or you'll have problems with a Big Bore B Series engine.
Most 1950cc kits use +.040 oversize 83.57mm domed Lotus TC pistons to produce an additional 8.2% (9 cubic inches) of displacement more than stock. These pistons use a standard thickness set of rings that lack the flexibility to compensate for flexure of the cylinder wall. They also have tops that are approximately .090" closer to their wrist (gudgeon) pins than standard MGB pistons, thus it is necessary to mill the deck of the block .100" in order to achieve a reasonable compression ratio of 9:1 with the 39cc combustion chamber of the heads used on the 18V engine. This will place the deck of the block very close to the cooling jacket, the consequent loss of rigidity resulting in a risk of cracking in some blocks. Because this reduction of the thickness of the deck of the block will decrease the number of threads available for the head studs, the depth of the threads will need to be carefully examined prior to redecking in order to determine that they will still be able to offer sufficient grip without incurring the risk of cracking and/or distorting the deck when the head is torqued. In addition, the use of these pistons also require the use of either the horizontally split connecting rods of the 18GH, 18GJ, and 18GK. engines that have bushed small ends to accommodate the use of floating pistons or the later connecting rods that have no balance pads with the small ends suitably modified in order for the wrist (gudgeon) pins and their bushings to fit properly. These later lighter connecting rods would help to compensate for the greater reciprocating mass of the larger pistons.
Unfortunately, the domes of the TC Lotus pistons interfere with both flow and combustion characteristics. If the bore is increased radically, then the squish area also increases and flame propagation becomes a problem, especially if domed pistons are used. Let's face it: A domed piston design and the Weslake kidney-shaped combustion chamber design aren't exactly in harmony with each other. Domed pistons present enough problems in a hemispherical combustion chamber, but in a Weslake kidney-shaped combustion chamber they're bad news.
County makes a cast piston that is adequate for a mildly tuned road engine. However, if you are building and engine that is intended to make use of high engine speeds in order to produce maximum power output, then a forged piston that is both lighter and has thin piston rings that will cope with bore flexure better, giving better oil and compression control, is the preferred way to go.
One of the better pistons for this application is the flat-topped Accralite BGT oversize 3.2874" (83.5mm) pistons (Accralite Part# 1196xc835). They make use of 13/16" wrist (gudgeon) pins, have a compression height of 1.6417" (41.7 mm), and a 6 mm crown thickness which together will enable the custom-tailoring of both the compression ratio and squish area, plus they weigh a very respectable 250 grams.
The best forged pistons to use for a Big Bore engine are flat-topped JE pistons. Being made from high silicon (13.5%) 4032 aerospace alloy, these are the only custom pistons sophisticated enough to have the same expansion/contraction coefficient as the Original Equipment Hepolite pistons, thus they can be fitted with the same clearances. These pistons come with peened and polished faces to remove dangerous hotspots and thus prevent preignition and detonation. Each set is delivered matched for weight, complete with the needed state-of-the-art thin ring assemblies that better compensate for flexure of the cylinder walls. Due to the height of their piston crowns being .040" greater than that of Original Equipment Hepolite pistons, when they are at Top Dead Center they are flush with the deck of the block and as such they will not require redecking the block to the point that there's a risk of weakening the block. In addition, their crowns are thick enough (.415") to allow the machining of the crown to a clearance height best for producing ideal squish characteristics (.012") as well as a dish diameter to custom-tailor both the size of the squish area of the crown and the depth of the dish to produce the desired compression ratio, as well as the contour of the dish to form the bottom of the combustion chamber to individual specifications in order to promote efficient flame propagation. It should be noted that the width of the squish area should be the same as the maximum distance that the mating surface of the head extends over the bore. While not everyone has access to such machining skills, fortunately there is a simple solution: JE offers the service of custom-machining their pistons to order, thus the piston can be made with a dished crown which, when coupled with a professionally reworked combustion chamber in the head, will accomplish the combustion chamber shape needed to promote efficient combustion while decreasing the tendency toward preignition when using a 10.5:1 compression ratio with 93 octane pump gasoline.
The combustion chamber volume of a Big Bore engine is relatively smaller in relation to the cylinder volume on a Big Bore engine than it is on a 1868cc engine, so the pressure rise within it is correspondingly faster than on the smaller bore 1868cc engine, the increase in compression / temperature ratio results in a larger squish area that induces too much turbulence for flame propagation to be smooth and even, inhibiting flame propagation in the areas near the roof of the combustion chamber, a factor aggravated by the dome of the Lotus TC piston. Due to the positional relationship between the circular cylinder and the kidney-shaped combustion chamber, the increased squish area increases the velocity of the turbulence in the direction of the spark plug, thus guaranteeing that the turbulence around the valves will be at the lowest in that location due to the direction of the moving fuel/air charge being biased toward the spark plug. The position of the spark plug also plays a big part in the detonation problem. The flame travels outwards towards the lobes of the kidney-shaped combustion chamber, creating a pressure wave. As the pressure wave at the border of the combusting fuel/air charge advances, the unburned fuel/air charge in front of it is compressed against the roof of the combustion chamber. When the pressure wave arrives in the vicinity of the hot exhaust valve, its velocity and pressure is at its greatest just as the remaining volume available for the unburned fuel is decreasing at its fastest rate. Because the area around the exhaust valve is the hottest region of the combustion chamber, its environmental conditions are best for producing preignition and detonation, and the arrival of the pressure wave compressing the unburned fuel/air charge against it triggers the event. While opening up the combustion chamber to decrease the squish area will alleviate the squish problem, the resultant increase in combustion chamber area can increase the likelihood of preignition in the vicinity of the hot exhaust valve at the expense of a lower compression ratio which in turn will prevent the potential of the engine from being attained. Obviously, it is difficult to reach a happy medium, so both the width of the squish band next to the dish of the piston crown and the distance between the piston crown and the head is critical to producing the correct amount of squish turbulence. It would seem that the most practical clearance is .012". This in turn will force a compromise when selecting a high-lift camshaft as some of them produce so much lift that it becomes necessary to relieve the deck of the block to a depth greater than that of the piston/head clearance. The edge of the compression ring may be directly exposed to the heat of combustion, in turn leading to premature ring failure and piston land breakage.
These problems could be minimized by using less spark advance, a lower compression ratio, and a mild camshaft such as the Piper BP270, but this solution would in turn result in the engine reaching its peak output at a less-than-optimum engine speed. Due to the increased displacement, higher port velocities are increased at lower engine speeds, resulting in a flatter power curve which reaches its peak at substantially lower engine speeds. What is really needed is either a Piper BP285 camshaft or a Piper BP270 camshaft coupled with a 1.69" intake valve in order for the engine to fulfill its power output potential and keep the power peak where it should be in order to retain the standard gearbox ratios and a compression ratio of 10.5:1 to keep the power output at a efficient level.
Steve S.

Geez! thanks a lot!
You convinced me not to go the 1948cc way. I'm now looking at 1868cc.
Can the standard con rods cope with it? I suppose so...
To achieve that displacement with .060'' pistons what should the exact bore size be in mm? Certainly I can pass the new liners with this bore, right? A Pressure test and that should be it?

Thanks!

Valter.
Valter >´99 VVC & '73 BGT<

My 1950 cc pistons are flat topped, although the block did need to be skimmed to get the compression right. When it was on the rolling road another car being worked on in the shop had been there for investigation as to why it would not produce the anticipated power after a similar upgrade. Tony at Redline did not take long to work out that without skimming the block you did not have a very high performace compression ratio! I have been very pleased with my engine, and when it came apart this winter the bores were still in good shape, I would hwever stick to 1860 or threabouts if I was building an everyday car with a bit more go.
Stan Best

When my 1950cc engine was built, Chris Betson ordered a set of custom made forged pistons that would NOT require the block to be skimmed to reach the desired CR.
If I am not mistaken, there are cast pistons that offer this as well.

Valter,
My 1950cc engine is used severely most of the time and hasn't used a drip of oil nor given any other indication of problems that could be caused by the large bore.
It does have a lot of torque (for a 4 cylinder car that is) and makes around 150 hp at the flywheel around 5500 rpm!
Alexander M

Alex's block WAS skimmed to achieve a piston height of 10 thou down with flat topped forged Accralite pistons. The CR was adjusted by combustion chamber size and is 11 to 1 - this is a crossflow Derrington head. The top of the block is more than thick enough to take a skim - all my standard blocks are skimmed by 20 to 40 thou to get a piston height of 15 thou or so.

The block was NOT linered and IMHO the concerns over wall thickness are over exagerated - most blocks will take it just fine!

Just get your machine shop to bore it out but not hone it - have a look at the bores and if the surface is clean and free of the mild steel straps lower down then go ahead and hone to finished size - only liner if the straps show or there are voids in the cylinder wall. The reason for not honing first is that this tends to hide the imperfections!

Head gasket sealing is still adequate at 1950 and even 1995cc - no need for special attention to head torquing - this is standard.
Chris at Octarine Services

Oops, sorry there Chris, my mistake!
Alexander M

Is there a difference between blocks for different years on casting thickness or iron quality? I have 3 blocks sitting in the garage ranging from pre 68 to 74. I'm running a '78 block at .040" overbore and thinking of going to .060 because of rust pockets in one cylinder.

Whether I should rebore that block or use one of the other blocks, is the question.

Barry

PS JP Pistons has a flattop cast piston that is available at .080" oversize.(and other sizes from stock) The deck height is apparently stock height, which is my impression is about .040 below stock deck height. It is a 4 ring full skirt piston.
Barry Parkinson

Barry-
I've never noted any quality control problems with the blocks that could be attributed to any given year of manufacture. They were all made by the same foundry. As Chris rightly pointed out, most, but not all, blocks will accept Big Bore pistons without having to resort to liners. Some blocks will present casting voids if you try to bore beyond +.040". Quality liners are more resistant to bore flex, thus reducing the liklihood of compression loss and high oil consumption. Have the blocks either sonic tested or X-rayed and you should be OK. As an aside, Chris noted that he installed Accralite pistons into Alexander's engine. I've been running Accralite pistons in my midget's A Series engine and can testify that they can take a hell of a pounding without so much as a whimper. Racing really does improve the breed. Good choice, Chris!
Steve S.

I've raced for several years with a Burgess 1950cc race motor and it's never consumed any oil.
Tony

Tony-
I don't know about your particular engine, but for some time now Peter Burgess has been installing liners in all of his Big Bore engines.
Steve S.

Peter does so because he has had some comebacks with oil burning which turned out to be porosity of the walls - manifesting itself as brown staining.

His decision to do so is simply based on the fact it is cheaper to fit the liners than to have a customer come back with an engine that needs to be taken out of the car and redone!

Chris at Octarine Services

Chris-
Exactly! Porosity weakens the cylinder walls, making them prone to flexure, but the sleeves restore rigidity. In addition, oil can't get around the ring's sealing properties via pores. Its the perfect solution.
Steve S.

Just to play devil's advocate, it seems that the costs of extra machining, liners, special pistons, etc seem steep given the small relative increase in displacement. Now that supercharging has been worked out to a large degree, what would be the adviseability of doing a more convential rebuild and put the savings towards a blower?
Mark Bates

I can build a whole new 1950cc engine including liners and special pistons for less than the cost of a supecharger alone AND it will develop more out and out power!!! Less power down the rev range, though.
Chris at Octarine Services

This thread was discussed between 17/04/2006 and 29/04/2006

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