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Technical Information (MGB 11)

Updated 01/28/03

MGB Piston Choice

Replacement OEM pistons and aftermarket pistons are usually supplied in the following over sizes: +.010", .020", .030", .040", and .060". These pistons are designed with a predetermined piston dish size cc (cubic centimeters) and theoretically should provide you with the correct geometric compression ratio (GCR), i.e. 8.7:1 1963-1972, 8:1 1972 onwards. Unfortunately, during the engine rebuilding process, we routinely see GCR's in excess of those outlined above.

NOTE: This technical article should be read in conjunction with MGB10

The following machining operations will increase the GCR during the engine rebuild process:

1. Engine reboring to oversize.

2. Cylinder block redecking.

3. Cylinder head resurfacing.

4. Cylinder head valve seat insert replacement along with new exhaust and inlet valves.

The following machining operations will decrease the GCR during the engine rebuild process:

5. Connecting rod big end journal resizing.

6. Cylinder head valve seat recutting along with exhaust and intake valve resurfacing.

British Automotive recommendeds 9:1 GCR for engines used for street and street performance applications. Obviously, knowing the piston cc dish will help you in accomplishing this GCR. However, as previously mentioned, the problem mainly lies in controlling excess GCR's in the rebuilding process.

Installing custom made pistons with a predetermined piston dish cc can satisfy this problem.

British Automotive,s JE manufactured pistons can be supplied in the following sizes. STD 010" 020" 030" 040" & 060" as well as 83mm & 83.5mm NOTE: Even larger capacity engines of 2000cc are being built by companies here in the USA and in the UK. However, this requires offset boring techniques, as well as increasing crankshaft throws. Normally, we supply JE pistons in a flat top configuration with a piston pin height of 1.700". This additional height, over the OEM height of 1.660", allows for additional material removal where necessary. Incorporated in the piston design is an unusually thick piston crown (.415"), this thickness allows us to machine the appropriate cc dish without piston crown breakthrough.

However, if you have already calculated the exact piston cc dish required, relative to the desired GCR, this can be carried out during the piston manfacturing process. The also applies to any changes that maybe required in the piston pin height.

To determine the piston cc dish and the piston pin height, the following information will be required.

We will use British Automotive's 1924cc Big-Bore conversion in the following example:

Swept volume (SV) (1924cc divided by 4) 481.00 cc

Cyl/head cc (spark plugs fitted) (must be determined) ------

Cyl/head gasket cc compressed 3.21cc (factory info) 3.21

Cyl/bore exhaust valve notch. (see information) .30

Piston below deck cc (see information) ------

Total of the above ______ cc

Swept Volume (SV)

To find the SV, we apply the following formula 3.142 x r2 x stroke, where r = bore dia. divided by 2. Stroke = 3.5". To convert to cubic centimeters, multiply by 16.39. This will then be the capacity for one cylinder.

Cyl/head cc (spark plugs fitted)

It is important that the cyl/head combustion chamber capacity cc be measured. This will be a total measurement of all 4 cylinders divided by 4 to give you the average measurement. Small variations in individual combustion chamber capacities is acceptable.

Cyl/head gasket cc (compressed)

This measurement was provided by the factory in numerous publications.

Cyl/bore exhaust valve notch

It is necessary to notch the cyl/block to prevent any Ex/valve contact problems that may occur when using cyl/blocks fitted with small combustion chamber cyl/heads (18V onwards), high lift camshafts and/or rockers. Even if the aforementioned is not contemplated, this notch will allow the Ex/valve to breath better. Although all 18V cyl/blocks had this eyebrow cut, we still recommend that they be recut to the following dimensions: use 1.625" dia. cutter, whose center line is 2.625," measured from the cyl/block face spark plug side and machined to the following dimensions: all bore sizes up to 83mm, .150" depth, .175" width (capacity .4cc), and 83mm and up, .150" depth, .150" width (capacity .3cc).

Piston below deck cc


Any necessary cyl/block deck machining must be carried out before proceeding. To determine this measurement proceed as outlined below. Measurements must be taken at the front and rear of the cyl/block, then divided by 2 for the mean measurement.

A. Cyl/block mean measurement + 1.136" = ?.????"

B. Crank stroke (3.5") divided by 2 1.750"

C. Piston clearance height (JE Pistons) 1.700"

D. Conrod length (center to center) 6.500"

E. Total of B, C, & D = 9.950"

NOTE: It is preferable to run the piston crown flush with the engine deck. Measurement A will determine the amount of material, if any, that has to be removed from the piston crown

Measurement A ?.????" Less Measurement E (9.950") Piston above or below deck = .0??"(+or-)

NOTE: If measurement E is greater than measurement A this will indicate that the piston is pretruding above the engine block deck. If you intend to run the piston crown flush with the deck, this is the amount that must be removed from the piston crown.

If you intend to run the piston crown at a pedertimined distance below the engine block deck, this distance must be added to the above deck measurement. Machine piston crown to this total.

Engine blocks that have been excessively re-decked will have a direct bearing on how much material is removed from the piston crowm and, the relationship of how much material can be safely removed from the piston crown to achieve the correct piston cc dish. Use the above measurement to calculate the piston below deck cc, applying the following formula: 3.142" x r2 x .0??" x 16.39 = ??cc.

With British Automotive's 83mm bore engine, for every .005" below deck we have approximately .7cc volume Now that we have the total clearence volume (CV), and also the GCR desired (9:1 for example), we can calculate the necessary piston dish capacity cc. and the amount we need to remove from the piston crown. Pistons will then be supplied in this configuration. To determine the GCR, use the following formula:


Example: assuming a CV (60cc), then the formula becomes: GCR = 481cc+60cc = 541cc = 9:1 (9.016666) 60cc 60cc

For every 2cc increase or decrease in CV (1924cc engine), there will be an increase or decrease in GCR of approximately .3:1. Please note that what we have discussed is the GCR only. The ECR (effective compression ratio) is determined by the intake valve closing position, therefore your choice of camshaft becomes very important before deciding upon the GCR value.

Conrod length

The measurement given in paragraph 5 is as per factory specifications, however, should the connecting rod journal be resized, then this center to center measurement will have to be recalculated.

Crankshaft stroke

For extreme accuracy, you may consider, when undertaking the crankshaft regrinding operation, having each journal index ground. It is not uncommon to find small variations in crankshaft journal throws. Personally, I don't believe rectifying small variations is cost effective.

From the information I have provided, one should be able to see the advantages of using custom made pistons and, coupled with today's low octane fuel, it is paramount that the recommended 9:1 GCR be adhered to, otherwise detonation will occur resulting in engine damage. British Automotive's Big-Bore Conversion 1924cc

British Automotive's 83mm JE forged alloy pistons require a piston skirt to bore clearance of .002". They are also designed to be used with any 5 main bearing conrod configuration. .002".

The following is applicable to 5 main bearing engines only: It is quite possible that your cyl/block has original factory cylinder liners installed (under no circumstances rebore these liners). These liners were installed to rectify cyl/block casting imperfections. Should this be the case, it will not be possible to bore the cyl/block to accept 83mm pistons, however, it is possible to use 83.5mm pistons P/N MGB1948cc (use 83mm boring procedure below), or alternatively, resleeve the cyl/block and bore to 010"/020"/030"/040" or 060", in preference, use piston P/N MGB1868cc (060"). If your cyl/block does not have liners installed, then careful boring of the cylinders should be undertaken to within .005" of the finished bore size. At this dimension, it can be determined whether or not the cyl/bore can be successfully honed to give the desired finish (please note, always start at #4 cyl/bore as this seems to be the troublesome area). In the event that your cyl/block cannot be successfully rebored to accommodate these pistons, it is unlikely that you will be successful in going to the next available bore size (83.5mm). Either proceed with liner installation and rebore or provide another cyl/block and start all over again!

Advantages of British Automotive's 83mm JE pistons

No cyl/block redecking required to facilitate piston compression height. No cyl/head modifications required for GCR work. After initial rebore, cyl/bore can be readily rebored to oversize (83.5mm). Pistons can be used with any type of conrod. No cyl/head gasket overhang into cyl/bore. Piston crown design can accommodate machining for the desired GCR. While there are numerous big-bore modification kits available, there are usually some disadvantages associated with their fitment, some of which are listed below:

Pistons can only be used with certain conrods. Piston crown thickness does not allow for any piston dish cc machining. Excessive redecking of cyl/block required due to the shorter piston compression height. To achieve the correct GCR, the cyl/head needs to be reworked to the correct capacity. Listed below are the necessary clearance volume(s) (CV) required for a particular engine size to achieve the recommended 9:1 GCR: Bore Size Engine Capacity CV Required Actual GCR Std 1800cc 56.5cc 8.96:1 +010" 1812cc 56.5cc 9.02:1 +020" 1822cc 57.0cc 8.99:1 +030" 1834cc 57.5cc 8.97:1 +040" 1844cc 58.0cc 8.95:1 +060" 1868cc 58.5cc 8.98:1 83mm 1924cc 60.0cc 9.02:1 83.5mm 1948cc 61.0cc 8.98:1

Effective Compression Ratio (ECR)

This is more important than GCR. ECR relates directly to the actual inlet valve closing (IVC) position relative to top dead center (TDC) and is expressed as after bottom dead center (ABDC).

To find this ECR, you must first install a valve timing degree wheel to determine exactly when IVC occurs. For convenient measuring purposes, I prefer to use .001" before actual IVC (under actual running conditions, this IVC position will be slightly different due to valvetrain component flexibility).

Calculating the ECR can be one by:

1 - Plotting a graph of piston movement to crankshaft angle.

2 - By applying the appropriate trigonometrical formula.

3 - The use of a PC program covering this subject.

Thanks to Dimitri Elgin of Elgin Racing Cams, Redwood City, Ca. for the following program information which,is applicable to British Automotive's 1924cc 83mm MGB engine: Intake Closes ABDC Swept Vol(cc) 9:1 GCR 10:1 GCR ECR ECR 57 395.108 7.562 8.383 1 58 392.09 7.512 8.326 59 389.024 7.461 8.269 60 385.911 7.409 8.211 61 382.751 7.357 8.152 62 379.546 7.304 8.092 63 376.294 7.25 8.031 64 372.998 7.195 7.969 65 369.658 7.14 7.907 66 366.273 7.083 7.844 67 362.845 7.026 7.78 68 359.375 6.969 7.715 69 355.863 6.91 7.649 70 352.309 6.851 7.583 2 71 348.715 6.792 7.516 72 345.081 6.731 7.448

ECR Applications 6.0:1 ECR - low horsepower engine 6.5:1 ECR - fair engine 7.0:1 ECR - street gas 92 octane 7.5:1 ECR - good tuning required, street gas 92 octane Over 7.5:1 ECR - requires 104 octane boost Over 8.0:1 ECR - requires racing gas

From the above information, it is easy to understand that we can optimize the resulting ECR by establishing earlier IVC. A comparison between 1 and 2 clearly shows these optimum and non-optimum aspects of IVC. So, if you are contemplating rebuilding your engine or already in the engine rebuilding process, you may want to review your camshaft choice. My recommendations are as follows: for street, performance street and rallying - use 9:1 GCR, choose a camshaft with around 220-225 duration, and with an IVC of between 35-37 deg. ABDC (both measured at .050" lifter rise). This should put IVC at approximately 60 deg. ABDC (+ or - 3 deg.) when measured at the intake valve. This puts us in the area of 7.0-7.5:1 ECR.