Technical Information (MGA 3)

MGA 1500cc ENGINE REBUILD (1698cc SPECIFICATIONS)

Last Modified - 2/26/00

If you are contemplating rebuilding your 1500cc engine and wish to retain your original engine block, then British Automotive's 1698cc conversion should be of interest to you; bore size 3.070" (78mm).

A conversion of this size is dependent upon whether or not there is sufficient engine block casting material surrounding the cylinder bore walls, especially, between adjacent cylinders. Minimum thickness should be no less than .100".

To determine if your engine block meets these requirements, you should first remove cam bearings, main oil gallery plugs and freeze plugs, then proceed and have the unit chemically dipped and thereafter have the engine block sonic tested. This test will determine if you can continue with this new bore size (3.070") or select alternate sizes such as 3.0" (1622cc), or 2.968" (1600cc) or any convenient oversize rebore of these 2 sizes.

To take advantage of this bore size (3.070"), we recommend using the MGB large intake valve (1.625") and exhaust valve (1.344") cylinder head and head gasket as fitted to 1972-1974 1/2 models (see Tech article MGB10 "MGB Engine Rebuild Information").

The above cylinder head has an improved and shallower combustion chamber .375" approximately and 39cc chamber capacity, cylinder head thickness 3.125" This also applies to the cylinder head that was fitted to the 1975 on models (1.565" intake 1.344" exhaust). Therefore, if you intend using either of these two cylinder heads, intake & exhaust valve relief notches should be milled into the cylinder block deck to prevent any possible valve contact and valve shrouding, especially with increased exhaust valve size (1.44"). Due to valve shrouding within the bore, we recommend staying with the 1.625" intake valve. We found, before we actually milled the relief valve notches, that #1 & #4 cylinder intake valves (1.625") made contact with the cylinder wall, the 1.565" size valve cleared but there was significant valve shrouding within the bore to warrant this valve relief notch.

The current replacement aluminum siamese port cylinder head for the MGA/MGB is fitted with 1.565" intake and 1.344" exhaust valves, thickness 3.105". The aluminum cylinder head that we had in our possession at that time had a combustion chamber depth of .375" and a combustion chamber capacity of approximately 37cc. However, since then, we have not been able duplicate this number, measurements taken by us on several cylinder heads, have shown a combustion chamber depth of .425" with a capacity of approximately 41.5cc. Always have your cylinder head cc checked before doing your calculations.

Due to shrouding problems within the combustion chamber, the 1.625" inlet valve size are not recommended.

The replacement aluminum cross-flow cylinder head has 1.565" intake and 1.344" exhaust valves, Head thickness 3.105" combustion chamber depth .340" with a capacity of approximately 35cc. What we have outlined in the previous paragraph also applies to this cylinder head.

It is important that upon installation of any of the above cylinder heads that the later MGB head gasket part# GEG377 be used. Do not use the original style.

Another alternative cylinder head would be from the MGA 1622cc engine, this cylinder head can be recognized by the raised number 16 at the rear of the head. However, it may be easier to locate and use the earlier style MGB cylinder head 1963 through 1967 (non smog) or 1968 through 1971 (smog). All these cylinder heads have a deeper combustion chamber .425" approximately, with a capacity of approximately 42.5 to 43.5cc, cylinder head thickness 3.175", 1.565" intake and 1.344" exhaust valves. Even with the fitment of any one of these cylinder heads we still recommend that the intake and exhaust valve relief notches be milled into the cylinder block simply because should you change cylinder heads further down the line to the later type, your cylinder block will be ready to accept the new head.

We gave a lot of thought as to whether or not we should upgrade the existing 1500/1600 conrods to the 1622/MGB1800(3 main brg) conrods with their larger piston pin diameter (.750" as opposed to .687") and beefier construction, However, we decided that the original conrod with a stronger .687" piston pin could be used as long as a Geometric Compression Ratio (GCR) of 9:1 was maintained and that the engine operating RPM be held within the parameters as indicated on the OEM tachometer. Your conrods should be magnafluxed, "shot peened" (stress relieved) and balanced. New rod bolts and pinch bolt should be fitted. If you are installing ARP rod bolt set #206-6003 make sure, on each connecting rod, that one of the bolts does not bottom out in the threads, shorten.

Before milling the valve relief notches into the cylinder block have the block deck thickness checked (sonic test procedure). Our donor 1500cc engine block showed an average thickness of approximately .450" in the areas designated for machining. We do not take the cylinder head gasket crushed thickness (.023" approximately) into consideration during our calculations, we prefer to use this measurement as a cushion against future cylinder head or cylinder block resurfacing. Our calculations are also worked out using a maximum valve lift of .450".

The following calculations were based upon the shallower combustion chamber cylinder heads (.375").

INTAKE VALVE
Cutter size 1.875" OD, 3/64" radius, whose center line is measured at 2.625" from the spark plug side of the cylinder block. Machine to a width of .100" and to a depth of .150". Volume .2cc approximately.

EXHAUST VALVE
Cutter size 1.750" OD, 3/64" radius, whose center line is as above. Machine to a width of .250" and to a depth of .150". Volume .8cc approximately.

Listed below are the intake and exhaust valve relief notch measurements, (same depth of .150"), for various oversize bores. We have also added the new cc displacement for each oversize.

To calculate your anticipated valve lift, multiply your camshift lobe lift by the rocker arm ratio (see tech article MGB16 "Rocker Arm Ratios"), then subtract the running valve clearance. Before final assembly, we recommend that you install the cylinder head (valves held only into position by placing the valve stem seal 'O' rings down the valve stems) with the cylinder head gasket in place, install and lightly tighten head nuts, then measure and record each individual valve lift. Obviously, these measurements should be greater than your calculated intake or exhaust valve lifts.

These forged flat top replacement pistons are manufactured for British Automotive by JE Pistons and are designed with sufficient piston crown material so as to accommodate piston cc dish machining. Short skirt design , Piston length 2.601" with 2 compression rings (1.5mm) and 1 oil control ring (4mm). The piston deck height (1.701") is greater than the OEM piston (1.656")** and upon final assembly the piston should be flush with the engine block deck.

* Quoted from AE Engine Parts Catalog

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. Below are the measurements from our donor engine block.

We can determine the height of the piston crown above the cylinder block deck by calculation. This is necessary so as to arrive at the desired GCR by way of machining the piston dish to the correct cc amount. This method of calculation is extremely close, providing that you do your homework and work out your calculations correctly. The example below uses measurements from our donor 1500cc engine block.

Therefore, for the piston to be flush with the engine block deck, this .018" must be removed from the piston crown for a new piston deck height of 1.683" (1.701" less 018"). This service will be provided by British Automotive as well as the piston dish cc machining which will be necessary to achieve the required GCR.

For the recommended GCR 9:1, we can apply the following:

Where SV = 1698cc divided by 4.

Or if you wanted to raise your GCR to 9.5:1 (not recommended)

CLEARANCE VOLUMES

Once we have determined the combustion chamber volume, we fill in the total amount on line A.

The amount on line C can now be machined into the piston crown to establish your GCR. To establish your Effective Compression Ratio (ECR) refer to technical article MGB 11 ("MGB Piston Choice").

As we have previously mentioned the pistons upon final assembly should be flush with the cylinder block deck. The OEM pistons (1.656" deck height) came up short. Leaving the piston below the surface of the engine block like this impedes the combustion process, today's technology dictates that the piston crown be flush, or near flush as possible with the engine block deck.

I don't have the original piston below deck figures for the 1622cc engine block. However, we can calculate this amount due to the fact that this engine had OEM flat top pistons originally fitted, part# 12H438 with 3 comp rings and 1 oil control ring. AE part# 16217 piston replacement had 3 compression rings and 2 oil control rings.

For every 0.010" piston below deck measurement we have the following:

OEM Bore size D2" x .7854" x .010" x 16.39 = 1.16 cc

Where D2 = 3" squared

Swept Volume = D2 x .7854" x S x 16.39 = 405.5cc

Where S = 3.5" (stroke)

Therefore 51.5cc is the required clearance volume to achieve 8.9:1.

So, if the OEM piston was flat top, then it stands to reason that 51.5cc minus 46.21cc = 5.29cc must be the volume of the piston below deck. Since we know that for every 010" below deck = 1.16cc volume, we can simply divide 5.29cc by 1.16cc to find this new measurement, which will be 0.045". In those days leaving the piston below deck by a predetermined amount was a common way of arriving at the desired GCR.

Using this 0.045" measurement, we can also calculate the original engine block height as follows:

The 3.21cc we allow for the cylinder head gasket volume is actually from the MGB OEM workshop manual, I don't have any information for the 1622cc engine head gasket, However, line A should be very close.

Using replacement piston AE part# 14985 with piston dish capacity of (7.75cc + or - 0.25cc) will help us in calculating the original cylinder block height. OEM piston part #12H178 with 3 comp rings and 1 oil control ring. AE replacement piston had 3 comp rings and 2 oil control rings.

For every 010" piston below deck measurement we have the following:

OEM Bore size D2 x .7854" x .010" x 16.39 = 1.13cc

Where D = 2.968" squared

Swept Volume = D2 x .7854" x S x 16.39 = 397cc

Where S = 3.500" (stroke)

Clearance Volume = 38.70 cc (Combustion Chamber factory information) 
                                  3.21 cc (Cyl/head gasket volume) 
                                  7.75 cc (Piston cc dish) 
                      Total = 49.66 cc

Required CV (cc) for 8.3:1 = 54cc + 397cc
                                                   54cc

So if we subtract 49.66 from 54cc we have 5.34cc. Since every 0.010" of piston below deck = 1.13cc, we can simply divide 5.34cc by 1.13 then multiply this amount by 0.010" = 0.047". So, we are right on track with the 0.045" we calculated for the 1622cc engine.

Using this 0.047" measurement, we can also calculate the original engine block height as follows:

Here again the 3.21cc we allow for the cylinder head gasket volume was extracted from the MGB OEM workshop manual. Line A should be pretty darn close.

Knowing the AE part# 13598 replacement piston dish capacity (4.85cc + or - 0.25cc), let us see if this 0.045" or .047" also applies to this cylinder block. OEM piston part# 1H1142 with 3 comp rings and 1 oil control ring. AE replacement piston 3 comp rings and 2 oil control rings.

For every 0.010" piston below deck measurement we have the following:

OEM Bore size D2 x 0.7854" x 0.010" x 16.69cc = 1.06 cc

Swept Volume = D2 x 0.7854 x S x 16.39 = 372.4cc
Where D2 = 2.875" squared
               S= 3.500" (stroke)

Clearance Volume = 38.70 cc (Combustion Chamber factory information)
                                 3.21 cc (Cyl/Head gasket volume)
                                 4.85 cc (Piston cc dish)
Total                      = 46.76 cc

Required CV(cc) for 8.3:1 GCR = 51cc + 372.4cc
                                                            51cc

So if we subtract 46.76cc from 51cc we have approx 4.3cc. (very close to the quoted figure) Since every 0.010" of piston below deck = 1.06cc, we can simply divide 4.3cc by 1.06cc then multiply this amount by 0.010" = 0.041".

Using this 0.041" measurement, we can also calculate the original engine block height as follows:

* The 1500cc donor engine block measured out at 8.860". So over this blocks' 40 year plus life cycle it had been decked, probably more than once.

Thanks to AE (Bradford England phone # 011 44 1274 723481 fax # 011 44 1274 308746) tech people for delving through their archives for the following information. Unfortunately, the following is the best they can come up with:

* All weights quoted in grams. NA = Not Available

I have put together the above information so as to do a comparison weight check between the AE and OEM pistons and that of the new 78mm JE piston.

The easiest way to destroy an engine is to raise the GCR to a point where detonation can not be controlled without the use of octane boosters, adding the correct amount each time you fill up.

Cast pistons will not survive very long under detonation conditions, Forged pistons do a much better job of surviving under these conditions. Invest in today's technology, it is not as cheap as yesterday, but then again I don't know of anything that is.

Piston set part# MGA1698cc

Piston Ring set part# 4265/STD or 2M4265/STD or TS4265/STD.

With the piston crown being flush with the cylinder block, it is important that we use the highest quality piston rings available. The environment that the compression rings work within is very hostile and no compromises in any other type of piston ring selection should be considered. Below I have taken the opportunity to explain the contents of each piston ring kit.

Part # 4265/STD. (HASTINGS)

Part # 2M4265/STD (HASTINGS)

Part # TS4265/STD (HASTINGS/TOTAL-SEAL COMBINATION)

While we have stated that there should be no compromises with piston ring selection, we also understand that budget restraints during the engine rebuilding process may well play a part in engine component purchases, Hence the alternative piston ring kits being offered. The obvious question would be; " Is one kit as good as the other"? Therein lies a very difficult question, with so many variables such as: Assembly techniques, piston ring end gaps, Bore finish, piston ring bedding-in procedures even rich fuel mixtures, these can all have an adverse effect on "blow-by" and oil consumption conditions. Personally, I would consider no less than part # 2M4265/STD due to the fact we are now running the compression rings in a more hostile environment than their predecessors, which had the top compression ring .250" below the piston crown.

All piston rings sold are presized in a 3.070" dummy bore, If for any reason the piston ring end gaps do not fall within specifications on your particular engine block, then, you had better have a word with the machine shop that did the reboring job.

Some potential MGA 1500cc owner/engine rebuilders may consider this type of rebuild extravagant, Please remember, we rebuild engines for a living and see many engines damaged by detonation due to higher than normal GCR's and oil ingress as part of the combustion process. Oil as no business in the combustion chamber. A good point to remember is that detonation ("spark knock"), as well as damaging your engine, will considerably accelerate oil consumption.

Cylinders heads and engine blocks by this time have usually been resurfaced and redecked one or more times. Therefore, custom forged pistons with the appropriate cc dish is the only way you are going to be able to control acceptable GCR's period.

WORKSHEET

Fill in the appropriate numbers:

A. Cyl/block mean measurement + 1.073"                  9.___"

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

C.Piston deck height                                                  1.701"

D. Conrod length (center/center)                                6.500"

E.Total of B C & D                                                 9.951"

                        Minus A                                           (9.___")

                        Piston above deck.                            .o__"

This total is the amount that is required to be removed from the piston crown to bring it flush with the engine block deck. When ordering the piston set quote this figure along with your cylinder head combustion chamber cc capacity. Your piston will then be machined the correct amount for 9:1 GCR.

PRICE LIST

PRICES SUBJECT TO CHANGE WITHOUT NOTICE

All other engine components are available under Moss Motors part numbers. Please supply list of required part numbers and FAX to (415) 883-7200. Remember 10% discount on all parts, with further discounts available on high $ orders.