GEMS 4.6L V8 conversion to distributor.

The GEMS/Motronic controlled engines are characterised by having a modified and improved timing cover and oil pump with a single Poly V belt (Serpentine) driving all the ancilliaries. A very compact and improved arrangement but with no provision for a distributor. There are several other improvements but none have any implications for retrofitting into 3.5/3.9 V8 applications. Assuming you have acquired a late GEMS or Motronic based engine and want to convert to carbs or earlier EFI systems, you will need to retrofit the earlier style oil pump and timing cover, pulleys, etc. If you intend to use a 3.5V8 manifold and SU's there are a few things to do here too.

Differences and suggestions

There are however a few important differences and things you need to be aware of before you start.

The block

Much has been said about the quality of the 94mm bore blocks (3.9/4.0/4.6 Litres). They all tend to suffer from porosities in the castings and in particular behind the cylinder liners where the alloy is quite thin, resulting in coolant leaks into the sump or into the combustion chamber. If you are going to rebuild one of these engines do get it pressure checked first as it is very very common for them to leak to some extent. It's not a disaster as it can be repaired by machining out the cast in liners, welding up any holes or cracks and fitting shrink fit liners. If you intend to use the engine harshly at all then it's recommended as shrunk in liners will not leak and are ultimately stronger. I would do it as a matter of course as it will only add around £400 to the cost of a rebuild and it will guarantee that you won't be pulling the engine down in 3 months to fix it again.

The Camshaft

The engine historically had a problems with end float on the camshaft, causing elevated wear in the timing gears. This has been solved by fitting a thrust plate against the front end of the camshaft. The upshot of this is that a standard or performance cam for a 3.5/3.9 will not fit unless it is machined slightly to fit the plate. Note that the cams are modified only to fit the thrust plate used on late 3.9 Litre Discovery's and not the 4.0/4.2/4.6 plate which has a much smaller hole in it. Do not just omit the plate as there are several suppliers who will modify the camshaft if necessary when you order it. Most performance camshafts are manufactured to fit anyway, but 3.5/3.9 standard cams may need modification. The timing gear on the camshaft is not interchangeable so either the standard or heavy duty aftermarket 3.5/3.9 V8 timing gear must be used.

The sump

The oil sump is different at the front to suit the new timing case and must be swapped for an older style such as the MG, P5, P6, SD1/Vitesse or any Range Rover pre 1996. The car sumps tend to have the main volume in the centre rather than to the rear as in the Range Rover type. The standard sumps will need a small amount of panelling to accomodate the increased throw of the 4.6L crankshaft.


The crankshaft nose is about 20mm longer and therefore no earlier pulleys will fit unless a spacer is made or bought or the nose of the crankshaft is machined down and the bolt hole deepened.

Crank position sensor

The web at the back of the block has a provision for a timing sensor mounted on a plate in a large circular aperture on the opposite side from the starter motor. You will need to make or buy a blanking plate for this aperture unless you want the bellhousing to fill with mud and stones.

Oil Pickup The oil pump pickup pipe is entirely different so the earlier type will need to be fitted to the block.


Additionally you will of course need all of the ancilliary (generator, power steering etc) brackets, belts, timing case, water pump, oil pump, sender(s), distributor and pulleys to complete the conversion. Everything bolted on the front of the block basically. One of the bolt holes on the cylinder heads is different to earlier heads, if looking at the front of the lefthead, you will see a pattern of holes used for securing ancilliaries such as the alternator or compressor. One of these has been changed from a UNC 1/2" AF bolt to a UNC 9/16" AF bolt. This can cause confusion when fitting the alloy carrier for the pump or alternator and a larger hole will need to be drilled in the carrier. Although the ancilliaries could be taken from any 3.5/3.9 engine of any age, it is suggested that a gash late type 3.5 (post 82) be used as a low cost donor as the oil pump is different and significantly better on these and later engines although early ones also fit. Bear in mind that the polarity of the key on the oil pump gears where they connect to the distributor keyway is reversed on early engines. You should fit new oil pump gears, pressure relief plunger and spring as well. Note that the later 3.5 heads, blocks and conrods have some trade value even if the bores are wrecked as they are suitable for repair or bigger bore conversions. Usable cranks also have value. It's relatively inexpensive to rebuild a wrecked 3.5 as a 3.9 as the only significant difference is in bore diameter and a block can be relinered for around £400.

Performing the conversion

Cleanliness is imperative. You will also need some space to arrange components in order. Try and clean as much crap off the donor parts as possible and rinse thoroughly in paraffin or similiar to remove any particles. Make sure you have all the bits you need, including a complete bottom end set of gaskets for a 3.5/3.9 engine.

Stripping the bottom end

Turning to the new engine, Drain the oil. You don't need to remove the oil filter. Assuming you have a complete 4.6 engine less the manifolds and GEMS sensors, strip and remove all the front end ancilliary components and put to one side. Remove all the bolts from the sump and put to one side. Remove the timing cover bolts. Remove the nut securing the oil pump pickup pipe to the centre main bearing cap. Lift away the timing cover complete with the pipe. A few GENTLE taps may be required to ease the cover off the dowel pins. Note that the two dowel pins may stay either in the case or in the block or one in each. You will need to check this against the new timing cover and swap them about as necessary. Remove the stud in the centre main bearing cap if your donor engine does not have one (some engines used this for locating a windage tray, it depends which sump you use). It is not funny if it works loose and travels around. If using the Range rover sump pan it must be removed as it will foul the welded in windage tray.

Stripping the top end

Remove the valley end seals and gasket. Remove the rocker covers and undo the rocker shaft bolts, raising each one a little bit until the rocker shaft is free. Now lift out the push rods and followers You will need to lay out the rocker shaft bolts, shafts, push rods and followers in exactly the position they come out of the engine. Note that despite appearances, pushrod ends wear differently in the rockers and must not be swapped with each other or transposed end over end. New followers MUST be fitted with a new cam and the remaining rocker gear may need renewal. See the FAQ - Rebuilding V8 Cylinder Heads, for advice on determining whether this is necessary. Remove the camshaft gear bolt and lift the gears and chain away. Put the bolt back in the end of the camshaft. Remove the two bolts securing the camshaft thrust plate to the front of the block and fully withdraw the camshaft taking care not to damage the cam bearings located in the block. Remember that you will need to fit the 3.9 Litre version of this plate with the new cam as the 4.0/4.6 version is rather different.


This is a good time to check the fit of the sump pan as the engine is easy to turn over at this time. If using a standard range rover pan, you will need to knock down the baffle plate/windage tray welded into the pan by about 5mm in the centre and along its entire length. You will also need to knock down the centre of the shallow front area by about 10mm and curve the area to make sure the rods and counterweights don't strike it as they approach the lowest point. It's useful to put a thin skin of plasticine on the thing and bolt it loosely to the block then rotate the engine. The plasticine will show where the rods and weights are too close. If you can get 60thou (0.060") or 1.5mm clearance or more this is adequate. 5mm is plenty. Do not include the sump gasket thickness as this will compress by an unknown amount. Don't fit the sump pan just yet.

CamshaftFit the new camshaft and the thrust plate now, using cam lube on the lobes and LM grease on the bearings. Grease the back of the thrust plate before fitting and use Loctite on the two securing bolts. Rotate the crankshaft until piston 1 (front right) is exactly at TDC and fit the chain and gears with the arrow on the cam gear at 6 o'clock. Fit the woodruff key, spacer and distributor timing gear. Torque the bolt correctly. Oil the chain and gears copiously or use a spray grease. Lubricate the followers and pushrods and refit with a dab of cam lube on the underside of the followers. New followers should be left overnight in engine oil before using.

RockersFit the rocker shafts ensuring that the pushrods locate correctly. Lubricate the bolts very lightly and ensure the bolt holes are clear before fitting the bolts to avoid hydraulic damage to the heads. Gently and progressively tighten the bolts, torque correctly. Check cam follower preload and add shims if necessary. See the Cylinder Head FAQ for this procedure.

Refit Covers

Refit the rocker covers with new gaskets. Fit a new composite valley gasket and rubber end seals. Grease the seals and gasket, do not use silicone.

Oil PumpFit the new oil pump pickup with a small gasket on the block face, noting that the gauze strainer is fitted correctly, is clean and free of damage. Loctite the securing bolts.

Refit Timing Cover

Fit the timing cover complete with new front seal, using grease on the gasket and remember those dowels! Do not use silicone. Torque the bolts progressively, remembering the concealed one beside the oil pump (this may be a stud on the donor which will have to be swapped over. If the water pump is already fitted to the timing case, do the long bolts through the pump housing as well.

Refit Sump

Fit the modified sump pan with a new gasket using a small amount of silicone on the joint faces. Torque the bolts progressively and alternately round the sump pan to avoid distortion.

Timing Pickup

Remove the timing pickup plate in the right hand rear of the block and fit the blanking plug using silicone adhesive to bond it in place. This usually lightly welded in place. You will need to grind away the welds to remove the pickup plate and correctly locate the blanking plug.

Oil Pump

Replace the plunger and spring in the oil pump body with new lubricated parts and fit a new copper washer to the retaining plug. Oil the timing case bore of the driven oil pump gear and the gear shaft. Fit the oil pump using a new greased gasket, new gears and pack the housing and orifices with Vaseline or Petroleum Jelly.

No other lubricant will do. Torque the bolts correctly. Fill the oil filter with 20/50 oil and fit it hand tight. Lubricate and fit the crank Woodruff Key, front pulley, spacer, washer and bolt. Torque correctly.


Fit the distributor with the vacuum advance mechanism at the 8/9 o'clock position and so that the rotor arm points directly to the front of the engine. You may need to rotate the oil pump shaft to get it right. I use a dowel with a slot in the end to turn it but it can be rotated by pushing the key on the end round with a long screwdriver with difficulty. The skew timing gear causes the rotor to turn as the distributor is pushed home. This can cause aggravation!


Refit the brackets and ancillary equipment. If the water pump is being refitted now, use a small amount of silicone on the faces before fitting the gasket. Tighten the large bolts correctly first then the small ones.

Rear and crank

Depending on what you got the engine from and what you gearbox you intend to use, you may need to make some changes to the rear end of the crank. Sometimes a dowel pin is located in the output flange. This may or may not be needed. It is OK to grind it flush to the rear face of the crank unless you can easily extract it. If you need to remove it, you will usually need to drill and tap it then force it out by winding a bolt and ball bearing into it. You may also need to fit change or remove the spigot bearing in the rear drive flange. This may be a small phosphor bronze or steel bushing pressed into the recess in the flange. Sometimes they can be hooked out quite easily, sometimes they will need to be cut with a dremel before they slide out. You may be able to devise a small tool to wind it out. There are 3 or 4 different size bushes depending on the gearbox and they all press in very easily if the recess and bush are clean and lightly lubricated. Normally they are pressed in until flush with raised rim of the recess.

Inlet Manifold

The standard non efi manifold with properly prepared SU's is a good piece of kit and has the added advantage of working correctly at odd angles due to the float chamber design. There are a number of mods which can help improve the performance of these, especially on a 4.6 engine where considerably more flow is required.


First off, the carbs should be overhauled properly with new spindles, seals, metering needles and tubes and float needles and seats. The metering needles will need to be something like BBW or even more rather than the standard BAF, BAK, BGC or whatever. The engine will run lean otherwise.


The standard airbox arrangement with the curved alloy ducts can be made to give more bottom end torque than K+N's bolted directly onto the carb intakes and if ducted to pick up cool air, will also result in more power. The standard squashed oval box with an intake in the middle and an outlet at each end attached to two curved aluminium ducts is a pile of crap as is the T pipe used on the 101FC. The rounded box used on SD1 is better and can be improved more. If there is not enough room for this box discard the air filters in the squashed type and duct it to a separate filter or put a large K+N on the inlet pipe. The round type is not bad and K+N filters are available for it, personally I would omit them and apply an external filter. If this is done, an internal baffle should be inserted in the middle to divide the intake air from the entry stub and stop the carbs "seeing" each other. A half moon shape or a rectangular plate across the middle will do.

Allow elbow

sThe casting of the curved alloy pipes joining the carbs and the airbox are often very messy internally. Clean them up so that the entry to the carb is tidy and smooth. Blank off any breather pipe attachments and ensure that the 'o' rings and rubber seals fit properly and are still pliable. A little vaseline will assist in fitting. The adapters on the front of the SU can be modified to give a better bellmouth shape to clean up the flow into the carburettors. Round the edge of the entry where the filter or curved intake pipe attaches as well.

SU mods

The throttle plate has an overrun bypass valve attached on a spring. Remove it, remove the stem from the valve seat and solder the valve seat onto the plate, with the seat facing upstream, ie towards the air filter side of the carb. Loctite the brass screws securing the plate to the spindle and trim the exposed thread. Ensure that the piston carrying the needle can move fully up into the housing, thus presenting no restriction to airflow on full throttle. The damper should contain 20/50 or EP90. Thinner oils such as ATF may result in hesitation on throttle opening.

SU Manifold mods

The manifold has plenty of scope for porting and can be bored to 2 inches where the carb attaches. If this is done the outlet of the carb will need careful matching, along with the paxolin spacer between the carb and manifold. Discard the swirl device anyway. If the manifold entries are opened to 2" there is scope for opening out the thing as far as the internal runners because of the easier access. Smoothing this area and getting a gentle radius on all curves will help. The port runners can be opened to the limits of the valley gasket and opened and smoothed as far in as possible. This should give quite a bit more flow overall and the increased volume should match it better to the 4.6L engine. It may perform as well or better than a 4 barrel carb at low to mid range, with it's limitations only showing from 3500 revs upwards but still with power possibly in excess of 200BHP with a mild performance camshaft and excellent low end and midrange torque in the region of 260ftlbs. It is simple and preserves the original appearance and standard fit. It is possible to get up to 230BHP at 5500rpm from this arrangement with a suitable cam but probably only on the 3.9V8 rather than the 4.6V8 where the extra low end torque is more useful than a lot of top end. Make sure the thin steel water outlet on the front face of the riser on the manifold is fully open. It's designed to allow air bubbles to pass out of the system and should be clear enough to blow through easily. Also make sure the balance hole between each inlet through the riser is clear. Use an 82 degree thermostat.

Starting and Running In

When the manifold is fitted and the engine is fitted in the vehicle, fill it with 20/50 oil and at least 25% antifreeze with water. Treat it as a new engine for at least 500 miles to let the rocker gear and camshaft settle. That means no revs above 2500rpm. Change the oil and filter then raise rpm gradually for short bursts over the next 500 miles. Do not do long journeys at constant speed, keep the revs as varied as possible. Look out for oil and water leaks. When the engine is first started, do not let it idle but raise the tickover to 1200rpm to ensure oil splash and sufficient pressure. Check thoroughly for leaks during warm up.

Summary of Parts Required

You will need the following new or reconditioned 3.5/3.9/4.0 V8 parts:

Other parts

You will need these special parts made or supplied:


You will need the following tools:


Suitable mild performance cams for all the Rover V8 engines include

All these cams are suitable for auto gearboxes.