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Torquing Technical - Lotus 1558cc Twin Cam Rebuild

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Torquing Technical - Lotus 1558cc Twin Cam Rebuild


Martin Lucas

The Lotus 1558 Twin Cam engine was fitted to many Lotus, and non Lotus cars. The engine was phased out in approximately 1975. Unfortunately these engines are no longer in their prime, and being of a "sporting" configuration most have not led a sedate life. At some point you may need to contemplate a rebuild.

The basic engine is very robust if not abused. The engine can be broken down into two areas (or more if you throw a rod), the cylinder head assembly, and the cylinder block assembly (bottom end). The cylinder head will last if looked after. All cylinder head parts are still available. The main points to watch are - corrosion around the waterways, warpage, and severe cracking of the spark plug holes. It appears that minor cracking is very common in this area and does not affect performance. All due care should be taken during strip down and re-assembly. Follow the Lotus manual.

The bottom end is another story, bearings, rods, pistons, etc are still available. Due to the block and crank etc. being courtesy of the Ford Cortina these parts are now no longer available new. However alternative solutions are available to Lotus owners. The preferred option may well vary for each individual, their decision may be based on cost, originality, gain in horse power desired, and/or the individuals mechanical skill.

From recent experience it appears that the weak link in the standard Lotus engine is the nodular iron crank shaft. It is very important to have this component crack tested during the rebuild process. These are getting old, and the repercussions to a newly rebuilt engine are disastrous should a crankshaft break because a cracked item was reused. Save yourself the possible future stress - get it crack tested.

Once the various parts have been measured and checked you will now be faced with a decision, depending on the results. If the crank shaft passed the crack test then it can be re-used (as long as it can be reground undersize if necessary). If the crank failed then you have some soul searching to do. Wreckers may have a standard 1500 Cortina crank, but don't count your chickens. Other options are: buying a steel crank (very expensive), using a 1600 crossflow block and crank assembly, or modifying an alternative crankshaft to suit.

If the block and pistons measure OK then you're lucky, just slap it back together. If however the pistons are worn these can be replaced. Should the block be worn then the decision process can be dependent on the amount of wear. Should it be small, oversize pistons may suffice. If excessive you may be faced with the decision of sleeving the block (may be necessary if originality is a prime concern), or a replacement from a wreckers yard may need to be sourced. Lastly you can do away with the Lotus 1500cc "L" block and use a 1600 Ford cross flow (if you do this a 5/16" spacer will need to be fitted between the head and the front cover).

Basically the above scenarios result in roughly three options:

  1. Rebuild the engine using standard or original components.
  2. Rebuild the motor using a 1600 crossflow for the bottom end.
  3. Rebuild the motor using a modified alternative crank.

Note: According to Miles Wilkins approximately 34,000 engines were produced. Production changes were listed as follows-

  • L-type rods disappeared, replaced by c-type in late 1963/early 1964.
  • Tappet sleeves fitted from mid 1964 at engine number 1576.
  • Four bolt crankshaft and A-type pistons used up to engine number 7799.
  • Six-bolt crankshaft, revised rear seal and c-type pistons, used from mid 1967, from engine number 7800.
  • Exhaust valve material changed, requiring different tappet clearances (0.009-0.011 in.), during 1967 from engine number 9952.
  • Spigot bearing changed to needle roller type, 1969, from engine number 18,500.
  • Non-shouldered head bolts (stronger than original type) introduced in 1969 from engine number 18,820.
  • Renewable oil-filter canister on new oil pump introduced in late 1970, from engine number 23,607.
  • Apart from gasket sealant and material specifications, no other running changes were made. The two main changes were the introduction of the Stromberg head, in March 1969, and the Big Valve head in October 1970.

Scenario One. Every Thing Checks Out Okay.

For this scenario it is assumed that on disassembly there were no surprises. Or if there were you were fortunate enough to have either a spare lying around the garage, or a salvageable replacement was sourced. The engine rebuild typically would be as follows. It always pay to try and read up on what you intend to do. The club has an extensive library. In particular Miles Wilkins book on the Lotus Twin Cam is extremely helpful as it lists all torque settings, limits, fits etc.

CYLINDER HEAD

Carefully remove and strip down. The inlet camshaft and sprocket are marked as is the exhaust, do not to swap the two. A wider than standard valve spring compressor will be required due to the width of the head casting. Its no race so take your time. Remove the tappets and shims keeping them in their respective order so that on reassembly they can be returned to the sleeve and valve location where they originally came from. Remove all valves (don't loose the spring seats they tend to blend in with the alloy casting when the oil is dirty).

Do all the normal checks, Visually assess it, does it look ok? Check the valves are any bent? (I have rebuilt two heads and each one had a bent inlet valve). What condition are the valves, tappets, springs, seats tappets, cams etc in. The manual will have the acceptable sizes. If you can not check these all good machinists/mechanics can do it for you. If you need any machining work undertaken on your engine it pays to select those that have had a lot of experience with these engines. It is always better to pay a few more dollars knowing that it will be done properly than go for an unknown. If during the machining an "accident" occurs a cheap price becomes irrelevant.

Check valve stem to guide fit. If excessive these can be "K" Lined. this is about ten dollars a guide and works out a lot cheaper than fitting new guides. Valves and valve seats should be relapped. Check the wear surfaces of the tappets (even the step which comes in contact with the shim, the cam contact surface can be "lapped" if slightly dished using 600 grit wet & dry. using a flat surface (Glass is good) put the wet & dry on top, use some kerosene as a lubricant and run the tappets flat face in a figure eight across the wet & dry. Turn the tappet frequently. Replace the tappet if it can not be salvaged or is questionable. Next comes the fun part of reassembly and setting the valve clearances. Use the club shim exchange, this will make life easier. One point to be aware of here is make sure all the valves open when the cams are rotated. This sounds obvious but in one instance I found that one exhaust valve would not fully open. It ended up that the spring retainer was hitting the top of the valve guide. The retainer and valve guide was machined to provide adequate clearance. This was interesting considering that the engine had done 90,000 km. like this. Reassemble the cylinder head as per the Lotus manual.

CYLINDER BLOCK

Check the overall condition again. Measure the bores, pistons, bearings, crank etc. for wear. Replace all worn parts. As mentioned earlier get the crank as well as the rods crack tested. We will assume every thing has checked out ok. The crank may need to be ground. Make sure that the crank grinder does not destroy the fillet radii. Correct fillet radii are vital to maintain the cranks strength and fatigue resistance.

It is wise to replace the chain, chain wear pad and water pump assemblies whilst the engine is stripped. You certainly would not want to reuse the original items only to discovery shortly after the rebuild that one or more of these reused items has failed. The oil pump may also need replacing. Beware, this is more than likely a high volume flow unit, different to the standard Ford item. Cost being one difference. Don't go for an inferior item here, if the pump fails so will the engine and a seized engine is not a pretty thought. (Mind you parts sales people seem to like them. Ed.)

On reassembly make sure every thing is clean. If you have access to a cleaning bath scrub the block etc. clean. If you do not have access to cleaning equipment most engine machinists will 'tank dip' parts at a minimal cost. Follow the manual on reassembly. All torque figures are for nuts and bolts well lubricated with oil on both the thread and the underside of the head. The actual bolt or stud tension will vary markedly compared to being torqued down dry as opposed to oiled. Use a ring compressor to install the pistons and rings. Don't forget to check the ring gaps. Install the pistons as marked as the piston cut-outs correspond to the inlet and exhaust valves. The deck height should be between 0.005 - 0.010".

Use a reputable brand of sealant. Do not use too much. Some sealants will ooze inside where it can not be seen. Set hard , only to fall off at a later stage, possibly blocking off an oil way. Do not reuse gaskets, this is a false economy as the overall cost for the correct gasket is minimal compared to the cost of the whole project. Reused gaskets will not seal or protect against oil/water leaks very well.

Scenario Two: Rebuild Using a 1600 Crossflow Bottom End.

Unfortunately Lotus/Ford "L" blocks are no longer available through the Ford or Lotus spare parts network. This item has been obsoleted long ago. Because of this the following guide will outline a practical alternative path that more and more Lotus owners are trekking down to resolve the main problem of the "L" block now being obsolete.

Luckily for us in one of those planning meetings many years ago a smart Ford engineer must have argued his case for standardisation very successfully. I say this because the interchangeability of Ford England parts is astounding. Within reason engines and gearboxes from the Ford Anglia to the Ford Escort range can be made to fit together. I am not recommending you check this out by fitting an Anglia engine and gearbox into your Escort or Cortina. However it certainly has the major advantage that should it need to be done, a Lotus Twin Cam with a written off bottom end can be resurrected using the major components from the Ford Kent 1600 Crossflow engine.

There are reasons as to why someone may decide to do this:

  • The original Lotus/ Ford "L" block may be destroyed beyond repair. This has been known to happen when an engine is over revved, and a con rod breaks. This tends to have the tell tale sign of the sudden appearance of a very large hole in the side of the block.
  • The owner may want to have a bit more power. This conversion is a cheaper option to extract more power.
  • The original block may have had a substandard repair carried out that has deemed its use unwise.

A variety of engine capacities can be achieved by mix and matching components. As can be seen from the following table.

Engine Type Capacity Bore Stroke Con Rod Length
Lotus Ford Twin Cam (standard pistons) 1558cc

82.550 mm

(3.250 in)

72.746 mm

(2.864 in)

121.922 mm

(4.800 in)

Lotus Ford Twin Cam

(oversize pistons)

1651cc

85.000 mm

(3.346 in)

72.746 mm

(2.864 in)

121.922 mm

(4.800 in)

Ford Kent Crossflow

(standard pistons)

1601cc

80.980 mm

(3.188 in)

77.62 mm

(3.056 in)

125.171 mm

(4.928 in)

Lotus Twin Cam / Kent (standard pistons) 1661cc

82.550 mm

(3.250 in)

77.62 mm

(3.056 in)

125.171 mm

(4.928 in)

Lotus Twin Cam / Kent (oversize pistons) 1761cc

85.000 mm

(3.346 in)

77.62 mm

(3.056 in)

125.171 mm

(4.928 in)

Once the decision has been made to utilise a Crossflow block the implementation is relatively straight forward. There are several ways to go.

The first option is to utilise as many of the Ford Crossflow items. Because the Kent 1600 Crossflow engine is of a "Heron" design the first thing you will notice is that the pistons are different. Don't worry about this as they will not be used. With both blocks stripped it will be seen that the Crossflow block is about half an inch higher (12.7mm for those that are metric conversant). The four engine mounting bosses on the exhaust side are 1/2" longer, and if your Lotus engine is an early 'four bolt crank' then you will have the added bonus of gaining the six bolt crank and lip seal instead of rope seal rear housing. Should this be the case then you will either have to source, or make a new flywheel (the Lotus clutch is larger than the Crossflow item), new gearbox input shaft bearing, and sump.

All you do is use the standard 711M block, crank and rods. You can go for either a special after market piston that brings the piston to within 0.010 to 0.020 inch from the top of the block face. The piston can be sized to either the original Crossflow bore of 3.188" or enlarged to the Lotus 3.250". The Lotus bore with the Crossflow crank will result in a capacity of 1660cc as the crossflow crankshaft has 3.056" stroke compared to the Lotus 2.860". Obviously a torque and power increase will be realised. However since the engine's internals have been changed, bore to stroke ratio, con rod length to stroke ratio, the engines running characteristics will also change. The engine outlined here would not be as "revvy" but it would develop more torque. By using a suitable after market piston that results in the correct deck height no machining of the block face would be necessary. The same engine can be rebuilt using pistons from a BDA. The main point to note is that the BDA pistons have cut outs for four valves. These pistons would have to be suitably modified to the two valve cut out detail as per the Twin Cam pistons.

An important note to remember when using a 1600 Crossflow crank is that the timing chain sprocket has a different offset compared to the Twin Cams. If the original Twin Cam sprocket is in good condition this can be removed from the Twin Cam crank and replaced on the Crossflow crank. If this is not possible, and a replacement can not be sourced, then a suitably sized spacer can be made to give the correct offset. This spacer is fitted between the shoulder on number one crank bearing and the sprocket itself.

In a similar way the above motor can be built still utilising the Crossflow block, rods, and crank. Standard or oversized Lotus Twin Cam pistons can be used. Standard Lotus 82.55 diameter pistons will still give a capacity of 1661cc, however if oversized 85.00mm diameter pistons are used then an engine capacity of 1761 can be achieved. If going for the larger pistons beware of porosity or "weeping" bores, get the water ways pressure tested to see if anything comes through to the bores. Now that Lotus pistons are being used on reassembly you will discover that the pistons will fall short of the ideal 0.010" to 0.020" deck height. This is no problem as all you do is establish the actual deck height and subtract 0.010". The remainder is what you will get machined off. This should be in the region of approximately 0.200".

The key to remember with both of these engines is that a higher than standard block height will result. So a new, longer timing chain will be necessary. This will be either a 121 or 122 link chain. Only use a continuous chain, chains with a removable link are not as strong and may fail. A spacer plate will also need to be made and fitted between the top of the front timing cover and the cylinder head. If you do not install a spacer expect one very large oil leak.

Lastly you can machine the Crossflow block to correspond dimensionally to the Lotus Ford 'L' block. This for all intents and purposes will give you a replacement "L" block. If the Ford block is thus machined then a Lotus or 1500 crank and rods will need to be reused. Obviously because if the 1/2" inch is removed from the block deck height, then the pistons will be sit proud of the face if assembled with the Crossflow rods and crank. Even though most people desire an increase in compression ratio, this is going to the extreme.

If an engine is rebuilt using a taller 1600 block then engine mounts and bonnet clearance will need to be checked.

Due to the additional 1/2" on the 1600 Crossflow block exhaust side mounting bosses, the original mounts will not work as they will be spaced out the additional 1/2". This additional 1/2" can be machined down off of the block mounting bosses, certainly a nice & tidy solution. If this is not possible the original engine mounts can be modified or replacements made.

The bonnet clearance is not a problem for Europas, Escorts and Cortinas. However for Elans and Sevens any additional engine height can lead to the cam cover fouling the bonnet. If this is the case then you may have to manufacture some new engine mounts. There have been articles written where "spacing" the body off of the chassis has been suggested. This does seem to be a rather crude solution, and would no doubt lead to body/chassis problems latter on. For this reason new or modified engine mounts seem far more sensible at what must surely be no additional work in comparison.

Scenario Three: Rebuild Using a Modified Datsun Crankshaft.

Scenario Three, the engine rebuild from hell. The most likely reasons why a Datsun crankshaft would be used instead of the original Lotus cast iron crank are:

  • The original cast iron crankshaft is cracked, and a suitable replacement can not be found.
  • The Datsun crank is steel, utilising this crank results in the obvious advantage of the increased strength of a steel crank versus cast iron. This conversion is alot cheaper than a 'genuine' steel Twin Cam crank from Lotus.

Whatever the reason for using a Datsun crank the procedure is the same. For this scenario it is assumed that the conversion is necessary because the original crank shaft was cracked and a suitable replacement could not be found. It is also assumed that the engine is fully stripped down.

Once it has been determined that a Datsun Crank is to be used, one of the first steps is to locate a suitable one. For a Lotus Twin Cam 1558cc, 'L' block, a Datsun 1600 crank shaft is required. Although not confirmed, the Datsun 1800 crank maybe suitable for the longer stroke of the Ford Kent 1600 engines. Your budget and access to resources/machinery will influence the final cost for the Datsun crankshaft modification. A point to remember here is of course the more you can do your self, the less the final bill will be. The simplest way to get the crank shaft is to ring wreckers, use the yellow pages and start at the first ad. There will be two prices for the crank, depending on who removes it. You can save money by removing the crank yourself. As a rough guide $60 - $100 if you remove the crank, $120 - $150 if the wreckers do it.

Datsun Crank Modifications.

Now you have your Datsun crank lay it along side the Lotus crank, here you will see what needs to be done. Access to a suitable Lathe will help save costs. If not find a friend who does, otherwise be prepared to pay up to $40/hour for a machinist to do the necessary work. The front nose for the spigot and sprocket is longer and of a larger diameter, this needs to be turned down as per the Lotus. Don't forget to drill and tap the nose for the Imperial bolt that holds the pulley on, or mill the keyway to locate the chain sprocket. Next step is to machine down all the counter-balances. The crank to block clearances are different for the Lotus block, so not only will the diameter need to be changed but also some counter-balances need to have their respective corners chamfered to avoid block casting protrusions. Do not be concerned about where the big end journals are relative to the Lotus crank. There is sufficient side ways clearance for the Lotus conrod to move along the Piston gudgeon pin. The mains are another story, these need to be widened, use the middle main (number three) as your datum. The rear flange (where the flywheel is bolted to may also require machining. Choices are make a phosphor bronze spigot bush, or machine the crank to accept a Ford needle roller spigot bearing. Also the Datsun rear flywheel mounting flange is slightly longer than the Lotus. Again options are to machine it back as per the Lotus, or since a new flywheel is required machine the required offset into the new flywheel. Having only machined one crankshaft, after work, learning as I went, the above took about twenty hours. Should I do another I am confident it could be done in eight hours. An experienced machinist should be able to do it in a similar time.

Now the crank is turned down it can be ground to size. Be aware that this will not be cheap as a lot of material must be ground away. Take the original crank along as well so bearing sizes and fillet radii can be checked and reproduced. It is recommended to get the crank ground to standard size and use new standard size mains and big end bearings. Thus as the crank wears undersized bearings can be used, extending the life of the Datsun Crank Shaft. When the Datsun crank is finished check its fit and rotational clearance in the block. If all is okay it can now be balanced.

Flywheel

A new flywheel is needed as the Datsun crank has a five bolt flywheel mounting pattern, whereas the Lotus has a six bolt pattern.

For the slight increase in material cost it is well worth while specifying a medium tensile steel (1040), or if you want extreme strength, a high tensile such as 4340 would be more than adequate. Mild steel may have the strength but a tensile steel gives a safety factor as well as the bonus of producing better surface finish when machined. This is what you want of course for the clutch face.

The dimensions for the flywheel are the same as the original, changes are required if the crank rear flange has not been machined down, don't forget the five bolt pattern. The starter ring gear is open to choice, since tensile steels can be hardened the new flywheel can be made with teeth machined in to it, or a groove as per the original can be reproduced, and a Lotus Ring Gear heat shrunk on. It would make sense to change the clutch cover mounting bolts from 5/16" UNC to 5/16" UNF. Since tensile steel has a greater strength than cast iron it can be drilled and tapped to suit a fine thread. A UNF bolt will be slightly stronger than a UNC as its thread is not as coarse (deep). This allows a greater hold down force to be obtained form the same tightening torque setting.

A final point to note is that steel has a slightly greater density than cast iron. Put simply for the same shape/size a steel part will weigh more than a cast iron one. If you make the steel flywheel identical to the cast iron version then it will be heavier. A heavier flywheel will reduce an engines ability to accelerate (rev). All is not lost though. Because you specified a tensile steel it is safe to have it lightened. The main point to remember here is that when lightening flywheels the most important weight to get rid of is the mass furthest away from the centre, as it is this material that affects the Rotational Moment of Inertia. In other words two flywheels each 5Kgs. One has 4Kgs on the outer circumference, this will be harder for an engine to spin compared to the other 5Kg flywheel with 4Kgs on the centre. And don't get too carried away with lightening the centre, this has to keep everything together.

You should expect to pay around $60 - $80 for the steel, if you can not machine the flywheel yourself then a machinist may charge in the region of $250 - $300.

When you have all the parts finished and in your garage, don't forget a trial assembly. Assemble with pistons and rods, again check every thing turns over, and piston to deck height is satisfactory. The pistons should be .005 to .010" under the block face (or more considering Unleaded 96). The pistons tops may need to be machined if there is insufficient deck height. don't forget piston cutouts for the valves, and a trial assembly (with gasket) should give a minimum valve-piston clearance of 2.5mm (0.10 inch). Use plasticene to check.

Approximate Costs. (NZ $)

Description DIY Outside Vendor
Datsun Crank 60.00 120.00
Machine modifications to crank 0.00 400.00
Crank Grinding 250.00 250.00
Steel for Flywheel 80.00 80.00
Flywheel Complete Machined 0.00 250.00
Totals 390.00 1,100.00

Either way the above still works out cheaper than a genuine steel crank supplied from overseas (can cost as much as $3,000.00).