If you are contemplating rebuilding a Lotus twin cam engine with standard components then this is going to be a rare event.
Fifty years after the first twin cam engine was first built, the fact is that most of them have experienced a hard life.
In reality, the majority of engines are now dimensionally different from when first leaving the Lotus Factory.
It is useful to make some key measurements prior to completely dismantling any engine.
It is imperative to keep the old worn out parts so they can be examined for wear and any inherent problems.
So if you are contemplating ordering replacement pistons, then its wise to check some key dimensions that directly affect any successful engine rebuild.
The cylinder block height should checked to establish if material has been removed from the top face.
Failing to do this where a cylinder block has been repeatedly machined may result in a standard compression height piston protruding above the top face.
Core-shift and internal cylinder block corrosion significantly influence whether or not a block can be successfully re-bored.
This can only be be determined by subjecting a block to ultra-sonic testing and checking the thickness of the cylinder walls.
The crankshaft should be cracked tested (preferably by magnetic particle crack testing equipment) to determine serviceability.
The crankshaft should also checked for straightness and for the actual stroke.
From experience, crankshafts have been seen to have been changed or off-set ground to increase the stroke.
The pistons and rings and rings should be examined to establish how they have worn during service.
The piston crowns and valve cut-outs should be looked at to see if there are signs of piston to valve contact.
If there has been contact then a slight outline of the valve maybe seen in the bottom of the valve cut-out.
This suggests the engine rev limit may have been exceeded or the valve springs are weak (promotes valve bounce).
The standard Lotus twin cam valve spring assembly has a seat pressure of 50 - 55lbs when installed at a height of 1.250" (31.75mm)
There are several publications to assist in the diagnosis of these types of engine problems including a series of publications from ACL
Engine bearings Technical Handbook - Published by ACL Bearing Company
Pistons and Piston Rings Technical Handbook - Published by ACL Engine Parts
ACL Engine Manual (Gregory's 413) - ISBN : 08556666803
NOTE CAREFULLY - IT IS THE RESPONSIBILITY OF THE ENGINE BUILDER to make the necessary measurements and checks before ordering new pistons or any other components.
Therefore it is important to cross-check the actual component dimensions against the original twin cam engine dimensions stated in the Lotus workshop manual.
Standard dimensions stated in the Lotus Elan +2 manual (part number X050T0327ZD) are as follows-
The con-rod centre to centre dimension is 4.799" to 4.804" (121.89mm to 122.02mm)
Piston compression height is 1.536" to 1.538" (39.014mm to 39.065mm)
Crankshaft stroke is 2.863" (72.74mm)
With a standard height block, the piston will usually sit down the bore by approximately 0.020" at TDC
To optimise squish, it is desirable to have the piston sitting flush with the cylinder block at TDC
To achieve this, there are after-market forged pistons that are machined to have a compression height of 1.543" (39.20mm)
This provides scope for the top of the cylinder block to be machined to clean up any severe corrosion marks
So with this in mind, a replacement piston to suit a lotus twin cam engine with a 72.74mm stroke crankshaft could be as follows-
Forged alloy pistons set to suit 3.267" (83.0mm) bore
Piston compression height 1.543" (39.20mm)
High quality forged pistons usually have generous valve cut-outs to accommodated larger valves and increased camshaft lift.
Modern-day piston ring sets supplied by companies such as Total Seal Rings, offer a narrow contact surface area that reduces friction between the ring and bore wall (results in an increase in power).
Piston manufacturers will usually supply piston rings, gudgeon pins, locking rings to complete the piston set.
In addition to these components, an installation guide is supplied so the cylinder block can be machined to the correct size.
It is also desirable to check the cylinder head combustion chamber volume with an Grade-A burette that has a 50cc capacity.
The combustion chamber volume will be influenced by the following -
1) the amount of material that has been removed from the cylinder head face.
2) the size of the valves and resultant position when installed in the combustion chamber.
3) the extent of of valve seat recession (dependent on both valve and seat wear)
After carrying out this exercise, it is important to determine the static compression ratio.
If the engine is to be rebuilt for road going applications and 98 RON fuel is going to be used, then a safe maximum compression ratio of 10.5:1 is recommended.
Foot-note
Failing to carry out critical measurements to determine the correct piston compression height will compromise the engine assembly and how it performs.
Recently an example of poor assembly techniques been high-lighted by one of our Melbourne competitors purchasing excessively tall pistons then machining the tops off to suit the installation.
This approach has a total disregard to piston crown thickness and subsequent reduction in strength, holding and machining the piston so the crown it's square to the bore (then having to re-balance all four pistons).
Any piston profile is NOT a perfect cylinder and in reality they are barrel shaped in the vertical axis and oval in the horizontal axis.
A special fixture that holds the piston by the gudgeon pin is required to accurately machine the crown detail.
Copyright © 2016 Lotus Marques
Fifty years after the first twin cam engine was first built, the fact is that most of them have experienced a hard life.
In reality, the majority of engines are now dimensionally different from when first leaving the Lotus Factory.
It is useful to make some key measurements prior to completely dismantling any engine.
It is imperative to keep the old worn out parts so they can be examined for wear and any inherent problems.
So if you are contemplating ordering replacement pistons, then its wise to check some key dimensions that directly affect any successful engine rebuild.
The cylinder block height should checked to establish if material has been removed from the top face.
Failing to do this where a cylinder block has been repeatedly machined may result in a standard compression height piston protruding above the top face.
Core-shift and internal cylinder block corrosion significantly influence whether or not a block can be successfully re-bored.
This can only be be determined by subjecting a block to ultra-sonic testing and checking the thickness of the cylinder walls.
The crankshaft should be cracked tested (preferably by magnetic particle crack testing equipment) to determine serviceability.
The crankshaft should also checked for straightness and for the actual stroke.
From experience, crankshafts have been seen to have been changed or off-set ground to increase the stroke.
The pistons and rings and rings should be examined to establish how they have worn during service.
The piston crowns and valve cut-outs should be looked at to see if there are signs of piston to valve contact.
If there has been contact then a slight outline of the valve maybe seen in the bottom of the valve cut-out.
This suggests the engine rev limit may have been exceeded or the valve springs are weak (promotes valve bounce).
The standard Lotus twin cam valve spring assembly has a seat pressure of 50 - 55lbs when installed at a height of 1.250" (31.75mm)
There are several publications to assist in the diagnosis of these types of engine problems including a series of publications from ACL
Engine bearings Technical Handbook - Published by ACL Bearing Company
Pistons and Piston Rings Technical Handbook - Published by ACL Engine Parts
ACL Engine Manual (Gregory's 413) - ISBN : 08556666803
NOTE CAREFULLY - IT IS THE RESPONSIBILITY OF THE ENGINE BUILDER to make the necessary measurements and checks before ordering new pistons or any other components.
Therefore it is important to cross-check the actual component dimensions against the original twin cam engine dimensions stated in the Lotus workshop manual.
Standard dimensions stated in the Lotus Elan +2 manual (part number X050T0327ZD) are as follows-
The con-rod centre to centre dimension is 4.799" to 4.804" (121.89mm to 122.02mm)
Piston compression height is 1.536" to 1.538" (39.014mm to 39.065mm)
Crankshaft stroke is 2.863" (72.74mm)
With a standard height block, the piston will usually sit down the bore by approximately 0.020" at TDC
To optimise squish, it is desirable to have the piston sitting flush with the cylinder block at TDC
To achieve this, there are after-market forged pistons that are machined to have a compression height of 1.543" (39.20mm)
This provides scope for the top of the cylinder block to be machined to clean up any severe corrosion marks
So with this in mind, a replacement piston to suit a lotus twin cam engine with a 72.74mm stroke crankshaft could be as follows-
Forged alloy pistons set to suit 3.267" (83.0mm) bore
Piston compression height 1.543" (39.20mm)
High quality forged pistons usually have generous valve cut-outs to accommodated larger valves and increased camshaft lift.
Modern-day piston ring sets supplied by companies such as Total Seal Rings, offer a narrow contact surface area that reduces friction between the ring and bore wall (results in an increase in power).
Piston manufacturers will usually supply piston rings, gudgeon pins, locking rings to complete the piston set.
In addition to these components, an installation guide is supplied so the cylinder block can be machined to the correct size.
It is also desirable to check the cylinder head combustion chamber volume with an Grade-A burette that has a 50cc capacity.
The combustion chamber volume will be influenced by the following -
1) the amount of material that has been removed from the cylinder head face.
2) the size of the valves and resultant position when installed in the combustion chamber.
3) the extent of of valve seat recession (dependent on both valve and seat wear)
After carrying out this exercise, it is important to determine the static compression ratio.
If the engine is to be rebuilt for road going applications and 98 RON fuel is going to be used, then a safe maximum compression ratio of 10.5:1 is recommended.
Foot-note
Failing to carry out critical measurements to determine the correct piston compression height will compromise the engine assembly and how it performs.
Recently an example of poor assembly techniques been high-lighted by one of our Melbourne competitors purchasing excessively tall pistons then machining the tops off to suit the installation.
This approach has a total disregard to piston crown thickness and subsequent reduction in strength, holding and machining the piston so the crown it's square to the bore (then having to re-balance all four pistons).
Any piston profile is NOT a perfect cylinder and in reality they are barrel shaped in the vertical axis and oval in the horizontal axis.
A special fixture that holds the piston by the gudgeon pin is required to accurately machine the crown detail.
Copyright © 2016 Lotus Marques