Lotus Elan

Crank differences

PostPost by: Tom Rae » Thu Sep 26, 2019 7:40 pm

I have acquired a twin cam engine - in boxes and I am trying to work out just what I have! I have a 2731 crank which certainly looks in good order. I have read Bryan Moorcrofts useful article on Lotus Cortina engines and he makes reference to the Lotus crank. Checking the crank dimensions in Miles Wilkins Lotus Twin Cam book and a Ford Cortina workshop manual, the dimensions for the 2731 crank and the Lotus crank look the same? I am obviously missing something but can you tell me what the difference is?

Tom
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PostPost by: promotor » Thu Sep 26, 2019 9:29 pm

The 2731 crank is a 6-bolt crank found in 1500 pre xflow engines in the early mk2 cortinas - it is different to a Lotus 3020e crank in that the 2731 crank has much smaller counterweights.
If using 125e con rods (rather than 116e / early "L" rods) you should really use a 3020e crank as it's likely the counterweights are there to suit the heavier 125e con rods.
I would recommend using 125e rods over the 116e / "L" rods every time so it would therefore follow that you should seek a 3020e crank to suit.
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PostPost by: Tom Rae » Fri Sep 27, 2019 6:48 am

Thank you for your comprehensive reply. I do have the 125e rods but finding a Lotus crank might be difficult? I also have the 2731 block but no caps so I iwill need new caps and therefore line boring. I am only planning to use the engine for road use only so could I get away with using the 2731? I also have the possihility of using a 120e block which has main caps. Thanks again.

Tom
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PostPost by: avro » Fri Sep 27, 2019 2:13 pm

Tom

If you choose to use a Lotus crankshaft with your 2731 or 120E cylinder block or any other block that has not previously been used in a Lotus engine they will require just a small amount of machining to clear the larger counterweights of the Lotus crankshaft at both ends of the block.

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PostPost by: promotor » Fri Sep 27, 2019 2:36 pm

Tom Rae wrote:Thank you for your comprehensive reply. I do have the 125e rods but finding a Lotus crank might be difficult? I also have the 2731 block but no caps so I iwill need new caps and therefore line boring. I am only planning to use the engine for road use only so could I get away with using the 2731? I also have the possihility of using a 120e block which has main caps. Thanks again.

Tom


Tom, if you want a decent quality Lotus crank Nick (who responded below) is your first port of call. One of the best in the Lotus world.

The 120e block would need a modification to the sump baffle if using it with a 6-bolt sump as the 120e block uses a screw-in pickup pipe and they used a different sump. It is IMHO that the screw in pick-up pipe is the better system though as it allows the oil pick-up basket to sit centrally in the sump. The push-in pick-up pipe sits to one side, and is a pain to remove from the block if you wish to do so.

avro wrote:Tom

If you choose to use a Lotus crankshaft with your 2731 or 120E cylinder block or any other block that has not previously been used in a Lotus engine they will require just a small amount of machining to clear the larger counterweights of the Lotus crankshaft at both ends of the block.

Nick
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PostPost by: Tom Rae » Fri Sep 27, 2019 2:52 pm

promotor wrote:
Tom Rae wrote:Thank you for your comprehensive reply. I do have the 125e rods but finding a Lotus crank might be difficult? I also have the 2731 block but no caps so I iwill need new caps and therefore line boring. I am only planning to use the engine for road use only so could I get away with using the 2731? I also have the possihility of using a 120e block which has main caps. Thanks again.

Tom


Tom, if you want a decent quality Lotus crank Nick (who responded below) is your first port of call. One of the best in the Lotus world.

The 120e block would need a modification to the sump baffle if using it with a 6-bolt sump as the 120e block uses a screw-in pickup pipe and they used a different sump. It is IMHO that the screw in pick-up pipe is the better system though as it allows the oil pick-up basket to sit centrally in the sump. The push-in pick-up pipe sits to one side, and is a pain to remove from the block if you wish to do so.

avro wrote:Tom

If you choose to use a Lotus crankshaft with your 2731 or 120E cylinder block or any other block that has not previously been used in a Lotus engine they will require just a small amount of machining to clear the larger counterweights of the Lotus crankshaft at both ends of the block.

Nick


Thanks chaps, most helpful. That said, I am coming to the conclusion that the cost is going to far outweigh the benefit for my planned use. I am in the process of restoring a basket case Lotus 7 (S2 1961) that came in a collection of boxes, completely stripped and it had been so since 1970! Unsurprisingly, not all the bits are there and I am trying to ascertain what I have and what I need. I have a 2731 6 bolt crank, a 2731 block but without main caps so it will need line boring. I have a head, cams, an alloy lip/6 bolt sump and a pair of Strombergs with manifold. Thanks to the forum I have had two realistic offers of 3020 cranks already but I fear the rebuild costs are going to be outwith my budget and overkill for my planned road use. I have been looking around locally (Scotland) and a friend has a 1500 GT pre crossflow which will probably be sufficient to propel me round the byways with the occasional European foray.

Tom
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PostPost by: 2cams70 » Sat Sep 28, 2019 11:23 am

Note it is possible to add weight to crankshaft counterweights by using special tungsten slugs. You may be able to bring a 2731E crankshaft up to Lotus 3020E weight.
If you are using lighter aftermarket pistons and rods however it may not be needed. The lighter counterweights on the standard 1500 crank may suit better!

https://www.hotrod.com/articles/what-is-mallory-metal/
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PostPost by: Tom Rae » Mon Sep 30, 2019 1:12 pm

2cams70 wrote:Note it is possible to add weight to crankshaft counterweights by using special tungsten slugs. You may be able to bring a 2731E crankshaft up to Lotus 3020E weight.
If you are using lighter aftermarket pistons and rods however it may not be needed. The lighter counterweights on the standard 1500 crank may suit better!

https://www.hotrod.com/articles/what-is-mallory-metal/


I have Hepolite pistons and 125e con rods so I am not sure if they require additional weight or not. Despite having almost given up on building a TC it seems any option is likely to be expensive so I may be better to go with what I have!

Thanks again.
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