Twincam Engine clearances

[rjaxe]

A question for all those knowledgeable twincam men out there. Has anyone considered revising the bearing clearances when subjecting their engine to a major rebuild programme.
If I look at the "modern" clearances specified for mains and big ends for example on a BMW 318 and a Ford Ka (cars which we happen to have) they are significantly tighter than those originally specified for the twincam. As I am about to embark on a rebuild programme for the engine out of my Plus two I would be interested in others thoughts on this subject.
I guess modern machining techniques and improved engine oils all contribute to the determination of the optimum clearances and there may be other applicable factors as well. Could a 40 plus year old design benefit from a revision??
Cheers

[persiflage]

Remember the old adage ...

"If it ain't broke, don't fix it"

[rgh0]

It is certainly possible to redesign the bottom end of the twin cam to achieve tighter tolerances and run it on the thinner oils available today and potentially get an increased engine output through lower losses.

However as always it is not a simple exercise to do - certainly not as a home builder unless you really like a challenge and have the time to dedicate to becoming an engine designer

For a change to Main and Big End bearings questions you would need to answer would be around things like:

1. What is the correct set of clearances and oil specifications that will work given the engine speeds and loads
2. What is the impact on oil tempurature and overall engine temperature and cooling system of the changes
3. What change is required to oil pump sizing, presure releif settings and design clearances. Any changes needed to the oil passages in the block and head and crank and cams
4. What tolerances are the crank and bearings required to be manufactured to, to achieve the desired bearing clearances and are bearings avaiable to the required tolerances
5. Are the main and con rod bearing caps sufficently rigid to maintain the tighter clearances in operation
6. Do you need to change the cam and layshaft bearing clearances as well and if so to what

Having answered all these questions and probably more you would then need to build a few prototypes and do some durability testing to ensure you have got it right.

Lots of fun but very very time consuming which is why car companies have hundreds of engineers and very sophisticated design programs to do this sort of work and even then they rarely make big changes like this to an engine design

cheers
Rohan

[GrUmPyBoDgEr]

Modern day clearances could be applied to the Twinc' but a lot of factors need to be taken into account.
I think that the Twinc' Block & Crank would be capable of running tighter Tolerances however.
The positional relationship of all of the Bearings will also have to be held to tighter Tolerances also.
The capability of the Machines doing the job & the ability of the Man operating those Machines must be good.
I know a fair bit about the manufacture of the BMW Engine mentioned & can say the following:-
The Blocks are extremely accurately line bored on the most modern machines & at the end of the process each of the Bearing Bores is measured & the Block stamped with a grading Letter for each Bore.
Similarly the Cranks are ground on top Machines to very tight Tolerances, they are then "Super Finished"; each of the Journals is graded for size & it is inscribed on the end of the Crank.
Upon reaching the assembly area those markings are automatically read off & the Bearing selection machine selects the best Bearing shells for each main Bearing.
There are 3 different Grades which differ only minutely to achieve the design requirement for the Bearing clearance.
Needless to say that during assembly extreme precautions are taken to ensure total cleanliness; tight Bearing fits cannot tolerate any form of Contamination.
A top machinist with top machines may be able to achieve those dimensions without the use of those graded Bearing shells.
(But he might need to take several Swings at it )
Nevertheless in order to achieve the tighter tolerances you are thinking of you will need to know what dimensional & geometric Tolerances are required in order to achieve that Goal.
I don't think any Engine maker would be prepared to provide you with that information.
I also think that you will probably need to start with "oversize" parts in order to achieve your Goal & also be able to use the "standard" Twinc' Bearing Shells.
Errrm yes you with also need to know the exact thickness of each of the Bearing shells that you intend to fit; oh & is that thickness consistent along it's length & width?
That's more or less covered the Main Bearings but of course there is Bore Honing etc. & all of the other items covered in Rohans Post to be considered.

Cheers
John

[rjaxe]

Very interesting and comprehensive response to my question. Clearly sticking with the spec clearances is the least risk option but I was interested to see if anyone had done any work in this area.
Thanks for all your replies.

[msd1107]

Rohan and John mention mention a few of the things that can be done. But a whole range of new technologies has been developed and/or become commercially available in the last years.

The average builder concentrates on the engine. But you can continue from the engine to the gearbox, drive shaft, differential etc with improvements for reduced losses, quieter running, or improved reliability. The depth of your pocket book and/or enthusiasm determine the extent of improvements. It may be very difficult to obtain some of these improvements since the TC and other Lotus components are not mainstream or because of cost.

One key spot is treating the differential to ISF and maybe RF85 processing.

Everybody should have their level of fun.

David
1968 36/7988

[rgh0]

There are a whole range of improvements to the total driveline that are practical and needed if your racing an Elan. Most are around firstly better materials and then around careful and individual assembly specific machining techniques. There are a few basic design changes but most of these are simple with easy engineering implementation. Fundamental design changes such as crankshaft bearing clearances are rare and very hard for a one off home based developer or even the major usual suppliers to the Lotus market place to attempt.

I have huge respect for the few individuals who do a major re-engineering of a complex component on an Elan in a professional manner. e.g John McCoy on the Stromberg conversion or Col Croucher on the CV drive shafts or the guy who first developed the Datsun crank conversion ( whoever that was - does anyone know?)

cheers
Rohan

[GrUmPyBoDgEr]

Hi Rohan,

I've heard mention of the Datsun crank conversion but don't recall seeing any in depth information on the subject.
Has it been covered on the Forum?

Cheers
John

[rgh0]

Its been discussed at times but have a look at these links

http://www.elanfactory.com.au/product_datasheets.htm
look at the datsun crank data sheet

or
http://www.cortina-mk1classifieds.com/f ... tsun-crank
there was geocities site based on this same info but it has gone now with the demise of Geocities- you may find if search some of the web archive sites still. I think perhaps this guy in NZ was the first to develop it?

cheers
Rohan

[GrUmPyBoDgEr]

Its been discussed at times but have a look at these links

http://www.elanfactory.com.au/product_datasheets.htm
look at the datsun crank data sheet

or
http://www.cortina-mk1classifieds.com/f ... tsun-crank
there was geocities site based on this same info but it has gone now with the demise of Geocities- you may find if search some of the web archive sites still. I think perhaps this guy in NZ was the first to develop it?

cheers
Rohan

Many thanks,

2 very interesting links.
Looks like the work required is quite extensive & costly but as the Man says cheaper than a "Special" Steel Crank & a load better than the standard C.I. Crank.
I didn't know that the Datsun Crank was Steel, which of course is a major plus.

Cheers
John

[rgh0]

Not only is the datsun crank steel but it is a very good quality steel forging. I have seen a lot of very abused datsun cranks in twink race engines and never seen one break !!!

Vibration at 9000rpm plus may be a little worse with the 4 counterweight design than the special 8 counterweight designs but at those revs you are getting into very esoteric territory with lots of reliability issues and a short time between rebuilds.

I limit my Datsun crank engines to around 8300 to 8500 rpm and get good long term reliability. A friend is running to 9000rpm and is having failures of the 4th main bearing base in the block casting. We theorise this may be due to the 4 counterweight design putting a bending vibration load into that 4th main bearing. If you look a the casting details the 4th main bearing base is thinnner thasn the 2nd main bearing base which sees the same load in a 120E 1500 block

cheers
Rohan