Lotus Elan

Weber head specifications as new

PostPost by: tedtaylor » Wed Jul 31, 2019 4:02 pm

can someone point me in the right direction to find OEM specifications on the Lotus twin cam Weber head?
i'm having an engine rebuilder/machine shop check mine, but need the original OEM specifications to check against. thickness, openings, etc.
thanks
TED
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PostPost by: 2cams70 » Wed Jul 31, 2019 11:29 pm

If your builder needs to ask those types of questions I'd tread carefully!!
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PostPost by: StressCraxx » Thu Aug 01, 2019 12:17 am

The Elan shop manual has much of the info, particularly good instructions on how to remove and install valve guides, tappet guides and seats.

I agree with 2cams. There are engine builders in your area who speak Twincam. They may save you both grief and money.

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PostPost by: tedtaylor » Thu Aug 01, 2019 12:58 am

CLARIFICATION = I am not getting the head rebuilt, I just only want a machinist to inspect for cracks, pressure test, measure against factory specs (I don't have the equipment or knowledge) to ascertain whether it is a good head or boat anchor.
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PostPost by: 2cams70 » Thu Aug 01, 2019 1:04 am

You would still be better off taking it to someone who has Lotus knowledge. They should be much better placed to know what to look for. The Lotus manuals were produced when the cars were new - i.e long before the experience of 50 years passing of time.
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PostPost by: prezoom » Thu Aug 01, 2019 2:03 am

Take it to Sandy at Quicksilver.
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PostPost by: StressCraxx » Thu Aug 01, 2019 4:47 am

prezoom wrote:Take it to Sandy at Quicksilver.


Sandy and his team are absolutely first rate. He stands behind his work.

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PostPost by: UAB807F » Thu Aug 01, 2019 5:31 am

tedtaylor wrote:can someone point me in the right direction to find OEM specifications on the Lotus twin cam Weber head?
i'm having an engine rebuilder/machine shop check mine, but need the original OEM specifications to check against. thickness, openings, etc.
thanks
TED


The head depth, together with maximum permissible metal removal, is given in the technical data of the workshop manual. Mine says;

Small valve, 4.638" to 4.643"
Big valve, 4.598" to 4.603" (higher compression ratios)

Max. Permissible metal removal,

Small valve 0.045"
Big valve 0.010"

No doubt heads have been skimmed over the years and it wouldn't surprise me to find a BV head below the limit. Equally if someone is using the head for racing they might want a higher CR and skim more than these figures.

I'm not sure what else you can check. Flatness would be my first call and following that answer you'll have an idea of how much you'd have to remove and what that'll do to the CR.

The head diameter for inlet & exhaust valve inserts and valve guides are also in the same section of the manual, together with oversized items. You'd only need this if you're doing a complete overhaul though.

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PostPost by: 512BB » Thu Aug 01, 2019 7:59 am

Big valve, 4.598" to 4.603"

Interesting figure this one, but misleading. I have a few genuine Big Valve heads that measure 4.610. They are not cut down earlier heads that start life thicker.

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PostPost by: 2cams70 » Thu Aug 01, 2019 8:56 am

There are ways to compensate for a thin head in ways the factory never thought possible. Please send it my way though if you're going to bin it!
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PostPost by: tedtaylor » Thu Aug 01, 2019 12:51 pm

i have the workshop manual "technical data" page 4&5 where cylinder head is, but nothing about thickness.
thanks for providing.
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PostPost by: rgh0 » Thu Aug 01, 2019 1:14 pm

Hi Ted
Attached is the spreadsheet I use for head rebuilds. The standard big valve column is the best data I have been able to assemble from multiple sources and measurement of actual heads. I don't know of any authoritative factory source for most of this data. The workshop manual has some, the Dave Bean Catalogue has some, most comes from actual head measurements and lots of thought. If you're machine shop cannot understand this then go somewhere else :lol:

The file is a .xlsx excel file but I have changed it to a .xls extension as the site software does not accept the later excel file extension. When you open you may get an error message but ignore that and it opens OK at least in my version of Excel.

cheers
Rohan

Standard Big Valve .xls
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PostPost by: UAB807F » Thu Aug 01, 2019 3:58 pm

tedtaylor wrote:i have the workshop manual "technical data" page 4&5 where cylinder head is, but nothing about thickness.
thanks for providing.

You're welcome Ted.

The data is from a copy dated 1970 and with any luck I'll be able to upload the pdf of the relevant pages for your manual with this post.

CylinderHeadData.pdf
Cylinder Head pages
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PostPost by: promotor » Thu Aug 01, 2019 4:15 pm

I have an original Lotus head that I bought as new old stock - I took some figures from it before having any work done on it and they were as follows:

The head thickness was 4.635".
Inlet port size at carb end 1.530"
Exhaust port at outlet face 1.140-1.142".
Inlet valve size 1.530" (exactly the same size as the inlet valve).
Exhaust valve size 1.325"

Hope this helps.

(Photos below are after some work was done) :

dsc05307b.jpg and

dsc05308b.jpg and

dsc05309b.jpg and

dsc05310b.jpg and

dsc05311b.jpg and
Last edited by promotor on Thu Aug 01, 2019 7:22 pm, edited 1 time in total.
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PostPost by: 1owner69Elan » Thu Aug 01, 2019 5:30 pm

Just as another reference for a Weber head.

These are the specifications for new SAS Weber head, "street ported":


New LM25TF Casting to original dimensions
Cam bore machined into head (No cam bearing required)
Inlet port machined and hand polished to 40mm Weber 1.3 diameter
Inlet Valve seat cut for 1.625 dia valve- (Inlet valve not included)
Exhaust port machined and hand polished 1.25 dia
Exhaust valve seat cut for 1.4 dia valve- (Exhaust valve not included)
Colsibro valve guides fitted finished for 5/16 valve stems
Combustion chamber machined and hand polished @ 4.640 head height
Standard spark plug 14mm
All core plugs fitted
Full pressure test prior to despatch
Head supplied fully machined with valve guides and valve seats cut, cam caps & cam studs
Valve seats are in sintered material suitable for unleaded petrol.

Interestingly the SAS inlet port size indicated is smaller than that measured by promotor (1.3" vs 1.53"), but perhaps measured at a different location. On the other hand the SAS exhaust port is indicated as bigger at 1.25" vs 1.14".

But, subsequently during build, my engine builder opened up both intake and exhaust ports to a larger dimension (unknown).
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