Lotus Elan

Cutting High Comp Pistons to Lower Comp

PostPost by: Woodboater » Mon Mar 25, 2019 3:44 pm

Looking for some advice, I have a set of good Cosworth high compression Twin Cam pistons that I would like to use in a Twink I am building, but, would like to run it on pump premium fuel. Can these be machined safely to use?

Also looking for a good Twink builder in the Twin Cities area.

Thanks in advance.

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PostPost by: RogerFrench » Mon Mar 25, 2019 4:07 pm

How high is the CR you expect from those pistons As they are? What head are you using?
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PostPost by: nmauduit » Mon Mar 25, 2019 6:17 pm

it is certainly possible to machine off some of the intruder, one would need to find an understanding machinist that either would have a jig to do so (holding the piston without marring the sides or causing a change of shape to the skirt area), and that would ensure that the machined intruders would be close enough in finished height so as not to cause any discrepency in compression ratio.
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PostPost by: types26/36 » Mon Mar 25, 2019 7:02 pm

I think it will depend on how much you want to remove from the piston crown, which in turn depends on the thickness of the intruder/crown.
I cannot comment on Cosworth pistons but several years ago I fitted a 1600 crank (77.62mm) in a Lotus 1500 block using standard Lotus pistons, the longer stroke popped the pistons out of the top of the block so I machined (in my lathe) the piston crowns to bring the pistons about 10thou below the block height.
The governing factors were the crown thickness and how close the top ring would be to the crown.
I thought that if the ring became too close to the crown then the heat may close up the groove and problems would be caused.
In the event it was fine although I ended up with a CR of approx 11:1 but the engine is still running today.
Think I also had the valve cut outs machined deeper but not sure now as it was a long time ago.
I believe some racing pistons are supplied with a tall intruder and you are expected to machine them to suit your application.
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PostPost by: denicholls2 » Tue Mar 26, 2019 3:51 pm

Other option is a thicker head gasket. Scary outcomes are possible unless the piston was designed with redundant meat (not in itself a great concept due to the increase in rotating mass; I would think that racing pistons would be designed to keep weight at a minimum and hence not have much flab for the cutting.)
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PostPost by: Quart Meg Miles » Tue Mar 26, 2019 11:09 pm

denicholls2 wrote: Other option is a thicker head gasket. )

This will affect your cam timing too but that can be trimmed out with offset dowels to realign the sprockets.

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PostPost by: 1owner69Elan » Wed Mar 27, 2019 1:20 am

As an example.
My engine build had an initial 11.5 CR with the pistons as delivered. This was lowered to 10.25 CR to allow use of pump fuel by machining .060 inches off the intruder.

During the machining process the piston was secured via gudgeon (wrist) pins extended out each side. Not by clamping the piston body or skirt. I believe 4 passes were made to achieve the desired final cut.
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PostPost by: rgh0 » Wed Mar 27, 2019 10:31 am

I have machined JE forged pistons intruders down to achieve the desired compression ratio as combustion chamber sizes can vary dramatically depending on head thickness . Generally forged pistons have the same bottom side and the intruder is added thickness so that their is adequate thickness whether intruder present or not.

Assuming Cosworth pistons are the same then there should be no issue with taking some off the intruder height.

The biggest issue is location of the cut outs versus the top ring especially if you have to make them larger or deeper than the piston manufacturer supplied. You need to check this as the bottom of the inlet cutout can get close to the top ring groove and lead to cracking.

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