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Old 11-12-2019, 05:05 PM   #1
nvmylh
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Default 131.6mm conrods + cosworth 2.6lt stroker pistons?

Hi all,

I might have access to some cheap, brand new cosworth pistons that came from a 2.6L stroker kit that never got used. I also have some brand new crower conrods lying around which are for N/A motors (131.6mm length).

Does anyone here know what the compression height is for these pistons, the dish volume and possibly the conrod length that comes in this kit?

So far I ran some numbers through a spread sheet, assuming the following:
- 9.2:1 compression, 52cc chamber cc's and 18cc dish volume. Then came up with the following combinations of rod length and comp height:

1) 130.6mm conrod length + 29.5mm compression height
2) 129.4mm conrod length + 30.7mm compression height

I've emailed cosworth and asked a couple of shops who sell/use their parts, no replies yet..


The last thing which might kill this idea is that when using the crower rods with 30.7mm comp height, the piston will stick out of the block by around 0.8mm. This MIGHT not be a deal breaker as we can easily machine the pistons (have done many times before).

Just hoping t weigh up the cost vs benefit vs effort etc as these will cost less than half of a normal set of forged pistons.
Cheers!

PS the idea is to build a reasonably low comp (for turbocharging), SOHC engine, possibly using a ej255/7 block or modifying a ej251 block that i have in the shed.
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Old 11-12-2019, 06:20 PM   #2
Turn in Concepts
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IIRC, and someone please correct me if I am wrong, but I believe Cosworth sold those rods and pistons paired due to moving the wrist pin. I THINK the wrist pin bore may also be different than normal.
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Old 11-12-2019, 06:35 PM   #3
nvmylh
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The cosworth catalogue says 22.998mm pin so should be fine there
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Old 11-18-2019, 03:42 PM   #4
nvmylh
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Well if anyone ever need this info in future, someone at cosworth replied with 11.9cc dish, 29.825mm compression height.

BUT using these figures are a 130.5mm conrod gets you 9.9:1 compression (51cc head), i've asked them to confirm the conrod length..

Last edited by nvmylh; 11-18-2019 at 04:29 PM.
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Old 11-19-2019, 06:12 PM   #5
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Cosworth was an 81mm stroke crank if I recall. I imagine they'd have kept the 130.5mm stock rod length, and sucked up the 1mm (81-79 = 2mm , then divided by two because stroke gives 1mm) in the piston *30.7 down to 29.7). So that aligns close enough with the 29.825.

As for your CR numbers, I don't know what you're using for variables, so can't comment.
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Old 11-27-2019, 07:28 PM   #6
nvmylh
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Here's the figures I used to get 9.2:1 comp:

Chamber volume: 57cc (maybe this should have been 55cc which works out to 9.5:1? 51cc heads = 9.9:1)
Head gasket thickness: 0.78mm (as per cosworth specs)
Bore: 100mm
Stroke: 81mm
Piston dish: 12cc
Piston comp. height: 29.825mm
Conrod length: 130.5mm
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