will this work on connecting a 6" cast iron to a 4" clay sewer line??

No, the 4" side on the P1056-64 is a nominal 4.46" inside diameter according to Fernco so it too small for 4" clay. The Fernco 1002-46 is designed for 6" PVC/cast iron to 4" clay and according to Fernco.com has a 6.39" by 5.18" inside diameter. Haven't worked with clay pipe but according to the sizing information from the Fernco site, any adapter with a clay connection requires that side of the adapter to be sized just for clay. Hope this helps.

Will this work on connecting a 6" dsw sch 40 pvc pipe to a sdr 35 4" pipe?

It will work but because sdr 35 is thinner wall than sch 40 be careful not to over tighten the connection to the 35.

What are the inner diameter dimensions?

Its difficult to measure as it is flexible and not perfectly round when not installed on a pipe. As close as I can get the inside diameters are 4 1/2 and 6 1/2 inches. Fits 4 inch schedule 40 PVC and the top fitting on a IBC container which is what I used it for.

12"coupling reduce to 6"

No. Mine fit loosely on an 8" to 6".

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Fernco, 1056-64, Flexible Coupling, For Pipe Size 6" x 4"

Fernco, 1056-64, Flexible Coupling, For Pipe Size 6" x 4"

Product ID: 444319158

Details

Brand
Fernco
Colour
black
Connector Type
Coupling
Exterior Finish
black
Item Weight
0.81 Pounds
Item dimensions L x W x H
12.2 x 17.3 x

🔧Fits 6" x 4" pipes seamlessly

⚙️Flexible elastomeric PVC for leak-proof seal

🛡️Corrosion-resistant stainless steel clamps

Description

🔗 The flexible coupling pros swear by—secure your pipes, secure your project!

BUILT TO LAST - Durable elastomeric PVC resists chemicals & sewer gases
PRECISION FIT - Dual internal diameters ensure perfect pipe matching
TRUSTED QUALITY - ASTM, CSA & UPC certified for professional peace of mind
PRO GRADE VERSATILITY - Connects PVC, cast iron, copper & more with zero fuss
EFFORTLESS INSTALLATION - No primers or glue—just slide & tighten clamps

The Fernco 1056-64 flexible coupling is a high-quality, durable plumbing connector designed to join 6" and 4" pipes of various materials including PVC, cast iron, copper, and steel. Featuring corrosion-resistant stainless steel clamps and elastomeric PVC construction, it ensures a leak-proof, chemical-resistant seal without the need for messy primers or glue. Certified to ASTM, CSA, and UPC standards, this coupling offers professional-grade reliability and easy installation for drain, waste, and vent systems.

Specifications

ASIN	B000BQM0JC
Brand Name	Fernco
Colour	black
Connector Type	Coupling
Country of Publication	USA
Customer Reviews	4.6 4.6 out of 5 stars (151)
Exterior Finish	black
Global Trade Identification Number	00018578000131, 00018578015586
Included Components	P1056-64 Flexible PVC Pipe Coupling
Item Dimensions	12.2 x 17.3 x 14.1 Centimeters
Item Type Name	Flexible PVC Pipe Coupling
Item Weight	0.81 Pounds
Manufacturer	Fernco Inc.
Manufacturer Part Number	P1056-64
Material Type	Polyvinyl Chloride Stainless Steel
Model Number	1056-64
Number of Pieces	1
Size	6 Inch
Thread Size	6.85 inches
Thread Type	NPT
UPC	018578015586 018578000131
Unit Count	1.0 Count

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