Sierra 18-1908 marine exhaust riser spacer block for MerCruiser center-rise exhaust systems from about 1985 through 2002 (verify part numbers in cross reference below). Replaces the 3 inch tall spacer that sits between the riser and manifold on many V6 and V8 applications. Does not come with gaskets, mounting hardware, or plugs, since those will vary depending on your configuration. May require 3/4" and 1/8" pipe plugs depending on your configuration (Sierra 18-4263 and 18-4258). Each purchase is for one spacer. Most V6 and V8 applications will require two spacers to do both sides of the engine.

Gasket Information
Sierra makes three different styles of gasket for different Mercury configurations (sold separately):

• Sierra 18-2849-1 Open Gasket (4 water slots)
• Sierra 18-0897 Restrictor Gasket (2 water slots)
• Sierra 18-0672 Block-Off Gasket (no water slots) - Used for coolant-cooled manifolds (closed cooling) to isolate raw water riser circuit

Riser mating bolt dimensions are 3-7/16" (87.3mm) by 3-1/16" (77.8mm) bolt center-to-center.

This part is new and high-quality cast iron construction. To determine if this is the correct replacement for you, look on your current part for casting numbers and match them to the cross reference below:

Cross Reference	Part Number
MerCruiser	93320
Osco Motors	3320
Barr Marine	MC-20-93320A3
Sierra Marine	18-1908
Aqua Power	n/a
GLM Products	50300

About Sierra
Sierra provides premium-quality aftermarket replacements for over 6,000 marine parts, tools, and components. Sierra products are engineered to meet or exceed OEM standards, and stand up in harsh marine environments. Every new Sierra part comes with a limited lifetime warranty that covers the useful life of the product against any defects in workmanship and materials.

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