See full version: Elbow restrictor gasket vs full flow gasket
2. next is all open water jacketed riser gaskets for manifold / riser block extensions that are installed in between the riser and exhaust manifold to raise your water line height to prevent water reversion and in some cases installed because of locations ie: thru transom exhaust placement and so on. Also this riser gasket is widely used for the cold manifolds cooling set up.
Ayuh. Merc has used two systems, the warm manifold system, 'n the cold manifold system.
1. There is a closed cooling system - riser gasket that is just a gasket with no water jacketed openings [links]
The T-stat housin's are also different, so replace what you've got now, with copies of what's there.
Actually Mercruiser has 3 types of riser gaskets for NON DRY JOINT EXHAUST MANIFOLDS & RISERS. ( like what exhaust manifolds & risers should be installed on the stock Merc apps for the 502 and 454 and other apps down.) More info on top of to add to the mix. [links]
Otherwise, you'll be havin' coolin' issues. here
Re: Mercruiser 5.7 engine riser gasket more
Look at this service bulletin, it explains which gasket you need for which type cooling system.
http://www.boatfix.com/merc/Bullet/99/99_10.pdf [links]
Re: Mercruiser 5.7 engine riser gasket
Multi purpose gasket- Fits more than one riser application. Be sure to use the right one for FWC and raw water cooled applications. here
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