When I removed the riser on my Mercruiser 350, it had a "restricter" riser gasket - 2 holes open for riser flow. I cut open the 2 closed holes on the replacement as water flowing sounds like a good thing.

I need to replace all 4 now (twins) in the spring as I install riser spacers to get new-to-me exhaust diverters to fit. I see 3 kinds of riser gaskets:

Blank - I would call it "blocked" for now flow - and off my option list
Restricted - 2 of 4 holes open
Open - 4 of 4 holes open

When do you use restricted vs open?

It sounds like restricted would increase the pressure of the water flow vs open. But open just sounds like the right thing to d - let as much water as possible flow through.

I'm sure this is an easy question for most of you. :-)

Thanks.

the ones that are blocked are for closed(fresh) water cooling systems where there is antifeeze in the blocks and manifolds, the raw water is dumped in the risers only...the restricted ones have never seen them..

Thanks - makes sense. You can see the different types here: http://www.ebasicpower.com/pc/GLM503...20Exhaust.html

Do you have the engine serial number?

And do you have any risers?

No serial numbers as the boat is shrink wrapped with a foot of ice/snow on top. Waiting for the spring thaw. Is there some secret Davinci code in the serial number related to riser water flow???

Standard Merc risers are on it - I need to add 3" riser spacers to get some exhaust diverters to line up with the original thru hull holes. I want the option to hear some noise this summer.

My small blocks with Alpha drives use the restricted gaskets, I have no issues with water flow or over heating. If you use the open you may have issues at low speed with too much flow thru the risers starving the engine for water. Use the good Merc composite gaskets the green paper ones dont last very long.

Will do. Thanks for the advice.

Having the Mercruiser serial number helps to ensure you're finding the correct part numbers for your engine, Drive, & Transom Assembly.

Also having the serial number should help a Mercruiser dealer find any possible changes that Mercruiser may have done to any of their parts over the years by looking up any possible service bulletins that pertains to your engine, ect.

Since the boat is covered up, By chance can you still read the sterndrive serial number on the outside of the boat? There is a 50/50 chance a mercruiser dealer can look up what engine you would have had from new using that number.

lol, So Yes, on some certain style engine models from Mercruiser, Sometimes there is kind of a secret Davinci code in the serial number related to riser water flow

[QUOTE=242LS;3323487]No serial numbers as the boat is shrink wrapped with a foot of ice/snow on top. Waiting for the spring thaw. Is there some secret Davinci code in the serial number related to riser water flow???

Thanks lakematdude - but the engines were replaced in '97, the one of them rebuilt DIY-style in 2008.

I'll stick with restricted as that is what was on there when I got it and they ran great for my two summers of use.

I am running the restricted ones as well. I have not had an issue.