Thread:

Sizing Guides for Marine Use

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Originally Posted by Ahsumtoy

Okay I'm no professional like several of you guys. I'm just a garage hack and build my own motors. I was not aware that the guides needed sized different for marine use, and I'm not suggesting that they don't need special clearances. However, given the number of engines I have built over the years, without any guide failures, leaves me wondering why they need special sizing, or why I haven't had any guide issues? My latest build (Dyno Results) used AFR heads but my head guy supplied the valves and springs. Since the new motor currently has less than an hour of run time, off the dyno, do I need to pull the heads to check the guides?

AFR specifically says in their tech sheet (included with the heads) that their guides are on the tight side and for blown/Nos or marine use, they advise more guide clearance.
Last year I had to do a complete rebuild on a set of AFR 325 heads for a customer who had someone else build the heads, supplied the ex valves, and didn't check the guide clearances. There were clear signs of the ex valves running hot and dragging in the guides. Fortunately (his was a centrifugal blown engine) he hadn't really stuck his foot in it yet, because from what I was seeing on the ex valve stems, he was one hot-foot run from sticking an ex valve. I tell you this in detail to let you know your engine can be running, but you could still be risking sticking a valve which we all know can be catastrophic. Also note: the valves AFR uses are a few tenths smaller than, say, a REV or a Ferrea, so when you stick an aftermarket valve in the head, like this last case scenario I explained, you end up with even less clearance than what AFR is already telling you is minimum.
As far as other heads, most have generous enough clearance that it's not an issue, but for me, it's always a good idea to check.
HTH

Originally Posted by cakeeater ed

pistons need more clearance in marine motors because the block never gets hot enough to expand to size, valve stems only care about guide material & fuel, ex valves that don't have enough clearance stick pretty quick, intake valves need to be on the tight side to keep oil out of the combustion chamber. valve stem seals are suppose to be a controlled oil leak. ski boat motors valve guide seals should be a pc on the intake & umbrella seal on the ex.

Guide expansion is effected the same as cylinder wall expansion. Cold water in the heads will lessen ex guide expansion especially, and increased egt's in marine engines can exaggerate the condition.

What valve stem to guide clearance is considered to be "marine spec" for your typical lake ski BBC motor. More Specific, Aluminum heads with bronze guides and Inconel valves? (Darts/Brodix) Does the spec change when we start talking about "GM iron" 990's, 781's?

I'm just a DIY garage hack that builds his own motors for fun and education too. This forum gets read by me every morning over coffee and is chalked full of great info. There are a lot of astute individuals that post here and much can be learned if one pays attention. Thanks for all the replies and good posts!

Ashumtoy, couple of reasons you may not have had any problems is, one your lucky. But as much as anything, it could be the way you drive the boat. Not to say you don't hammer the sumbitch. But you do it after its good and warm.

When water cooled 2 stroke bikes first came out, they had a bad problem with sticking the piston big time. What was happening was that the rider would "warm" up the bike, and them hammer it. Well the water wasn't warm, and really, neither was the cylinder. But when it got hammered, the piston would go to temp in a heart beat and the thing would stick. An exhaust valve and guide isn't a whole lot different.
The whole engine has a much better chance of surviving less than ideal clearances if the thing is fully warmed up before the fuel is layed to it.

Your heads may have simply been looser without you knowing it. Steelcomp had an issue with loose guides from Patriot heads, even after returning them.
Also, long overlap cams and high compression helps alittle. Low compression and short overlaps tend to create more exhaust heat combined with less intake cooling during the overlap event. Also, even though the guide doesn't expand as much in a lake cooled engine, a huge part of the exhaust valve's heat is rejected thru the seat and the stem. Both are much cooler in a marine engine. Even though a marine engine generates alot more combustion heat due to load, the exhaust valve is able to reject much more heat due to the cooler seat and guide.

Bottom line, all marine engine should have the guides set accordingly, but blower and NOS marine engines seem to be much more problematic than N/A deals. JMO.

I try not to worry too much about this crap and just let Steel handle it. That way if I stick a valve, I know whos ass needs whooping.

Okay, you've convinced me. I'll make the 260 mile trip to Havasu and pull the heads to check the clearances. What should the clearances be?

Originally Posted by Ahsumtoy

Okay, you've convinced me. I'll make the 260 mile trip to Havasu and pull the heads to check the clearances. What should the clearances be?

Int. should run about 0013-.0015, exhaust .0016 -.0018 and I'll "allow" as much as .0020 on big power adder stuff. Like to try to keep it about .0018.

Originally Posted by steelcomp

Int. should run about 0013-.0015, exhaust .0016 -.0018 and I'll "allow" as much as .0020 on big power adder stuff. Like to try to keep it about .0018.

Will these clearances work for both cast iron and aluminum heads? I have 049 irons that need to be sized for marine use. I will be running e85 if that makes a difference.

Originally Posted by jrider44

Will these clearances work for both cast iron and aluminum heads? I have 049 irons that need to be sized for marine use. I will be running e85 if that makes a difference.

IMO, yes.

Originally Posted by stewvette

I bought a set of assembled heads from afr (bbc 325's) and told them i was running marine with a blower. After reading a few threads on here I took them to Tim at Morgans Machine since he is right down the street from me. He said the guides were way too tight and probably wouldn't have lasted long on my application. So again thanks to the guys here on performance boats for the the helpfull advice. So i would also recommend having the heads checked out by a professional.

I may take some smack for this, but if you want to buy a set of ready to bolt on AFRs, the best place to get them is thru Teague. He gets them bare, does all prep, and final machining, uses manely Severe intakes, Extreme exhausts, and Isky springs and retainers, and they are only slightly more than getting them from Summit, Jegs or AFR.

HOWEVER...if Steel is up and running yet, getting AFRs thru Cstraub is another excellent place as well. Well Scott, are you doing heads yet???? Are you sitting up Canfields yet?