rear seal / front cover ? 351W

We have all heard the advantages of forged pistons. However, when you consider the disadvantages of forged pistons it becomes very clear why GLM cast pistons are the ultimate pistons for your rebuilding and repair projects.

The major disadvantages to forged pistons are actually a result of the forging process itself. Forging results in a piston that is considerably heavier, than cast pistons, and is limited in the aluminum alloys that can be used to produce the piston. Additionally, the forging process also limits the design configuration of the piston itself.

The forging process and its limited choice of aluminum alloys result in a dramatically higher expansion rate for the forged piston. This means that the set up characteristics are very different from the original engine manufacturers' and that break-in and warm up periods are crucial for the forged piston.

An example of the dramatic expansion of forged pistons is the piston skirt clearances in the cylinder. A typical cast piston has a skirt clearance of approximately .0007 to .0009. A forged piston has a skirt clearance of .005 to .007. The forged piston has 10 times more slop in the cylinder. This results in less ring stability against the cylinderical wall, more piston noise and extra blowby.

In all fairness, after the forged piston has reached operating temperature, its dramatic expansion makes up for these extra clearances. However, this should remind us of the typical customer that a dealer services. Can we expect the casual weekend boater to strictly observe the extended break in period and the critical warm up procedures required for a forged piston? Let your own experience answer this question.

Most forged pistons are quality products, but they are better suited to racing and professional applications.

The alloy from which a piston is made not only determines its strength and wear characteristics, but also its thermal expansion characteristics. Hotter engines require more stable alloys to maintain close tolerances without scuffing.

Many pistons used to be made from "hypoeutectic" aluminum alloys like SAE 332 which contains 8-1/2 to 10-1/2 percent silicon. Today we see more "eutectic" alloy pistons which have 11 to 12 percent silicon, and "hypereutectic" alloys that have 12-1/2 to over 16 percent silicon.

Silicon improves high heat strength and reduces the coefficient of expansion so tighter tolerances can be held as temperatures change. Hypereutectic pistons have a coefficient of thermal expansion that is about 15 percent less than that for standard F-132 alloy pistons. Because of this, the pistons can be installed with a much tighter fit – up to .0005˝ less clearance may be needed depending on the application.

Hypereutectic alloys are also slightly lighter (about 2 percent) than standard alloys. But the castings are often made thinner because the alloy is stronger, resulting in a net reduction of up to 10 percent in the piston’s total weight.

Hypereutectic alloys are more difficult to cast because the silicon must be kept evenly dispersed throughout the aluminum as the metal cools. Particle size must also be carefully controlled so the piston does not become brittle or develop hard spots making it difficult to machine. Some pistons also receive a special heat treatment to further modify and improve the grain structure for added strength and durability. A "T-6" heat treatment, which is often used on performance pistons, increases strength up to 30 percent.

Machining hypereutectic pistons is also more difficult because of the harder alloy. Consequently, hypereutectic pistons typically cost several dollars more than standard alloy pistons. That’s why most OEMs (except Ford) have gone back to eutectic alloy pistons in their late model engines. High copper eutectic alloys offer most of the advantages of hypereutectic alloys without as much cost.

JoeinNY wrote: cheapest available parts including cast aluminum pistons that are going to be the worst case for thermal stability,

Actually forged pistons expand the most out of the bunch.

Generally piston manufacturers will dictate the necessary piston to bore clearance. What's in the shop manual would only apply to OEM pistons.

I used Speed Pro hypereutectic pistons in my rebuild along with Hastings moly rings. The Speed Pro pistons specified a minimum piston to bore clearance of 0.0015" my machine shop set them to 0.0025" from experience (not knowing much about marine engines). The engine lasted about 20 hours. I then honed it out to 0.003-0.004" clearance and she's been running strong for 50hrs now. The only down side of having piston fit on the loose side is a bit more piston slap until the engine warms up.

If you use standard fit rings you shouldn't have a problem, because with the slightly larger bore size you'll automatically get slightly larger ring gaps which is ok. Now if you're using file-fit rings then you might want to allow for a slightly larger ring gap. Bad things happen when the rings expand beyond filling the gap.

Follow the manual when your not using pcm rings and pcm pistons and your taking chances with your time and money. I listed the specs required to be used by keith black/silvolite when using thier pistons in normally aspirated marine applications. Some pistons have skirts less prone to scuffing than others, and all rings are not created equal, in addition to the material differences I talked about earlier. It would be my guess and experience that an engine rebuilt by an autoparts store rebuilder would use the cheapest available parts including cast aluminum pistons that are going to be the worst case for thermal stability, if thats the case more clearance is better than less and whats in the manual will give you an engine very likely to have a short life.
-Joe.

here is the link to the manual if anyone cares to check the piston to cylinder spec and comment on it's or my accuracy.

http://www.correctcraftfan.com/ftp/new/PCMsvcmanual.pdf

I just want to make sure I do this the right way.

Oh, I totally forgot about the direction of grinding having an impact on bearings....That's a biggie.

The larger piston to bore clearance on a marine engine isn't a ring gap issue it's a piston to bore issue. You can adjust the ring gaps all you want and you'll still scuff cylinders. I run standard ring gaps and haven't had a problem.

There are several reasons why you need more clearance in a marine engine. First, marine engines run at higher loads for longer duration than a car engine. This results in higher piston temperatures (i.e. more expansion). Marine blocks run semi-recirculated lake water through them and generally run colder than autos (i.e. less expansion). Since the pistons expand more and the block expands less you need more room between them to ensure bad things don't happen. Although marine and auto engines appear similar they are very different applications.

Marine engines typically run 143 or 160 deg. thermostats (cars run ~180-195). Eventhough the stat regulates temperature an auto engine is going to have a more consistant temperature fluid flowing through it. In a marine engine cold lake water is mixed with hot coolant to keep the temperature near constant. The result is you have slugs of hot and cold water flowing through which can cause some strange expansion phenomena.

As for aligning the front cover there aren't any dowell pins. You have to do your best to center the front seal on the nose of the crank and to get the oil pan rail lined up close enough that it won't leak. I've had mine apart twice and haven't had any trouble.

There are different style timing covers and the blocks are machined a little different depend on the year and application thus the issue with the dowel pins and if you look closely the bolt pattern most likely is slightly different as well. If you are using an automotive crank it is ground wrong for a reverse rotation engine and you will have premature bearing failure because of it. With a smooth journal the seal won't be an issue as long as the ribs on it are going in the correct direction for a reverse rotation engine. The issue with the ring gap is the block doesn't expand as much in a marine application but the piston will thus the need to increase the gap of the rings.

From what I understand up until the late 70's early 80's ford cranks had the grooves in them. The one I've got is smooth and with a reverse rotation rear seal I should be ok, right?

I don't understand why if a motor ran at 185 deg F water temp in a car and the same in a boat you would have different thermal expansion properties on each. I'm not trying to argue just understand. Thanks

Apparently I was sold a marine gasket set and the guy did not tell his suppiler it was a reverse rotation, hence the same seals. I was told the only dif in the gasket sets are the rear seal and timing cover gasket which come seperatly. They are on the way.
Everyone I've talked to says the front oil seal is the same. It's running 10 to 1 on the front seal being the same. Any thoughts on that?
I have not seen my new marine reverse rotation seal but the crankshaft is totally smooth.
Any thoughts on the missing dowel holes on the front cover and lining up so front seal won't leak?

Ok Tim, explain the front cover?

I thought Awhite70 covered it pretty good to start with.

When I rebuilt my 351 I bought a marine gasket set and it came with standard rotation front and rear seals (not all marine engines are reverse rotation). I had to get the RH rotation seals separately. When I got the seals I couldn't tell the difference between the two but they had different part numbers so I have to believe something was different.

More importantly if you bought a standard rotation automotive short block you're in for trouble. There are more differences than just the camshaft (and if you're old engine was in that bad of shape you should probably replace the cam anyway).

The crank is largely the same for RH and LH rotation the only difference is the grooves that the rear seal rides on. If you use a standard crank in a rev. rot engine those groves will be facing the wrong way and will pump oil out of the rear seal rather than hold it in the oil pan, the engine will run fine but you'll have a messy oil leak.

If your piston to bore clearance is set to automotive specs it will likely be too tight. Marine engines have more clearance to allow for thermal expansion. If you have automotive clearances I give your engine 20-30hrs before you scuff a few cylinders and have to rebuild it again. This is the absolute truth myself and others on this board have lived through the pain.

I'm building a 351. The crank and bores were shot so I got a short block from auto parts store/machine shop. Purchased the marine gasket set and the 2 piece real seal that came with the short block has the same part number on it and looks exactly like the 2 piece seal that came with the marine gasket set. This engine turns backwards, I did get old camshaft into new block.
Any thoughts on that the seals? I thought they were supposed to be different. The old engine from the boat had a one piece rear crankshaft seal and not the two piece. I'm getting a little confused. Help!

Also the front timing chain cover has no room for dowels to line it up the the front seal. The block has bigger holes to accept dowels but just bolt holes on the front cover? How does the front cover line up so the front seal will seal?

Any thoughts would be greatly appreciated.
Thank you!

sorry for the double post, this t*tle is more accurate.
thanks again