Are clearances between moving parts greater on a marine engine vs a car engine?

[soggy_feet]

I had a local speed shop tell me you can't use a truck engine in a boat because the clearance between the pistons and cylinder are greater on a marine engine, reasoning that marine engines run cooler thermostats and under load the block won't get as hot and therefore won't expand to the same degree as the piston, possibly causing the engine to seize.
He said the difference was .001" between bores.

I'm having a hard time believing that .001" is going to make a difference. Ever.
You still have oil transferring heat between parts, and you still have the clearance designed into the piston/bore fit.


Anyone have an opinion they'd like to add?

 

[GA_Boater]

Is the shop trying to sell you an engine?

 

[glenn property of pam]

a used truck block with 60k miles is going to be worn at least .001. would make more sense if they said the ring gap should be .001 larger. We use to have to heat race car engines before we could start them but ive never seen a truck engine that would seize in the winter months. I may be wrong as I normally tell the machine shop what I want and give them the oversized rings and pistons so they can machine the block and set the ring gaps

 

[soggy_feet]

I was there to ask some questions, and depending on the answers, I was going to see if he had a relatively clean block - just a block- that he'd sell me. The donor boat I bought this spring for the engine and outdrive has two big cracks in the block. I was told the boat needed a head gasket, the autopsy says it wasn't winterized.
With the low hours of the boat (for intents and purposes, new), I was going to try to find a block that didn't need to be bored over that I'd be able to swap my internals into. I'm sure that's a whole different topic to be debated, but that was my plan.

I was hoping I'd be able to pick up the block for less than $100, but he told me it'd be $250, and then started listing off all the things I'd have to do and what I'd have to replace to get a running engine. The dollar signs kept adding up until I told him I wasn't going to be doing a full rebuild for $1300+. I just wasn't willing to spend that much for the engine.

A day later I found a complete Chevy 350 on craigslist for $125, expected a beat up motor but I went to look at it anyway. It was a backup motor for one of the local circle track teams. It's seen 16 races since it was rebuilt. The team changed classes, the motor was taking up space, and it was about 2 days away from being sold for scrap. That's why I wasn't willing to spend thousands on a rebuild.

I was still left with that bug in my ear from his comment about different running temperatures. There are tons of articles online about converting a motor for marine use, nothing ever mentioned anything about tight bores.

The old 1970 250ci straight six that will be coming out of my boat had a 140° thermostat in it, this 350 I'll probably equip with a 160°. That's pretty close to car temps right there, which also adds to my belief that the guy at the speed shop was talking out of his ass.

 

[GA_Boater]

Once you change the core plugs a truck block becomes a marine block.

 

[mla2ofus]

More likely he was blowin' smoke up yours.
Mike

 

[dingbat]

I don't believe it.....temps between parts would be relative. Even If the marine motor ran 25 degrees colder, every thing would be relative to the temperature.

 

[MikDee]

More likely he was blowin' smoke up yours.
Mike

Err! What do ya think he was smoking?

By the way, I agree, a truck engine with stainless head gaskets, and brass core plugs, is a boat engine, Period!

 

[soggy_feet]

That's pretty much what I concluded.

And I was going to run it anyway, even if there was a difference. Just keep that temp gauge out of the red, but that applies to every engine ever made.

 

[Fun Times]

This is not quite the information your looking for but maybe you'll find some of the general information from GM interesting about marine engines they produce.

http://www.gmpowertrain.com/MarineEn...roduction.aspx

Also moving this topic down to Non-Repair I/O and Inboards.

 

[Howard Sterndrive]

you can look at the specs in the Mercruiser manual and specs in an automotive manual for the same engine, and you'll find basically all the same clearances specified.

 

[JustJason]

All of the clearances are the same. The only differenced between a marine shortblock and an automotive is the marine block has brass core plugs and a different cam profile. Everything else is the same.

~~reasoning that marine engines run cooler thermostats and under load the block won't get as hot

The combustion chambers, which is the guts of it, on a marine engine runs hotter than their automotive counterparts simply because a marine engine is a constant load engine, where an automotive engine is not.

. The heat range of the spark plug helps regulate those combustion chamber temps. Most automotive application 350's run R45's. Marine engines run R43's, which is a cooler plug, and helps to bring down the combustion chamber temps. If R45s were run in a marine engine, the valves and pistons would melt down.

 

[MBAKER]

If you are planning to use the used engine as is, if it has been run much at all it likely already has .001 wear in it.
If you are plannign to re-ring the used engine, by the time they hone it you will have at least .001 extra clearance.
If you are planning to get a block and bore it and put in new pistons just have the hone it .001 and forget about it.

While there is some truth in his theory in reality for a stock engine the clearances could be the same. If its a built engine that has been modified for more power then yes an extra .001 is probably called for. Many times a few thousandths extra piston to wall clearance actually helps make power, but this is on race engines where people are looking for 5HP at a time and will be freshened regularly.

The piston type will have a big role in it as well, some aftermarket cast pistons require different clearance than stock cast pistons, forged are completelly different yet. My thoughts.....If you put this together with factory clearance at a bare minimum and up to .001-.002 over you will never now the difference in a
boat....



On a side note.....that is another good reason to let your engines warm up before hammering on them, be it automotive or marine.

 

[soggy_feet]

All good information.

On another side note, this is going in a 40ft steel houseboat. A displacement hull in every sense of the word. Once I hit about 10 knots, it's about all she has.
I don't plan on pulling any water skiers, so I really don't expect to be hammering on the boat too much.

Well, I say that, but once I give the boat a shakedown with this new setup, I plan on opening it up just once to see if my top speed goes up at all. There's a chance with the shallow draft that I'm bordering on a semi-displacement hull and I'm curious. That's an expensive proposition to be running the boat like that all the time so I'll probably settle in with my 2400 RPM cruise throttle I've always used. About 6 knots, and it gets me where I'm going in one piece.

 

[HT32BSX115]

There's a LOT of boats out there in radio-land running wrecking-yard truck engines.

The boats don't seem to be able to tell the difference.....

A difference that makes no difference, IS no difference...................

 

[bruceb58]

The heat range of the spark plug helps regulate those combustion chamber temps. Most automotive application 350's run R45's. Marine engines run R43's, which is a cooler plug, and helps to bring down the combustion chamber temps. If R45s were run in a marine engine, the valves and pistons would melt down.

Spark plug heat range is for how the spark plug itself dissipates heat. Has absolutely zero affect on the combustion chamber temp.

 

[JustJason]

~~
Spark plug heat range is for how the spark plug itself dissipates heat. Has absolutely zero affect on the combustion chamber temp.

NGK must be wrong then. You should give them a ring, and see if you can get them to correct themselves..
http://www.ngksparkplugs.ca/tech-info-spark-plugs.cfm

 

[bruceb58]

NGK must be wrong then. You should give them a ring, and see if you can get them to correct themselves..
http://www.ngksparkplugs.ca/tech-info-spark-plugs.cfm

The heat range is controlled by the ceramic insulator size. It will control pre ignition. Maybe that is what you are thinking. Pre ignition will cause things to heat up if the explosion is too soon. If that's what you meant then I agree with you.

here is a better explanation from NGK
http://www.ngk.com.au/spark-plugs/te...ge-explanation

Basically the heat range is trying to get the tip of the spark plug to its optimum temp. If it's too hot, then preignition can occur. It's not soaking up heat from the combustion chamber.

 

[tpenfield]

I had a local speed shop tell me you can't use a truck engine in a boat because the clearance between the pistons and cylinder are greater on a marine engine, reasoning that marine engines run cooler thermostats and under load the block won't get as hot and therefore won't expand to the same degree as the piston, possibly causing the engine to seize.
He said the difference was .001" between bores.

I'm having a hard time believing that .001" is going to make a difference. Ever.
You still have oil transferring heat between parts, and you still have the clearance designed into the piston/bore fit.


Anyone have an opinion they'd like to add?

I think that is a very creative explanation thought up by the folks at the speed shop, despite its fictional nature. They should be commended for their imagination.

 

[muc]

Do truck engines have windage trays? Boat engines do. When looking for more HP, clearances do get adjusted. Does .001" make a difference? Yes it does, you can see it on a Dyno.