replacing engine

[Art1848]

1975 Slickcraft, OMC 225. Now I need opinions, Chevy 307 block is cracked.
307 blocks are hard to find. so it seems my options are,
A--find a 307 block, put in a marine rebuild kit and transfer parts
B-- get a 307 marine short block ( $1000 ) and transfer parts
C-- find a 350 block, and put in a marine rebuild kit and transfer parts.
D-- other?? open to any and all ideas,

If money was no problem it would be easy. I do have engine rebuilding experience, Thanks for input

 

[whitepointer23]

1975 Slickcraft, OMC 225. Now I need opinions, Chevy 307 block is cracked.
307 blocks are hard to find. so it seems my options are,
A--find a 307 block, put in a marine rebuild kit and transfer parts
B-- get a 307 marine short block ( $1000 ) and transfer parts
C-- find a 350 block, and put in a marine rebuild kit and transfer parts.
D-- other?? open to any and all ideas,

If money was no problem it would be easy. I do have engine rebuilding experience, Thanks for input

4 bolt 350.

 

[PAR]

All first gen small blocks will bolt in directly. You don't need a 4 bolt 350, unless you're going to also replace the drive with something that can tolerate a lot more HP. A 305 (2 bolt) will easily produce 220 HP without breaking a sweat and they're everywhere for cheap. The 350 is as well, though 4 bolts aren't quite as common.

A good (running), used 305 or even a 350 can be had for a few hundred bucks. Change the cam and other marine parts and drop it in the boat. I can get a 305 block for less than 100 bucks. I can get a 2 bolt 350 for about $100. If you go the 350 route, you'll need a new rotating assembly, as the 307 stuff isn't going to work, except the cam. The 307 wasn't produced for long and it's the old "small journal" block (2.45" crank). The 305 is the "medium or large journal" crank similar to the 350, but is weighted different, so can't be used in a 350.

I mention the 305 simply because it's the most common, cheap SBC replacement you'll find. All the external pieces will bolt on and no one really wants one, so they're cheap. You can even buy a crate 305 for not much money, say in the $1,500 range at full retail or about $700, from a local rebuilder. These (retail crates) will likely be the second gen SBC version, but your stuff will still bolt on.

Do some shopping at the local bone yard and see what you can find. Don't call them up, because they'll just say, "sure, we got plenty". Go in person and have a look at what they actually do have and remember, they have a lot less invested than they'll be asking for, so haggle them down to a reasonable price.

 

[Art1848]

Par, Thank you, I do not want to over power the drive, so My thought was, find a used 350 pre 76, use the crank and rods, get a rebuild kit with piston and rings .010 or .020 over ( what ever block needs to clean up) rebuild the 307 heads I have, and use them with the 4 barrel carb and intake from the 307. then the 307 exhaust. Rebuild kit will have Marine cam, bearings, lifters, HV oil pump, brass freeze plugs, and all gaskets. I will also use a all new SS bolt kit. Will the water pump, flywheel and starter from 307 go on the 350?

 

[PAR]

307 heads are the "accessory" run of 283 and 327 two barrel with 1.72" valves. You can use 1.94" valve 350 heads on a 307 with zero problems, but you will need the 64cc chambers and flat top pistons to keep any compression. If you use 307 heads on a 350 you'll probably run into compression issues because of the chamber volume differences, which can be addressed, but why go through this headache.

You need to understand what a 307 is. It's a 283 with a 327 crank. The heads work, but nothing to brag about and are fairly restrictive. The short stroke crank, isn't the best choice for a boat. Again, the 305 is commonly available, has a better bore/stroke ratio for a boat, though the 307 heads will bolt on, valve interference and compression issues can crop up. The 350 is a great engine, but easily can blow out that drive, with anything more than a docile cam and 2 barrel carb.

Since you're working with 307 parts (dead block), you only have a few choices, a new (used) 307 block which was only made from '68 through '73 (there's some exceptions), or using the innards for another 307 or a ******* 302/283. These engines (307's) aren't well regarded by most, mostly because they had small valves, small intake runners, etc.

The 307 water pump and starter will go on a 350 (and a 305), but the crank and flywheel will not. Simply put, as a rule all the "accessories" will bolt right on, from one 1st gen SBC to the other, but pistons, cranks and rods, which are the expensive items, need to be checked for appropriate size/length. This is why I recommend you get a long block 305 or even a 350. The innards will be assembled and fit, without figuring out how much head gasket thickness you need to get a reasonable compression ratio or if the valves will pound the picton tops, etc. on a 307 parts swap to a newer version of the SBC. The 307 is literally that last of the first gen SBC engines. The 350's appearance in 1968 was the "new" version of the 1st gen, with flanged crank, different journal sizes and a host of other changes from the 3 7/8" bore (265, 283 and 307) family of the 1st gen.

 

[whitepointer23]

Seems a lot of trouble to go to when you can just bolt in a 350.

 

[PAR]

There are a few bone yards near me and I can get a running first or second gen 305 for $400. This is a complete, running engine. Of course, it'll need the marinized parts swapped over, but you're literally good to go, depending on how much HP you want. If your 307 cam is still in good shape, just swap it out (takes less than an hour).

350's at these same bone yards cost more ($500 - 600), mostly because folks looking to do a transplant, typically want the displacement upgrade.

If you go for the 305, get the second gen ('86 or newer), which has the one piece rear main and the '87 or newer might have roller lifters, which is free HP too.

Don't get me wrong, I'm a 350 guy, as it's a lot easier to get more durability and performance from it, but given you'll be building a budget minded, relatively low HP SBC, the 305 can get it done cheaper. Hell, I've seen complete 305's for $100 in the newspaper (yeah, they still print these), usually from guys that have swapped out for a 350 and have a hunk of iron lying around to get rid of.

It's simply leg work, so call around and see what's available. Better yet, hit the local bone yards at 8:00 am on a Saturday, with a box of donuts in hand and see what some butter, can do for your bread (old trick, works good).

 

[Art1848]

Any engine I get will be pulled apart and get new bearings, rings, lifters, and if needed bored with new pistons, Couldn't just get one and drop it in, to much work to gamble on. I would like to stay in the 225-250 hp range, with good durability. Up here, Green Bay, Wisconsin, 350 seems easier to find than 305. but still looking, 0 on Craig's list. but a # of 350's. I have not looked into the hp ratings of the 350 or 305, going to have to do that next. Would torque also be something to watch? Also will the Driveshaft coupling from the 307 flywheel bolt onto the flywheel of 305 or 350? I see that using the block, heads, intake, from the same engine is the way to go, the 307 I have has a 4 barrel carb. If the replacement has a 2 barrel. Can a 2 barrel auto carb be changed to Marine use? It seems I should also change to a marine cam, would that be advisable. Thanks so much for all the help, I just want to cover all the bases before I start.

 

[PAR]

The 307 crank is different than the 305 or 350, so nope, you can't interchange the flywheels.

A first gen 305 is usually rated at 200 HP and the 350 about 225 HP. A second gen 305 is 225 and the 350 anywhere from 240 to 260 HP.

Torque is based on a lot of variables, but the 305 runs from about 250 ft. lbs. to 320 (in the usual variants), while the 350 can offer much more. This is one of the issues with the 305 and the 307; narrow and restrictive intake runs to the valves. The 307 head has some room to make adjustments to get more CFM, but the 305 doesn't, other than grinding off the "smog bump" in the entry of the intake port and slightly relieving the valve bosses.

Pulling apart a perfectly good running engine doesn't make a lot of sense, unless you'll be asking for more power. At 225 HP for a 305 or 260 HP for a 350, the engine just isn't working very hard and it's RPM range can't take much advantage of a bigger CFM intake or head passages. In other words, the 200 HP Crusader 305 is a 2 barrel and the 225 Crusader is a 4 barrel or TBI. There are some subtle timing differences, but I'm pretty sure they use the same cam. The same deal with the 350. A 225 HP 350 is a 2 barrel, while the 250 HP is a 4 barrel or TBI.

I guess what I'm saying is, if you have a 305 or 350 that seems to run fine, check the oil and just change out the parts you need to. A marine cam doesn't load up the engine a lot (260 HP or less), so renewing valve seals, top end gaskets, etc. would be all I'd do to a junkyard engine that sounded good, and looked good once you pulled a valve cover. You can tell a lot by the condition of the valve train, cylinder walls the smell and look of the oil, etc. No sense making more work for yourself than you need.

On the other hand, if you just want to be sure, so you don't have to tear it apart again, get a short or long block. You'll pay more, but it'll have a guarantee and the parts will be new. These motors are so plentiful that you can get good, rebuilt long blocks for less than the bother of doing one yourself.

 

[Barry]

Any engine I get will be pulled apart and get new bearings, rings, lifters, and if needed bored with new pistons, Couldn't just get one and drop it in, to much work to gamble on. I would like to stay in the 225-250 hp range, with good durability. Up here, Green Bay, Wisconsin, 350 seems easier to find than 305. but still looking, 0 on Craig's list. but a # of 350's. I have not looked into the hp ratings of the 350 or 305, going to have to do that next. Would torque also be something to watch? Also will the Driveshaft coupling from the 307 flywheel bolt onto the flywheel of 305 or 350? I see that using the block, heads, intake, from the same engine is the way to go, the 307 I have has a 4 barrel carb. If the replacement has a 2 barrel. Can a 2 barrel auto carb be changed to Marine use? It seems I should also change to a marine cam, would that be advisable. Thanks so much for all the help, I just want to cover all the bases before I start.

Don't assume that the larger hp will cause issues with the drive train AT NORMAL OPERATING CONDITIONS

If you cruise the boat at say 30 mph and 3000 rpm, the propeller at that rpm will determine what the engine has to produce.

ie a 305, 350 454 etc will all produce the same hp at the same 3000 rpm with the same prop

Certainly, if you firewall the throttle, a higher hp engine will produce a higher max at WOT and the horsepower through the drive will increase but who usually drives their engine at WOT

A rule of thumb for carb engines was 10 gallons per hour of fuel usage for every 100 horsepower produced over an hour

 

[Art1848]

Par, I was asking if the coupler that bolts to the 307 flywheel, will bolt on the flywheel of the 305 or 350. 2nd if I pick up a 305 or 350 with a 2 barrel carb can that carb be switched to marine use? Or should I look for one with a 4 barrel and use the 4 barrel I have?

 

[PAR]

The 307 and early 350 are two piece rear main blocks, but the cranks are very different, in weight, balance and flange type, which directly affects the flywheel configuration. Then there's the starter and tooth count issue. The 307 is a 153 tooth flywheel, with a straight across starter bolt pattern, while the 350 is a 168 tooth gear and the starter has diagonal mounting holes. To directly answer your question, the 305 and 350 use the same coupler, but I don't know about the 307, but I bet it does, assuming your have the right flywheel/starter setup.

An automotive carb can be converted, but it's not as easy as installing some J tubes. Intakes are cheap for the SBC, less than $100 at a swap meet and about $120 on sale at Jegs or Summit. These will be aluminum, with nice runners and ports, so the 4 barrel will actually deliver some HP. The stock iron intakes pretty much suck, unless you know what to look for. You'll also save about 40 pounds by switching to the aluminum intake too. I don't think I've ever put a stock intake back on any SBC engine I've rebuilt, except for a badass 302 in a Camaro some years back. It was an all original 302, with double bump heads and everything. One willing engine when completed. When that engine went south, I sold the steel crank and heads separately for 2k, that's how special they are.

 

[Art1848]

"The 307 is a 153 tooth flywheel, with a straight across starter bolt pattern, while the 350 is a 168 tooth gear and the starter has diagonal mounting holes."

So my 307 starter will not bolt onto a 350? Is that correct

 

[PAR]

Not necessarily, they often are drilled for both patterns, at least many of the first gen 350's are. Once they made the major changes in the mid 80's, the old straight across pattern was dropped, so if you get a pre-'86 SBC, it may have both patterns or will be drilled for one, with a center punch dimple for the other, that can be drilled.

You can also change the "nose" on the 307 starter to the diagonal bolt arrangement, which is a stronger setup too.

 

[Art1848]

One more, This boat has the OMC elec. shift stringer drive. The Seloc book says to unbolt the bell housing and mounting bolts and move engine 6" forward and lift out. Does the drive from bell housing to prop have to be supported? Doesn't the drive hang off the back of the engine with no transom support?