4.3L Broken Crank Questions

[Sugo]

I have a 4.3L Mercruiser with a cracked block (Not winterized correctly by the previous owner). I purchased a 4.3L motor with only 150 hours on it with a broken crank. I inspected it upon purchase. The only damage appeared to be to the crank and 1 piston. The block had no marks or noticeable damage. The cylinders have no ridge, no marks and everything is very clean. The crank bearings even looked good. My plan is to swap over the crank and 1 piston. Two questions. Could a crank break without causing any damage to the block? The second question: Since the replacement motor is in such good condition and the same year as my 4.3 can I just replace the bearings swap over my crank and 1 piston with new rings (From the same cylinder #) or do I need to swap over all my pistons? Thanks for any input

 

[dan t.]

Re: 4.3L Broken Crank Questions

Either way, if you swap all the pistons and rods you are guaranteed that the balance matches , you will be keeping all the moving parts together.

 

[achris]

Re: 4.3L Broken Crank Questions

When you swap pistons into another block you MUST check bore clearance and ring end gap....

Chris.......

 

[Fordiesel69]

Re: 4.3L Broken Crank Questions

You need to swap ALL the pistons, rods, and crank from the cracked block engine to keep the balance.

The broken crank can be from hydrolocking (check the rod for a bend) and also from a poor crank casting. It may have not been a forged crank which is fine BTW. It happens. Could be from a block that is not line bored correctly at the factory. Assemble all the mains and check for proper alignment between the main webbs.

 

[Sugo]

Re: 4.3L Broken Crank Questions

Thanks for the input guys. When I am checking the ring end gap am I doing this to determine if the rings need to be replaced ? Is the alignment between the main webs referring to checking the clearance on each side of the mains with a feeler gauge once the crank is torqued down to ensure they are the same?

 

[joewithaboat]

Re: 4.3L Broken Crank Questions

I think you need more than you will get post by post here as far as re-assembly, best to get a assembly manual or book of some kind. Also if you have a truely low hours motor the undamaged pistons will be in better condition than your old stuff. I would buy one new piston and rod for the newer combo. That is IF the other 5 are undamaged. Any good automotive machine shop can check your piston rod combos for weight match.

One key thing to check with newer block is mains as mentioned above. Take your old crank and main bearings, clean them, oil them and put in the new block torque to spec and spin it. It should spin pretty smooth. You can use your old crank as is if not too worn, I would get new bearings.
Good luck!

 

[dollarten]

Re: 4.3L Broken Crank Questions

when you are checking the side clearance on the mains , you are checking the thrust bearing clearance . There is only thrust bearing be 4 6 or 8 cylinder . It just keeps the crankshaft centered in the block where is is supposed to be After you have the crank torqued to the proper spec and you can turn the crank freely , your main bearing alignment should be OK . Line boring comes into play for real high end race engines and also where an engine has a spun main bearing and the bearing housing got hot and distorted . It is more than likely a bad crankshaft and the rigors of marine engine life was more than it could have handled .................... almost forgot your fist question , checking ring end gap is a way of measuring ring wear More important is checking the cylinder wall for wear and the section that wears the most is just below the top ring area because this is where the piston exerts the most force on the cylinder

 

[Sugo]

Re: 4.3L Broken Crank Questions

All great info, thanks. I do have a good shop manual, but in my experience if helps if I understand the basics before I get into step by step in the manual. Some times the manuals are not an easy read.

 

[joewithaboat]

Re: 4.3L Broken Crank Questions

I would also get the new block magnifluxed and or black light checked to look for hairline cracks in the main webs. Check that the caps snap in place well. Its not common to break a crank, is it hairline cracked or "broke"? Not sure if bad machine work caused it to break and then messed up a rod and piston or did it get water in one cylinder... bend a rod and break the crank? Either way the block needs a good looking at.

The ring end gap that is important to you is the new assembly even if you are using used parts. If you have too little end gap, esp in a boat, the pistons can seize up. You put the ring only in the cyl ,squared up, and measure the end gap with feeler gauges. Best to use mercruiser marine specs here. Any good manual should show this, if it does not I would use a differnt one.

 

[joewithaboat]

Re: 4.3L Broken Crank Questions

Oh, and you should not use new rings unless you have the cylinders lightly honed. You should re-use the remaining 5 pistons and rings unless they are damaged, kept in the same holes. Replace one piston, rod and ring pack. Best to lightly hone that clylinder as to get proper ring seating. You wont get good results from old rings in the other motor being reused if you swap all the other stuff to new block.
my.02

 

[Sugo]

Re: 4.3L Broken Crank Questions

I don't know how the break came to be as I purchased the motor with the crank broken already. The crank is still in the block with the back 4 piston still attached. The break in the crank is clean and is located ant the rear of the journal where the front two connecting rods attach. The only visible damage is to the one piston. (The lower half inch of the skirt is broken off one side of the piston.) The bearing on the front main does not even appear to be flawed in any way.
If I only replace the one piston, will it be out of balance? Is the number stamped in the top of the pistons have to do with the balance. (I did noticed these stamped numbers were the same for some pistons and different for others.) Would it be better to use the complete set of pistons with new rings and lightly hone all the holes?

 

[achris]

Re: 4.3L Broken Crank Questions

What are the numbers on the tops of the pistons? They could be oversize numbers....

 

[dollarten]

Re: 4.3L Broken Crank Questions

unless the numbers are .020 ,, .030 and higher would indicate oversize piston .It could be some kind of prodution number used in the factory . Yes you can just replce one piston . It will not affect the balence as they a very close . Just make sure that you uses the right piston as regarding the year of the engine so you will have the same compresion ratio . I;e; same valve reliefs , top dimensions of piston(s)

 

[Sugo]

Re: 4.3L Broken Crank Questions

The numbers on the pistons are .030 Since the pistons in my old motor are not oversize can the difference be made up using different rings if I am going to swap all the pistons over or am I going to have to replace the damaged oversize pistons? (turns out to be 3) I think I am going to take the block to the machine shop to confirm the condition before considering much more investment. I spent $200 purchasing the block (I should have educated myself a bit more before going down this road) I have been looking for a good 4.3L replacement for a few months without any success. A new short block will cost me $2400 + 12% tax and a long block is $3000 + 12% tax. The boat is a 92 so I was hoping to avoid overspending on an old boat that's why I am hesitating on the new motor option. The block will probably cost me $100 to get checked. Should I continue down this road or should I toss the block in the recycle bin and start from scratch?

 

[joewithaboat]

Re: 4.3L Broken Crank Questions

No you cant make up the difference with larger rings. Im sure you can get .040 pistons and have it bored and honed again if it is good shape. Always a chance you could just have it honed a little and buy a new set of .030 pistons, if it wasnt run long. Are engine cores that hard to get in Vic. BC. Your tax sure stinks there. Good news is they should be the least exspensive pistons in the market, they are the same as 350 chevy.

Dont forget you have a host of other things that might have freeze damage from your original set up.
Good luck!

 

[Sugo]

Re: 4.3L Broken Crank Questions

Upon further inspection, my pistons are oversize .060 so I will take the block and get it tanked, they can check it for cracks and machine the cylinders from .030 to .060 then I should be in business.View attachment 119972View attachment 119973

 

[Sugo]

Re: 4.3L Broken Crank Questions

I just realized that the block has a spot for the mechanical fuel pump but there is no hole for the fuel push rod through to the cam . Would I have the machine shop drill that or is there an aftermarket electric fuel pump. What would probably be a more economical route?

 

[zbnutcase]

Re: 4.3L Broken Crank Questions

4.3s are such thinwall castings I would NOT run over a .030 oversize, especially in a highly loaded marine app. Bores will flex, leading to chronic oil consumption-or worse. Also 4.3 dampers ARE NOT the same as a V8; they are balanced differently, and if replaced with a V8's, crank could very well break. Only have seen 2 cast Chevy cranks break, but both were caused by damper issues.

 

[joewithaboat]

Re: 4.3L Broken Crank Questions

I just realized that the block has a spot for the mechanical fuel pump but there is no hole for the fuel push rod through to the cam . Would I have the machine shop drill that or is there an aftermarket electric fuel pump. What would probably be a more economical route?

Its not a simple drill bit type operation, it will need to be done on a mill and reamed or honed to final size. Not just any shop will be able to perform that one. Prob. best to just use an electric pump.

Im not sure i would run off and bore it to .060 just so you can use a used set of pistons. I would have the bores looked at and see what they look like. Maybe buy a new set of .030 over or jump up one size to .040. I have no specific knowledge of that block, which is what it will take to tell you if .060 is ok to run in a marine application. If you are using a performance type shop the may have a tester to measure cylinder wall thickness.

 

[Sugo]

Re: 4.3L Broken Crank Questions

All of your comments have potentially changed my course of action. Thanks for all of your input. I am now considering another option. A guy that works at the parts store has a 4.3L short block for sale for $100 (it was in a truck) I do understand the differences between automotive and marine however if I am only using the short block and swapping over everything else I don't see a problem. I was going to hone the block and replace the rings and bearings. I am assuming I can use the automotive short block as the part #'s for the rings and bearings seem to be the same for marine or automotive on this block - casting # 267 I could even swap over my cam and lifters as well. With new bearings, rings, seals, core plugs and all the rest, this option solves my issues for less than $500 Anyone have any input? Thanks