How do I fix this??

Brian

Brian

Well-known member
You may recall a while back I posted about issues with the main bearings on my Scat 221 4" stroke crankshaft. I fixed that little problem, posted on here, advised Scat; they didn't want to know!
Well, during a mock up assembly yet another problem has surfaced. I honestly don't know how to fix this.....Look at the photos; these are shots of numbers 1 and 5 cylinders. No, the rods are not bent; they are NOS 21A conrods. The crankpins on the crank have been ground on a different centre to the cylinders, consequently, the rods are hard over one side. I cannot run it like this; what to do?
I know what I'd like to do with that crank, and it wouldn't be pretty!!
 

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Gary, on Long Island, only chooses Eagle crank, and Scat rods. Only, other wise too much modification for clearance, and balance. Sorry, you are in a jam.
 
"Is Scat one of those Chinese made cranks?"



Definition;
scat3
skat/
noun
noun: scat; plural noun: scats

  1. droppings, especially those of carnivorous mammals.
    "fresh bear scat"





Origin
ggAAAABJRU5ErkJggg==

1950s: from Greek skōr, skat- ‘dung’.


Which means it is ****!!
 
Hi Brian, difficult to tell from the photos without actually having the unit in front of you but it appears the crank is not located correctly front-to-rear in the block??

If you look at all the rods are they all "off" in the same direction, is it right and left banks, or is it random??

The "fix" to at least that problem to make it work isn't that difficult but the rods and piston must be separated. You could "side-grind" the pin ends. We do this occasionally on the belt sander! You just need to be real careful not to "upset" the pin bushing!

Thanks, Gary in N.Y.

P.S. What exactly was the main brg issue??? We don't use Scat cranks (for other reasons) but we do use their rods. This isn't your issue though.
 
GOSFAST said:
Hi Brian, difficult to tell from the photos without actually having the unit in front of you but it appears the crank is not located correctly front-to-rear in the block??

If you look at all the rods are they all "off" in the same direction, is it right and left banks, or is it random??

The "fix" to at least that problem to make it work isn't that difficult but the rods and piston must be separated. You could "side-grind" the pin ends. We do this occasionally on the belt sander! You just need to be real careful not to "upset" the pin bushing!

Thanks, Gary in N.Y.


Why do you think this happens? Crank or pistons?

Lonnie
Click to expand...
 
Thanks,
Gary; to try and answer your questions; There is a definite pattern, inasmuch as all conrods are closer to the rear of the piston boss. The worst one is as pictured, cyl's 1 & 5, where the clearance is 0.020" at the back. How much is the least amount of clearance I could get away with? I could ,as you suggest, grind a bit off of the small end, but, Geeze, I shouldn't have to!
I think I can get away with all the other cylinders; minimum clearance on the rest would be 0.080".
Lonnie,: this is definitely a crank issue.
Incidentally, the pistons I ordered as specials from Ross, 4" stroke 3 3/16" bore flat tops. Same as 49-53 Merc but with flat tops not domed. It wasn't until I went to do the initial mock up, that I realised they hadn't machined inside the gudgeon bosses, and the conrods wouldn't fit. I was at work until midnight last night machining the required clearances. I notified them of their oversight, they said 'sorry'. At least that is a [ever so slightly] better response than I got from Scat when I told them about the main bearing issue. I need to follow Tony's suggestion from that earlier thread, I am over building these old things!! Too much agro, things were so much easier when I was young and knew nothing!!
 
Why do you think this happens? Crank or pistons?

Lonnie
Click to expand...

Difficult to tell at this point? I would suspect the crank, but it's not really fair to without actually measuring it! Could be a whole bunch causes or it could simply be "stacked-tolerances", this happens when the mfr's make the pieces!

Brian said:
Thanks,
Gary; to try and answer your questions; There is a definite pattern, inasmuch as all conrods are closer to the rear of the piston boss. The worst one is as pictured, cyl's 1 & 5, where the clearance is 0.020" at the back. How much is the least amount of clearance I could get away with? I could ,as you suggest, grind a bit off of the small end, but, Geeze, I shouldn't have to!
I think I can get away with all the other cylinders; minimum clearance on the rest would be 0.080".
Lonnie,: this is definitely a crank issue.
Incidentally, the pistons I ordered as specials from Ross, 4" stroke 3 3/16" bore flat tops. Same as 49-53 Merc but with flat tops not domed. It wasn't until I went to do the initial mock up, that I realised they hadn't machined inside the gudgeon bosses, and the conrods wouldn't fit. I was at work until midnight last night machining the required clearances. I notified them of their oversight, they said 'sorry'. At least that is a [ever so slightly] better response than I got from Scat when I told them about the main bearing issue. I need to follow Tony's suggestion from that earlier thread, I am over building these old things!! Too much agro, things were so much easier when I was young and knew nothing!!
Click to expand...

Hi Brian, it really would be nice to know the EXACT cause and not guess at it, but that's difficult right now! Easiest way to check crank would be to compare it dimensionally to the original, not all that difficult. I would say maybe the pistons but not likely because chances are it would not be a one-sided issue, it would be where the rod-eye was tight on both sides!

On a side note, this is an issue that wouldn't even be noticeable with the H-beam rods, the rod eyes are a bit narrower than the OEM ones!

Anyway, "Good Luck" with your fix, should be OK in the end!

Thanks, Gary in N.Y.

P.S. About 15 or 20 years ago we got an $1800.00 4340 Lunati BBC stroker crank in the door for a build. When I first removed it from the box the rear thrust flange "wall" caught my eye, it looked overly thin. Layed it in a block with some brgs and EVERY counterweight side was hitting the main webs, crank was too far forward in the block. Lunati asked no questions (we knew Joe really well), sent a replacement with a "call-tag", and took care of it all. The crank was definitely "defective"! The rear thrust is the "datum point" for the crank machining, it determines where the rest of the crank sits front-to-rear in the block!
 
Brian , Mike at H&H built an engine similar to yours earlier this year using the same crank as you. I believe he used the Scat rods also & that would possibly overcome the situation you have right now. He did tell me that a few other mods had to be made to the crank to get it to work in block . I am not sure what but did tell me if I purchased one of these cranks he would tell how to make it work & that it was no big deal. Maybe it would be worth a call to Mike .
Cheers
Tony
 
None of the conrods are centred in any of the pistons. The worst 'set' is as I pictured, with only .020" clearance. I still have the milling machine set up from when I machined the piston bosses, so could take a bit more off of the two pistons on one side, alternatively could take a bit off of the side of the small end of the conrods. Which would be preferable? I ask again- what is the minimum side clearance required?
 
Brian said:
None of the conrods are centred in any of the pistons. The worst 'set' is as I pictured, with only .020" clearance. I still have the milling machine set up from when I machined the piston bosses, so could take a bit more off of the two pistons on one side, alternatively could take a bit off of the side of the small end of the conrods. Which would be preferable? I ask again- what is the minimum side clearance required?
Click to expand...

Hi Brian, on higher HP units you would want .060", I believe you could get by on the Flatheads with a .040" minimum.

Having said that, if you're going to make a correction just take it right to .060".

Thanks, Gary in N.Y.

P.S. During the course of working with almost ALL crank mfr's, from the budget pieces (Eagle/Scat, both Chinese) up to the "Made-in-the-USA" high-end ones, we see many pitfalls with most. Recently had one hell of a time balancing a fairly expensive Crower shaft. Ended up welding our own flange to the existing flywheel flange on the Crower. I called out there and really complained, they didn't care at all and said "if you don't want the crank send it back". Definitely not fair to the end customer??
 
Hi Gary, Thank you. It is morning here, I've thought on this and decided I'd machine another .040" off the one boss of each piston to give me a .060" clearance. Just came on here and you've verified that figure, so thank you!
Knowing now what I do, I'd have been better off in all respects if I'd purchased a genuine 49-53 Merc crank, and machined it to suit my early block application. Hindsight, ain't it great?!!
 
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