Engine Knock on a 1950 8ba

George/Maine said:
From what I have read this guy doesn't know how to read a scale.
Top of dome to deck is 3 7/8", 4" stroke minus 1/8 drone height = 3 7/8"
I think one of the new goodie has to go loose the pistons. get flattops.
Click to expand...

As above. it's a little more complicated than just reading a scale. The key term here is "OFFSET ground".
 
The thread is getting hi jacked lets find out what the findings are for the original post and not who knows what about pin heights and off set journals that can be a new and separate thread. Just sayin!! :)

R
 
Ronnie said:
The thread is getting hi jacked lets find out what the findings are for the original post and not who knows what about pin heights and off set journals that can be a new and separate thread. Just sayin!! :)
R
Click to expand...

I agree. I just wanted to explain how a 3 7/8" stroke was possible. I can't add anything further, so I'm done.
 
The way the stroke is measured with dome piston.
TDC how much piston is above deck say .125,
Now bottom of stroke is 3 7/8"plus to first measure.
3 7/8" plus .125 =4" stroke.
I think its just a simple engine and reading more into it.
 
tubman said:
Think about it again, more carefully. With the piston/rod/crank at TDC, the distance from the top surface of the crank to the deck surface is exactly the same as stock. The extra 1/8 of stroke is picked up when the combination is at BDC. A 4 inch Merc still has the large rod journals, requiring a different piston. Remember, the 1/16 inch is taken off the BOTTOM of the journal (toward the center of the crankshaft).
Click to expand...

I think you better think about it again. When you offset grind, you are changing the centerline of the pin. Remember the small rods were on a 1.99" journal and the offset ground pin is 2.139" dia. journal. It is impossible to only make a difference at BDC. Must be the new math.
 
Yep, changing the stroke happens at both top and bottom unless you change the centerline of the crank and I don't think that would work out too well. A 1/8" stroke change is 1/8" at both top and bottom (should have wrote "overall" since that's what I meant, see post #48 for clarification). Long on top & short on the bottom.

The French manufacturers of the SUMB army truck engines made some shorter con rods that would drop the piston height some but I don't know if anyone makes pistons for a 3 7/8" stroke.
 
Last edited:
Ho hum are we done yet!! :D
 

Attachments

  • offset grind.jpg
    offset grind.jpg
    21.4 KB · Views: 33
I think where folks get confused is the difference between how much the stroke will increase on the top of the stroke but they can't see that you have to add in what happens on the bottom too. A 1/4" stroke increase on a crank like the 49 through 53 Mercury 4" total stroke which is actually 1/8" on top and 1/8" on the bottom. The Simca Marmon 255 CID engine used the 6 7/8" con rod so that both the 239 and 255 engines could use the same piston. I guess they figured it would be better overall for parts supplies since the con rods generally last longer than the pistons. That is, as long as they don't put 6 7/8" rods on a 3 3/4" stroke crank. That sucker might be a tad on the weak side.
 
rotorwrench said:
Yep, changing the stroke happens at both top and bottom unless you change the centerline of the crank and I don't think that would work out too well. A 1/8" stroke change is 1/8" at both top and bottom. Long on top & short on the bottom.

The French manufacturers of the SUMB army truck engines made some shorter con rods that would drop the piston height some but I don't know if anyone makes pistons for a 3 7/8" stroke.
Click to expand...
An 1/8th stroke change is 1/16" at the top and bottom. 1/16 + 1/16 = 1/8.
 
use a piece of tubing to ear,isolate knock,if in rear, the bellhousing is like a "bell" magnafing noise. before tearing engine down- check bottom flywheel cover-bent housing- or pressure plate arms. for contact, diafram presure plate is thinner and allows clearance .leave transmission bolts a little loose don't start. but turn over slowly to check contact, then tighten and turn over to check contact. could save a teardown. keep cool head.
 
If this crank is 3 7/8" most likely done years ago. The guy building it gets the Speedway catalog and orders all the speed parts and the added dome piston hits the heads. A hot rod from the past. I would recheck the stroke and be sure of it 100%
 
I don't think this is an accidental 3-7/8 stroker incorrectly assembled.

"It runs like a clock but has an annoying knock from the rear"

One assembled up as people have been describing probably wouldn't even turn over, let alone run like a clock.

Mart.
 
Guys relax. Stroke was measured through the head at tdc to bdc with a depth gauge. 3 7/8" is what we measured on the cylinders. Pulled heads and there were 2 copper head gaskets!!! Pistons stuck out of the block 1/8"!!! With only the cylinders 1-4 hitting we miked top of block to top of pistons on each of the 8 cylinders. They are a match so no one milled block more on one side. We cannot find the Merc dimple on the crank. Only said it was a 48 merc crank because that is what we were told. The rods are marked 8ba. Pistons are pointed dome +.030 4 ring pistons. This engine was an assembled crate engine from a speed shop in Indiana. That is all we know as the original owner has deceased. I have researched the net and found that some guys, although is not correct, would grind the 3 3/4" crank and use stock rods creating massive compression although they would have to use double head gaskets that eventually blew at high rpms. This is a bummer and it is only what I have researched. We know one thing though, that is what we have. We are thinking about using stock head gaskets and double them up with a .0625 steel shim (identical to the layout of the headgasket) between. It certainly would be cheaper than a set of stroked pistons and the metal to metal between gaskets would not be any different than block/gasket/head. This engine is not going to the strip and will only be a show runner. Should work. What do you say Ole Ron?
 
If you have an eighth of an inch proud of the deck with piston at TDC and 3 7/8" from deck to piston top at BDC then you have a 4-inch stroke crank with 3 3/4" stroke Ford pistons. The Mercury crank in 1948 was the same as the Ford crank. In 1949, the Mercury 255 came along with the 4-inch stroke crank. It is part of the 8BA family so the crank is drilled for oiling of shell type con rods. The 48 & earlier long snout cranks have only one oil hole for each con rod crank pin where the 8BA family of engines have two oil holes per crank pin. All the earlier engines used the floater con rod big end bearings.
 
How did you measure stroke through the head with a depth gauge???
Should be with head off!

R
 
I think the best thing to do is get a crank to match the pistons.
Sticking up to high most likely 255 piston so get a 255 crank and start over.
This engine was done years ago how knows 50 years ago. And the builder don't know what to do and put 2 gaskets.

On second thought get a stock 239 crankshaft 255 would put piston even higher.
 
George/Maine said:
I think the best thing to do is get a crank to match the pistons.
Sticking up to high most likely 255 piston so get a 255 crank and start over.
This engine was done years ago how knows 50 years ago. And the builder don't know what to do and put 2 gaskets.
Click to expand...

I agree with the new crank idea, but 255 pistons would not cause this.
 
With high compression heads don't think the need for dome pistons is good put flattop and get it over with. OR put stock heads. that you may not have.
 
You must log in or register to reply here.
Back
Top