360 stroker, cylinder wall thickness

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Swede408

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Hi!
I'm new to this forum, and start with a small block engine question.

I'm building a 4" stroke LA 360 (408).
Unfortunately, the -71 360 block I managed to find here in Sweden was quite rusty.
It had to be bored to 4.060" to clear up.
I have read all about the early 360 having extra thick cylinder walls, but also that it all might be a myth.
The general advise people always get is to have the block sonic tested regardless of what year the casting is.
So... I did that, and got the numbers, but what is really considered safe?

See the attached picture ( I hope it's possible to read)
Minimum major thrust wall thickness is 0.165"

What do you experts say? Good or bad for a 408 stroker (actually 413) with aftermarket heads, about 10:1 compression, flat tappet cam, RPM dual plane intake... I aim for close to 500 hp.

/Magnus

IMG_4390.JPG
 
IMG_3062.JPG
I know certain 318's cant be bored out....not me personally, i found it this way.

IMG_3062.JPG
 
Could always sleeve that bore if it worries you.
 
273 said:
Could always sleeve that bore if it worries you.
Click to expand...

Unless you're talking wet sleeves (darton) that won't make the walls any thicker; just replaces the original casting with sleeve material

Cylinder Sleeves > Home

"Sleeves are available in 3/32” and 1/8” wall for bore diameters up to 5-1/8”. Bore diameters larger than 5-1/8” are stocked only with 1/8” wall thickness. Melling Cylinder Sleeves also offers 1/16” and 2mm wall thickness’ for limited applications."
 
IMO, the block will work for you. That's a good example of why these "special block" stories are a load of crap. The pass side core shifted leaving the results you have. It's thinner than I'd like, but the thin spot is up near the deck and gets thicker as you go down. It wil be fine.
 
Swede408 said:
Hi!
I'm new to this forum, and start with a small block engine question.

I'm building a 4" stroke LA 360 (408).
Unfortunately, the -71 360 block I managed to find here in Sweden was quite rusty.
It had to be bored to 4.060" to clear up.
I have read all about the early 360 having extra thick cylinder walls, but also that it all might be a myth.
The general advise people always get is to have the block sonic tested regardless of what year the casting is.
So... I did that, and got the numbers, but what is really considered safe?

See the attached picture ( I hope it's possible to read)
Minimum major thrust wall thickness is 0.165"

What do you experts say? Good or bad for a 408 stroker (actually 413) with aftermarket heads, about 10:1 compression, flat tappet cam, RPM dual plane intake... I aim for close to 500 hp.

/Magnus

View attachment 1715095580
Click to expand...
 
I am Currently Planning a New Supercharged 360 with Low 8.3 CR I have a virgin 71 360 and I am going to sleeve it , We considered an R3 Block as it was the more cost effective option but we were limited to W9 Heads , We wanted to use edelbrock Victors , 5mm is the min bore I read somewhere
 
moper said:
IMO, the block will work for you. That's a good example of why these "special block" stories are a load of crap. The pass side core shifted leaving the results you have. It's thinner than I'd like, but the thin spot is up near the deck and gets thicker as you go down. It wil be fine.
Click to expand...

That's good to hear!

Even if my block had been cast perfectly centered, I still wouldn't have more than 0.18" on cyl #4. With stock 4" bore, this would have been 0.21" thickness. With the core shift, my block had 0.195" wall on cyl #6 when new.

I could turn the question around, If I start with another block it will most likely be a newer casting (old 360 Engines are not very common over here) that will be fine wth a 4.030" bore, what are the odds the bores will actually be any thicker? How many stock bore 360 blocks have more than 0.18" walls in the thinnest spot (thrust direction)?
 
Assuming the sonic tester is accurate, my personal decision for street any even drag racing use would be to use it since 1.2 HP per cubic inch is not highly aggressive. You have > 85% of the minimum original wall thickness in the major thrust area, and your minimum thickness now is about 75% of the average wall thickness. And you have > 80% of the minimum original wall thickness in the minor direction. Plus, the minimum thickness you have is 30% thicker than the thickest sleeves commonly found for bores of this size (1/8" thick).... (although sleeve material is stronger).

But I might decide differently if I was going to race it on circle tracks at high constant revs and high constant HP.

And your question about getting a different block is a good one...with .015' thicker walls in a .030" overbore, you could well end up with about the same minimum thickness in one spot.
 
This story is still not finished...
I ended up not using the block after all, since I already had the 4.030" pistons and eventually gave up trying to get them exchanged for 4.060".
Instead I found a new block. This time a "pre Magnum" from -89. I also consider the roller cam a bonus.
To make sure it was OK, I did the same measurement as previously done on the -71 block.

I also found it very interesting to check if the old myth actually was true, were the -71 - -74 blocks really cast with thicker walls??

The results can be compared in the pictures. Keep in mind that the -71 block was already 4.060" bore when measured, and the -89 block still stock bore, so to compare, the -71 figures must be increased by 0.03" or 0,76mm

-71.jpg


-89.jpg
 
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That's why I call them all "myths". The factory's intent was to have some thicker for certain reasons (cold weather 400 blocks...) and some people guess because the casting was similar that it must be the case that some are thicker. With the "230" 400 blocks the casting quality hurt the resulting block castings. And on the 360s I'd wager that the first few 360 blocks were thicker because they used the 340 block cores... Right up until the QC guys figured out why the base iron was getting used up faster than they expected. Then it get cut back - but the myth was born.
 
Thanks you for the data and all the work Swede408. That is very interesting and useful info. They are hardly any different once you equalize them to the same bore.
 
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