Cylinder wall thickness

[oldseddie]

I am building a srtoker for my '91 Comanche. I first aquired a '97 4.0 and had it sonic tested. It was clean original bore and could have gone back together with just new rings and bearings. It was very thin even without much core shift. I heard the renix blocks were heaver so I aquired one of unknown heritage. I have had it tested and it is thin as well. Are some years better than others, or are these blocks just that thin?

[1bolt]

I haven't found a 4.0 block that came in over 0.250 (a quarter of an inch) yet. Renix included... in fact the two 4.2L blocks I looked at were thinner than the 4.0's (in spots much thinner).

I have checked most major block revisions, 2000+ NVH, 96-99 NVH, Renix...

They all had very little difference... Oh and they all came in as Iron with high nodularity (a good thing, and one reason our I6 blocks rarely crack and always last so long). Also roughly the same nickel content, or the sound velocity would have been different between blocks... This goes for old AMC 4.2 blocks as well.

[Cheromaniac]

I've read somewhere that the minimum cylinder wall thickness should be 0.180" on the thrust (cam) side and that's the reason why it's not recommended to overbore a 4.0 block beyond 0.060".

[1bolt]

probably Mopar Jeep Engines, I seem to recall the same number. Mopar also said that a block with core shift to the major thrust side is actually desirable over a block with no core shift at all. The extra meat on the major thrust side more than compensates for less meat on the other side, and keeps the bore more perfectly round during the power stroke, leading to less friction and higher power output.

Guess why I was sonic checking blocks? I haven't found one yet, but at the rate that I accumulate free I6 cores I hope to eventually find one.

I know I'm repeating myself but I found it very remarkable how little each block I checked varied from one to another... And a little worrysome how many thin spots I found lower in the bore, especially on the 4.2... One .030 overbore 4.2 I checked actually had spots that were 0.15 If I wasn't already anti-4.2 this would make me that way. At one point these thin spots had me doubting the hardware, but its an old Military grade rack mount piece from back when they cost ten thousand dollars a pop, and it checks out on everything, including accessible parts of the same 4.2 block that can be checked with a micrometer and the sonic gauge in the same location.

I've been meaning to do a write up as I accumulate measurements but honestly the sameness between the blocks keeps making me loose interest... Every once in a while I see 5-90 (Kelly) over on NAXJA say that Renix blocks are thicker and made of better iron and that motivates me to do a little more research, but I don't have the heart or the free time to argue with him and so I loose interest again.

In my opinion the NVH blocks are unquestionably the best I6 blocks Jeep made, they are the same bore thickness, same high nodular iron alloy or very nearly so (based on sonic velocity without a metal assay) but with visibly improved webbing on the main bridges and crank skirt.

[oldseddie]

I have a friend that builds Nascar engines locally that is doing the machine work for me. He said he liked to see .250 minimum to build a performance motor as the cylinder walls bulge when under pressure causing ring seal loss if the're any thinner. I don't intend to run the cylinder pressures of a nascar small block but I don't argue with him about such things. I've been looking but I'm not sure that Jeep made them that thick. Anybody got a torque plate they want to get rid of cheap?

[Alex22]

I've taken apart engines that have been running fine with a cylinder wall that is only .020 to .040 thick the entire bore while all the rest will be .100 to .200 thick. 1Bolt, I'll try to remember to get the ultrasonic checker out at work next time and check my block out (98 out of a grand cherokee). Not many engines come with anything close to a 1/4 inch thick cylinder wall and as you said, there is nowhere the amount of cylinder pressure or power of a NASCAR engine.

~Alex

[1bolt]

Alex man that's more or less 1/32nd of an inch (.031 more or less) maybe you're confusing the amount the engines were bored over, for the actual cylinder wall thickness...

[Alex22]

I'm sure of the measurement. I sonic checked that Ford FE block myself about 2 months ago. What really did that engine in, on top of a bad casting was the fact that it was a boat engine cooled by the water it happened to be floating in. The block was junked, a sleeve wouldn't have held. I forgot to check my 4.0 today, I'll have to write myself a note.

~Alex

[Alex22]

I sonic checked my block today. The thrust surfaces were .220 to .240 thick and .110 to .130 thick on the sides of the bore that the wrist pin ends face. I'm going to assume that the bores are thinner between the cylinders to allow for more coolant flow. I used a Dakota Ultrasonics PB-8 2. For a stock application you would want the minimum thrust surface would be .090 and for a race engine you would want at least .140. Some of the other guys I work with have seen engines running fine with .050 thick walls.
Hope this helps your research 1bolt.
Casting # 53010341 and its out of a 98 grand cherokee.

~Alex

[1bolt]

Very interesting sounds like you have a thin walled block with core shift to the major thrust side.

[Alex22]

I should rephraise that, both the thrust side and the side opposite it are about the same thickness (.230ish)

~Alex

[oldseddie]

I found the sonic test sheet on my better block and it is very simular. I have around .230 average on the sides and the thinnest measurement in the ends is .129. I don't want to bore it any more than necessary to clean up the bores, but I wanted to do a 4.7 stroker. I would rather have a good dependable engine than maximize the potential displacement. Cylinder walls as thin as these call for the use of a honing plate. Anybody have one they want to part with?

[Alex22]

The cylinder boars don't appear to be torque plate honed from the factory. If they were when the boars are measured on a stock engine the bore should be out of round at the top when there is no head/torque plate on it. One way to check would be to put the engine on a stand and torque the head down, flip it over then put the bore gauge in to check if its out of round. I'm either going to buy a BHJ or a Hesco torque plate when I do mine.

~Alex

[1bolt]

I should rephraise that, both the thrust side and the side opposite it are about the same thickness (.230ish)

~Alex

No my fault I misread thrust sides for major thrust side.

[Alex22]

What ultrasonic checker are you using and did it give you a value when you calibrated it to your block? Just curious. Mine was 1850 micro seconds per inch.

~Alex