I have a C2AE short block to rebuild, and have read references to sonic testing for cylinder wall thickness. I would like to take it to as large a bore as is possible. There is a local shop that sonic tests blocks for around $50. I found out that sonic testing gives a general assessment of wall thickness, but not absolute. The reason being that relatively few points are tested in each cylinder, like say 8-10. So if there is a design characteristic resulting in a particular thin area, chances are it wouldn't be found, unless the testor happens to be familiar with the block he is examining.
So after all that, the question is: "is there an area(s) in one or all of the cylinders that should be especially examined"? Mike

 

What you are looking for is core shift. When it happens it will be equally displaced throughout that hole so 8-10 readings should be sufficient. If you find 1-2 holes that are thin you can always sleeve them and bore out.

 

My machinist explained that uniform cylinder wall thickness may not be fact, even in one cylinder wall. He used the example of a 400 block with one particular location where cylinder wall thicknesses were always thinner by up to .150 thousandths due to designs. I was wondering if there are similar areas of concern in the Y.

 

The 400 SBC is a case where the existing cores were modified in an attempt to produce a quick solution; that is, Chevrolet needed a high torque engine for heavy sedans & wagons, without using the 396 BBC (for a variety of reasons). The bore spacing was essentially at its limit before the bore increase, thus the siamesed walls. This engine had some overheating and head gasket issues as delivered from Chevy, all of which have been thoroughly addressed in later years. The SBC is considered a "thinwall" casting design, though that term wasn't used then.

The Y-block, on the other hand, is in no way a thinwall casting by any stretch of the imagination. The cyl. walls tend to be very uniform, at least on the ones I've seen. 286Merc has done several more than I have and might have a slightly diff. opinion. As long as the block in question looks good in terms of core shift, the walls should be "reasonably" uniform in thickness. As I mentioned in the other block post, you need to look most at the major thrust faces and their dimensions. Also, keep in mind that sonic testing isn't incredibly precise; machinist skill is a big factor; also, depending on who you ask, casting slag and rust/corrosion affect things.

Again, truthfully, if you're not intending to use the N2O fogger kit between every stoplight, it's not likely an issue for you. If you want to really increase the cubic inches, you'll have to alter the stroke. The practical limit on a Y seems to be around 350 c.i., give or take a few, and generally a little less. You reach a point of diminishing returns after a while, and get into custom pistons (expensive) for the big bores. Not trying to discourage you- I just wonder if you are really going to be able to tell the difference between a .060 overbore and a full "bore-until-the-block-walls-are-as-thin-as-frosted-glass" on a engine used for the street.

Hope it helps, Mike

 

Homespun, thanks again, and also thanks to 312 and 286 merc. I appreciate the reality check. It is the type of suggestion that I have given to others and forgot myself. The motor will not be a lot of HP. It would just "Be all it can be" short of stroking and port and polishing. All of my current goodies of blue thunder, 113 heads etc. can just bolt up. At this point with both of your inputs I will measure for core shift, and then sonic test, and go for a .060. Wouldn't be doing this at all if it wasn't for that .080 overbore, that apparently resulted in some weird pistons with improper pin height and a deck height of .144 or so.
That was all done before the internet, and before I even knew what that much deck height would mean. Mike

 

46 your 56 block is only 030 over 312.only a very few would not bore another .030.292 and 312 both use the block.no difference in cyl,walls.

 

Hey Mike,

One thing I ought to mention is that my comments are slanted by three different factors. The first is that they are made as kind of a general statement about all engines. The Y, as well as nearly all the first generation valve-in-head engines, has nice heavy cyl. walls & thick dimensions everywhere. So, while a .030 or .060 overbore is nice and relatively safe on later engines (with some notable exceptions), on a Y-block you can almost always go much more with no sweat, as 312 & Carl both mentioned, & still be easily within limits.

The second "slant" is this. I tend to be fairly conservative & save metal for future rebuilds, especially with older designs that are no longer in production. The Y-blocks aren't exactly rare, at least as far as the 272 & 292 engines, but they aren't SBCs, either. However, you may not ever need to rebuild it again if it's used for a cruiser only & not a daily driver, so you may not care.

The last factor is this: most "rebuilder" cast pistons, as well as older forged pistons, like the Jahns, M/T, Forgedtrue, etc. that can still be found at swap meets, have an excessive amount of deck height. I've heard a number of plausible reasons for this; regardless, your compression is nearly always less than whatever the piston box claims. If you bore to the max or close, you get two options: a) get modern pistons custom made by Diamond, Ross, J&E, etc. & pay somewhere around $500-800 for them; or b) scrounge up old Jahns or whatever & pay the price in terms of lower compression, or deck the crap out of the block to get the deck height back in spec.

If you conservatively bore your block to fit one of the new hypereutectic Y-block pistons, & deck the block to square, the new pistons are designed to have an increased pin-to-piston deck surface height to gain compression. Compression is good.

.144? Man, you almost need to hookup an air compressor to the dipstick tube just to make it run.

The other Mike

 

The current motor is finally running pretty good with the little 2V Autolite I put on (the engine only has about 20 miles on it). But the low compression and the carburetor are more compromise than I can handle. But it is running and I can drive it. The pistons are the problem with the current motor. Do 312 pistons have the same pin location as 292s? If they do I could just bore another .010 and put a set of them in. $500-800 for specially made pistons is too steep. Didn't know they cost that much.

The motor required a .080 bore to clean it up! It was in pretty bad shape.

A mechanic friend, without seeing a Y block rod, said it might be possible to relocate the piston pin in the rod and decrease the deck that way. Sounds like a long shot.

The truck won't be a daily driver. I don't want it to feel a drop of rain. So a well built motor should last long enough for my son to inherit it. Mike

 

Mike,

As you probably know, when an engine has an increase in stroke, an auto engineer has three choices in order to keep the piston in the block. He/she can a)change the pin height in the piston; b) increase the height of the block (actually the "true" deck height); or c) shorten the rod. In the case of the 312 they elected to keep the same pin height & block dimension and use a shorter rod, 6.252" center to center length. The 292 & all earlier Y-blocks had a rod of 6.324" length. The compression heights on the 292 & 312 pistons are identical @ 1.768".

The HD truck 292 was offered, I believe, from 1961 on. It used a shorter rod, the same length as the 312, 6.252", different forging though. I don't recall why they changed this from the earlier 292; often HD trucks have an additional piston ring to help heat transfer from the piston head to the cylinder. Carl or 312 might know. The compression height for the HD292 piston is 1.830".

What I am wondering, and you may have already determined this (I seem to recall you or someone posting about this some weeks ago but I didn't get time to catch the thread), is if you have a combination of short HD truck rods, or 312 rods, with std. "passenger car" type rebuilder cast pistons with the different deck height. Relocating the pin location in the rods might be doable although that's quite a move. However, if you were to tear it down for that, then I'd just grab a set of early rods which are easy to find, have 'em rebuilt, & go from there. Assuming all else equal, your piston will sit .072 higher with that alone. By all means, bore it out again if needed or wanted. It may be difficult to bore it only .010; many machinists prefer to either hone it that much (time consuming and won't compensate for any taper), or, bore it to the next appropriate size, like in your case a .030 over 312, a total of .040 over your existing bore.

Food for thought.....Hope it helps, Mike

 

Food for thought indeed. Sometimes in these forums it is difficult to admit your lack of knowledge, like pin locations of 292 and 312s. Guess I was focused on the 292s. That could simplify things a little. Although I hate the idea of tearing into the running engine it wouldn't need line boring or new cam bearings, they have been done. It still would get a new crank because I don't like the idea of the 50/30 that is in there.
Yes it is possible that the rods are the culprit, although I have chosen to believe it to be pistons. If in the tear down the rods were responsible, great. It may not be a good assumption that they are standard 292, as the heads were one ECG-C and one 113 (which I changed to two 113s).
I really do appreciate all the effort and information, and wonder how people like you, 286merc, pcmenten,wildbuch,bigwin,312 and others ever got that much in their heads. Mike

 

Mike, this is just my opinion, because on most things i just use common sense sort of like the time i figured out how to work on ac systems, and do retrofits on the new 134a freon, on the older vehicles.

Seems to me, a person would want to check the area that received the most stress and heat, the area right at " The combustion stroke" , we all know that's where the hottest point is, as well as the most pressure, I would concentrate mostly on that area........Janet

 

the long 292 rods are ebu.the heavy duty c2ae.short rods are ecz h.d. cite.you could drop base and check.

 

Mike (46Yblock ),

I think I may have a short answer to your question about where to sonic test. If/when I go to sonic check a block, the first place I'll be looking at the front and backs of the bottoms of the cylinder walls. When I say front and back, I mean in the axis of the crankshaft; back of the cylinder means toward the flywheel, front means toward the timing cover.

Why? Because I've seen tilted cylinders on an ECZ-C 312 block and on a C2AE block. Of course, you can get a feel for front/back core-shift/tilt by using the drill bit trick. If the block fails that test you can skip the sonic test.

I think I'm going to take the C2AE block back and see what else I can find.

 

pcmenten, would you mind explaining a little more how you are measuring for core shift? I just don't get it.

Regarding rods, the C2AE block I have had 7 C2AE-A rods, and one ebu rod. It looks like there could be quite a bit of weight difference between the two styles. Mike

 

46Y, I pulled the freeze plugs out of the block and grabbed my drill set. I pulled the drill bits out and tested the gap between the cylinders using the 'wrong' end of the drill bit. I checked for the distance between the cylinders and I checked to see if the gap was even from top to bottom.

As I mentioned, one of my ECZ-C 312 blocks had a pair of cylinders that were not parallel. The gap at the bottom was wider than the gap at the top. The other 312 block was very evenly spaced and the gap between cylnders was just less than 1/4".

The C2AE block had a pretty good tilt on one of the cylnders, wider at the bottom. As I think about it I'm wondering if it isn't the same cylinder on both blocks; number 4. I'll have to go check that again.