I have a 392 Hemi that I bought to put into my chopped Model A over this winter. Its been a long story about this motor. It was supposed to be a built motor, on going to look at it that turned out to be not completely true. It turned out to be ( I think) a motor out of an old drag car. After I got it apart it did have a new solid lifter roller cam, a fresh and just turned crank ( .010/.010) new rod and main bearings yet it had the old cam bearings in it. It also had a brand new set of Jahns 12 to 1 pistons, that were unmarked as to size, so I assumed they were standard. I went about putting it back together . At that point I miked one of the pistons and it turned out to be 4.120, yea that's right .120 over. So I hauled it to the machine shop and sure enough it has in fact been bored to .120 over. I tried my best to find a shop with a sonic tester with no luck. So I bought one myself, great little gadget to mess around with too by the way. I have measured every piece of metal in the shop. But to the block. I have now sonic tested this thing and know how thick it is, but I do not know how thick it needs to be, or what is considered safe. So my question is does anyone have a clue how much metal you need in those cylinders?
They made engine blocks with thick cylinders in those days, so they could be rebored 2 or 3 times during the life of the car. .125 was a typical limit, some Olds blocks could be bored .250 yes that is right, a quarter of an inch overbore. Thinwall castings with thin cylinders and overbore limit of .060 began in the early to mid sixties. So how thick are your cylinders? And are they uniform? Corrosion and core shift can make a difference. Chrysler engines were among the best built and seldom had problems that way.
Dave, Thanks for the bump I am a little surprised that there hasn't been some opinions or advice on this subject. Seems like on most threads like this the motor Guru's come out of the woodwork to voice there opinions. Maybe nobody knows what is or is not on this subject.
Don't worry, folks will chime in. Guys are out having fun since most of the country still has nice weather.
This is one of those "how long is a rope" questions. Sonic check the thrust sides on every single cylinder, and report the results. Yeah, 0.120 over is a lot. What matters is exactly what you have left. Casting is a science, with a sprinkle of art. Nobody can say what you have. Only the sonic tester can.
on the thrust side .215/.205 heavy use RACE... for the street cruse use, .180... error to the THICKER side... ah you know that...
123456 ..... here's the rub... it takes me time to "PRETEND" i'm SMART... but in reality i have to go look it up....j/k i can answer the same questions live from memory , i started this car,hotrod shit wayto early in life. lol..... most internet commandos, know it alls,-n-, gurus don't know SHIT without the internet to back up there [typed pretend knowledge] so... still with me...? like FG, i won't be outed, tested, or criticized right out in the open, it makes NO sense i'm trying to help, Fg is avoiding the DRAMA of the *i'm smarter than you* crap that you see here all to often... other than that the ...... <---- row of dots, saves my place to see results of the thread and how things go, if the question is answered , with several smart posters FG,Jim,31V.. ah the list is LONG there All good people, then it holds my place to GO back to and read. oh ya Check the thickness ABOUT 2-1/2 to 3" below the DECK. P.S. *i'm seldom right, but i'm never wrong*<-- come on now laugh at that.
Now I understand I often do the same with just posting the word "subscribe". I was/am interested in this because we do have one well equipped shop here that has a sonic tester, but trying to get anything out of the machinist/owner other than "it's OK" or "It won't take that much more bore" is almost impossible
I would want at least .160 on the thrust sides and .090+ on the non thrust. (inner walls of the left bank and outer walls of the right bank) You need to sonic check using a radiused transducer. You should be checking at least 16 spots per cylinder. (front, back, and sides) Early hemi engines had a lot of core shift.
Guys thank you all for your comments and advice. George I plan to ask you a lot of questions from time to time, thank you for your interest and advice. TR, The transducer that came with this sonic tester does have a very small amount of radius to it. I tried to slide a feeler gauge between it and the cylinder wall the smallest gauge I had was .0025 and it would not go so that is just going to have to be good enough. I did take multiple reading in each cylinder many more than 16. The two thickest cylinders were #2 and #7 which on the trust side had .210. The two thinnest were #3 and #5 which were at .198 I guess taking into consideration the advice here and with these readings I am good to go. Thanks Again Guys!
Couple questions. Did you run it before taking it apart? Did you measure the gap(s) with an old ring(s)? What do you mean by safe? Safe for doing what with what kinda odds are you looking for? And if you decide it's too thin to meet your safe criteria, what are you gonna do about it if anything? Jack E/NJ
Jack, Man those are some damn good questions. You know I actually made a mistake with the part of that post with the .060 end gap. I have two 392 engines, one is the .120 over motor and another one that came along and I bought it just to have and because I found out that this .120 motor was that big. And I even confuse my self sometimes with which motor got what done to it. Now with that said the .120 motor came with a new set of Jahns pistons and rings already on those pistons. I just went out into the shop and pulled a ring off one of those Jahns and checked the end gap. It is .016. No I never did run the motor as it was when I got it. As to if it was to thin, well in that case I don't know what I would do with it. I have some money tied up in it as it sets. I could not sell it to some one without disclosing exactly what is going on with the motor and in that case I would have to ask myself the question. Is it worth what I paid for it? in all honesty I would have to say NO it is not, so the chance of me getting my money back out of it and maintain my honesty is just not going to happen. I guess I could get it back together and just dress it up some and LOOK at the damn thing. After all a lot of people spend tons of money on stuff that is just for looking at. I have spent a lot of money for parts for this 392, maybe I should say for "a" 392, yea that sounds much better. I have purchased a set of Hot Heads aluminum heads, and all of the other stuff to build this into a good street motor for my little Model A. This .120 motor came built in that it does have that set of new pistons, a new roller cam, a fresh .010/.010 crank, fresh valve job on the iron heads. My plan as of right now is just to put it back together as is--iron Heads, cam etc just like I received it. I am going to run this motor at least on my test stand just to see what it does. After that I guess I will just look at it...
Did I misunderstand? I thought the piston/wall clearance was .016/.017. that's why I kinda freaked out. If that's actually the ring end gap, its not a real issue.
George, I have managed to completely screw this up. The problem is in the fact that there are two separate 392 engines. The first engine I got is the engine that is bored .120 Over and has the new Jahns pistons and rings. This motor I bought as a rebuilt engine so it was all together. It had been setting in the guy I bought it from shop for several years, so I was going to pull it apart and put it back together using the new heads and the aluminum parts like timing cover and chevy water pump new gaskets etc. The other 392 I bought when I discovered that the first motor was .120 over. I really didn't think with that much over-bore that it would be suitable as a street motor. I mean the Jahns pistons are 12 to 1 compression ratio and that along with the .120 over-bore I thought that motor was done. I then realized that I had a ton of money into this Hemi deal the .120 motor I paid $3,000 for add to that the $8,500 bill I paid from Hot Heads. Now I realize I can and will use the Hot Heads parts on the new engine so that is not a total loss. I still have a $3,000 piece of cast iron setting on an engine stand in my shop. The second engine I bought and had shipped to me from New Jersey. when that one got here I knew it needed to be built. Once I got it apart it had a set of factory pistons that were marked as STD. as were the main and rod bearings. There was no ridge at the top of that block so I thought and hoped that I could just re-ring it, install new bearings put on my aluminum stuff and be done. I desided to check the end gap of the piston rings just to get an idea of were I was at. That end gap on those rings was big I mean like .060. I then took this block to the machine shop and had them mike it. The machinest said man somebody has honed the hell out of this block. The bores did look brand new and had a nice cross hatch pattern. Turns out the block was .017 big. So I had that block bored and it cleaned up at .040 over. I bought a new set of .040 pistons and rings. What I think happened is someone started to build that second engine on the cheap. It probably (?) had a ridge at the top of the cylinders and they honed that ridge out of the block leaving it with the .017 over size. Once they did that they must have seen that the pistons were just to small with there .017 clearance, didn't want to spend any more money and sold the damn thing to a stupid SOB in Alabama. That still leaves me with the first motor I bought the .120 over engine. I would just hate to not be able to use that motor in something and see it go to waste. I cannot sell it ethically like it is, and just pass the problem on to some unsuspecting guy. I will not do that so I am hoping beyond hope that it is not beyond using. That is the reason for this post. As long as there is a chance that it will work like it is I am willing to spend the money and finish the build.
If it was mine, I'd try to figure out if my sonic tester was accurate or not. If it IS accurate, then your .200 or so wall thickness should be just fine - even if you were running a blower and mild boost. Personally, I find it a bit doubtful that the walls are that thick with a .120 bore . . . but that is just my perspective. I'm not a big fan of the old Jahn's pistons - they are probably the cast ones that were used back in the day. With a compression of 12 to 1, you'd need to run premium gas and make sure that you're not detonating it.
I have tried to insure it is accurate, it is a mechanical device and thus I am sure there is some percentage of error. I have checked it against many, many pieces of known thickness material ( that is material that I can get to both sides to measure) and it is damn close. Notice I say close not perfect but close, very close. Remember that as a friend on here reminded me .120 over bore is actually .060 per side of the cylinder. The Jahns are the old school pistons from back in the day. They are cast as all Jahns pistons were. I spoke with the folks at JE Pistons ( they purchased Jahns long ago) and they are the ones that informed me that the #1108 Jahns piston were 12 to 1 compression. The fact I will need to run premium gas doesn't bother me at all, I mean I don't even have a car to put this Hemi in right now as I am going to build the other motor I have for my Model A.
Interesting thread. A couple of thoughts. If you are satisfied the cylinder wall thickness is sufficient for a street engine, can the Jahns pistons have some material removed. From the top to bring down the CR or, better yet, buy some new pistons more suitable for the purpose? If the wall thickness is suspect, what does it take ($$) to sleeve the engine and new std pistons. My point being, the block is not junk and you have an investment to consider. Perhaps you would end up with more money in it than hoped, but it can be made useful and the investment recovered thereby. Ray
Ray, You make some very good points. Your idea about having some of the domes removed from the Jahns is one I have considered. I may do that at some point in time. New pistons---Man I have talked to every one I can about getting a New as in different set of pistons. Trouble is the .120 over part. no one today has a set that is .120 over and most of them are not interested in making me a set. There is a poster on here who sells some early Hemi parts, he has a web-site and I have spoken with him on the phone. I bought a fixture from him for taping the crank hub. He can get a custom set of forged pistons in any size I want, but the price whewwwww. Spending another $1,000 at this point in time is not something I really want to do. It may come to that, and I would spend the money for a set of those pistons before I would junk this motor. This motor is going to run again, at least for a while, but it will fire off sometime early next year. Wither or not it ever gets in a car is another story. I am an old guy but I have at least a couple of more cars in me I hope. I have had an idea of building a Model T sedan channeled and unchopped for a while now. A simple car real old school, so maybe this old .120 over hemi will get a new home after all.
Well boys there you have it. You need .160 - .180 minimum thickness and he has .198 on the thinnest cylinders. The thicker ones are only .012 thicker which to me indicates pretty precise casting. Considering that the motor will not be supercharged, and will be loafing in a light model A driven on the street it should last forever. Can you double up on the head gaskets for lower compression?
I think you can order Cometic gaskets in different thicknesses - which brings up the next point - .120 over gaskets may not be off the shelf either. Clark Gaskets can make whatever you need I'm sure. If you go copper like that, you don't want to run much radiator pressure though. The Hot Heads combustion chambers on my heads came out to 111cc which if bigger than most iron heads at 106cc so you might not be too high on the compression. I'd run the piss out of it if it was me
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