i sent my gm 151 head to the machine shop and hey found a crack....they informed me that they couldn't find another. so i had a look and found some on ebay for $100.....rebuilt.....i thought this was too good to be true but bought it anyhow. i could rebuild it i just need the casting. it was too good to be true ...the heads had been machined down .050 and the change i deck height was holding my valves open and compression on the cylinders was up to 100 psi with a wrench turning the crank, so maybe much more with a starter cranking/running How can i reduce the compression? i looked for thicker gaskets and thought briefly about doubling them up. any other suggestions? i was going to super charge this with a tiny b$m blower i got so i hope to reduce this compression quite a bit but need it strong. thanks
Sorry to hear that, bct. And you'll be sorry to hear, that I doubt you have any other choice, but to buy another set of heads. And another thing. You don't just have compression issues. I imagine that because of the large amount of milling, the intake manifold will now not seat well against the heads...it's changed the geometry too much. Bite the bullet, and take solace that you've learned something here. Good luck, pal.
IT's a 4 cylinder head! I'm sure someone can figure the CR with that .050 removed. You will need .050 shorter pushrods to keep the valves from being held open, and you should check for valve to piston interferrence. I'm going to guess about 10.5:1 compression but don't quote me! No need for a supercharger!
Out of curiosity, if the change in deck height (not really change in deck height, but lowered volume of combustion chamber) is holding your valves open, how can you be getting any compression? Open valves and 100 psi is not possible.
Changing the pistons is the easiest way to go . Now you could get the thickest head gasket you can find and see how that works out but like someone else already posted , dished pistons would be the easy way out . Retro Jim
good question....only seemed to hold valves open on the two middle pistons....at first i thought it was the cheap head/valves but they are good....i put washers under the studs to space em up for comp test.
i used smog pistons that are dished already. could they be dished more.?? i guess it is a matter of material thickness ....and some custom pushrods.thanks very much for the info.
I had one of those in a boat years ago and the guy who did the engine work on it for me built them for sprint cars. He used 283 pistons as I recall, so maybe you can find four of those with lower deck height. Just a thought. Don
If you have dished pistons , you can have a machinist make the dish circle a little larger BUT don't cut any deeper . If you do it might be too thin and well you know what will happen then . But even if you make the dish larger in size , that will only take is down maybe from lets say a 10 to 1 to a 9.8 to 1 , so in the end you are still where you started from but with a costly machinest bill now ! Honestly the best thing to do is get a thick head gasket and a new set of pistons . That way you know things are correct and not a guessing game of parts swapping ! Shorter rods will work too ! BTW what engine are we talking about ? Retro Jim
I have seen (but not used myself) dead-soft copper head gaskets at a whopping 0.125". I know, THICK, and I did not believe it, until I saw it. 0.050" should be do-able.
seems like a felpro permatorque head gasket should make up most of that . 050 milling. .045 on the felpro gasket comes to mind.
Have you checked with a gasket maker like Cometic? Seems like they could make a MLS gasket to your thickness requirements. Won't be cheap but probably cheaper and easier than some of the other options.
@ .050 mill, I would start to question head deck strength. Was it angle milled or straight milled? You could try a long duration cam to lower dynamic compression.
There are a few heads that were used on those engines. Is it a later 151 with a corssflow head like used on the S10's? With dished pistons the stock compression ratio is around 8.7. I don't know the combustion chamber cc's but thinking it's similar to a SBC head should be about 64 cc's. Using an online compression ratio calculator and typical SBC numbers it comes out to 8.35.Usually compression ratio is listed higher than actual to allow for this or that.Shaving .050 off the head should bring it up about .75 to around 9.25 to 9.5 actual. This won't cause any real problems or anything close to what you're having.I believe you have a mechanical interference problem..
I'd be looking for another head ... the head IS the cause of your problems right? Why mess around with pistons etc to compensate for "sumpin that just ain't right" to begin with. It's not likes it's an Ardun, or a Ford SOHC, there must be other good/uncompromised heads out there somewhere.
Can you have the pistons machined so that the outer rim is removed ? In other words make them flattops that are way down the hole. Somebody do the math and figure out how many cc's is in a .050" thick 4" bore gasket. As mentioned earlier, a .050" milling job shouldn't raise the comp excessively. You could also maybe unshroud the valves and get some cc's that way.
Don, Some times a couple of inches is important. I would guess that you were running a 153 cubic inch Chevy II and not a 151 Iron Duke (different engine, just looks about the same on the outside. Charlie Stephens
Iron Duke is the later engine..intake and exhaust on opposite sides of the head.153 has both on the same side like a typical inline 6
The early 151s had both intake and exhaust on the driver side as well; '77 through mid '79, I believe.
No way you can jump to conclusions without cc' ing the heads and pistons, and running the piston height, gasket thickness etc.. 100 with a wrench SOUNDS high, but with a heavy coating of oil or assembly lube on the cylinders and new everything, there is no way you can assume that you have "too much compression" judging by this reading alone.
good point, 39 all ford. it is a 1980 head with the cross flow casting # 552 i believe. the ebay deal was the only head i could find .... thanks for all the responses,.. with them i have several options at the moment ....i'll start with the free ones and see what happens.
like the others have stated from what you have said you shouldn't have too much compression the motor is assembled and there is oil in it install the starter turn it over without the plugs to get rid of excess oil then install a compression gauge and check before doing anything
A gasket with those dimensions would yield a volume of 0.63 cubic inches per cylinder or 10.30 cubic centimeters per cylinder.