Inliners International Forums General Discussion Engines New to inlines, critic my build plan!

Did a little measuring with an electronic caliper. Not saying it's perfect, but it's just to give me an idea of how much the chamfer takes up.

Overall diameter - 3.90"
Height of chamfer - .085"
Diameter to inside edge of chamfer - 3.75

What I did is calculated the volume of the overall diameter and height of the chamfer, then calculated the volume of the diameter of the chamfer and the height of the chamfer. I subtracted the latter from the former and divided by 2 since it is at an angle. I came up with .08 cubic inches and converted that to 1.3 CC's.

My math or how I went about it could be completely off, feel free to correct me if you'd like. Also, the overall height from the top of the piston to the top of the first ring groove is .25", and the height from the top of the first ring groove to the bottom of the chamfer is .165". I subtracted those 2, that's how I got the height of the chamfer.

I imagine the only way to calculate the volume of the valve reliefs would be to fill them with a liquid of some sort, I don't really have anything accurate enough for that.

Bummer, the chamfer means that you can't get true quench off which limits dynamic compression ratio to below 8.25:1 with premium fuel.

That is too bad.
Those pistons will probably give you 7.5 - 8.0 compression.

I would not run them, but that's up to you.

You should be able to get the correct pistons w/out that big chamfer.

I personally had used those same style pistons but were forged TRW's. At the time (I was young) I had told a shop to order me 307 high compression flat top TRW forged pistons, well, they did not, I got stuck with running those pistons & was really disappointed in the engine output, sluggish throttle response etc.

Part of the reason I tried all those different manifolds, 2 & 4 bbl carbs, carb spacers etc, trying to get the combo to work, well it didn't.

It did not run good until I installed the correct 307 flat tops, then I was over 10:1 compression. My head had been shaved/milled/surfaced.

Those knuckle heads did not know anything about how to get an inline 6 to run good.
I was young & ignorant, lesson learned.

MBHD

That's definitely not the news I want to hear! I still have to have my head work done, would milling it down help any? The 250 pistons are dished, I can't imagine that they are going to be much better on compression and I don't want to dish out the money for forged pistons just to get a higher compression. Maybe I'm SOL.

I just talked to my machinist and he hasn't decked the block yet. Does anyone know the measurements of max allowable decking? Sorry if my terminology is incorrect. Trying to squeeze all the compression I can! Max I want is 10:1

You should keep the decking of the block to a minimum, because it is already pretty thin compared to other engines, and has a weak spot at the front left head bolt that usually is cracked. The head can be cut a good bit, but expect other issues to start compunding it you cut an excessive amount. In a race prepped engine, a half a point of compression can make a difference, in a stock street engine, the difference couldn't even be measured. Since your stuck with the dished pistons instead of being able to upgrade to flat tops, there really isn't much gain to be had at this point that will be of any benefit.

Can you return those cast pistons & get the better 307 cast pistons?
It would be worth it to try.
Your compression is going to be really low, lower than stock.

MBHD

I agree with Hank. That would be the best scenario for getting your compression higher.

What/where are the better 307 cast pistons? I've checked Summit and Jegs with no luck. I have no problems sending the pistons back (if Summit will allow) as long as I can find some replacements.

This thing is starting to get closer to seeing boost...

I do not have any catalogs here.

If the pics are correct. Then these are the ones to get.
http://www.ebay.com/itm/Chevy-307-Sealed...7c71a72&vxp=mtr

Maybe these?

http://www.ebay.com/itm/68-73-Chevrolet-...4b2734d&vxp=mtr

Or ?
http://www.ebay.com/itm/1968-1973-Chevy-...0c847c6&vxp=mtr

Or ?
http://www.ebay.com/itm/Federal-Mogul-TR...0934453&vxp=mtr

Seeing some boost could be a good thing, & an easy way to get a bunch more power, just keep the boost low w/cast pistons & other preventative measures to prevent detonation.
MBHD

None of your links worked for me... I'll have to try them at my home computer later.

Strange, they worked before , but not now.
HMMM

Alright, I asked the first Ebay listing if it has a chamfered edged. It's listed as Sealed Power piston and our book only shows 1 option, the ones I have. The second listing is the same part number I have and the same stock photo as on the Federal Mogul site for the piston number I already have so I'm going to pass. The third listing I asked if he would split the set into 6 pistons since it is around $100 more than the ones I currently have, although I don't currently have rings. Hope to hear some good news, although I leave for my military training tomorrow I'm hoping I can stay on a computer and get this stuff straightened out.

First listing already replied! Unfortunately, it does have a chamfered edge.

Second listing response

I cannot guarantee that on this piston. F-M tends to change styles from time to time without telling us..........just go with it being square cut, and if there is a champfer then that's a plus. It's a stock type piston at a fair price. Thanks, Don

Third listing response

No Sir, our pistons for V8 engins are offered in sets of 8 only.

Grumpy's



I was hoping for some better news, back to the drawing board.

They probably phased out the lesser popular 307 piston over time so they wouldn't be wasting resources offering 2 types and only offer the one style(chamfered edge)piston now. Think about it, how many people actually spend money to rebuild a 307.

Probably just the straight restoration crew.

Well, it just so happens that I received my "Modern Engine Blueprinting Techniques" book in the mail this afternoon. While looking through block decking I found the formula to figure out zero deck. 1/2 stroke + rod length + piston compression distance. I'm sure this isn't news to many of you, but it seemed perfectly timed to me.

My inputs came to be (.5 X 3.53125) + 5.7 + 1.655, totaling out to be 9.12065. I'm not around my parts, so the stroke was based off Wiki for a 250, rod length was from this page
http://users.erols.com/srweiss/tablersn.htm#Chevy
and piston CD was from Summit Racings specs on the ones I just ordered.

Should I tell my machinist to deck it to 9.12? Would that be a good starting point?

I'm sure some of my writing may seem simple to the more experienced inline crew, but I'm just learning as I go here. I would like to thank everyone for the help so far, it's been great!

That really isn't how shops do it, and by doing so can open up a huge door of liability. You have to physically install the crank in the block and all other components in their respective cylinder bores once everything is final machined and check it that way.

I personally do not like to take much off the deck. That is why the Forged flat tops I offer have a taller CH of 1.670.

This usually means .015 is needed to be shaved with my pistons.

With a stock type piston, can you imagine taking .035 or more from your block? These blocks are thin, don't sacrifice strength in the deck.

I do not know the original deck height.

So, a shop would do it how? Install crank, put piston at TDC, measure deck, remove what is necessary?

I currently have my block at the machinist, but my crank, rods, and pistons here. I'm sending the pistons up to him for boring the cylinders, but I don't have the 250 crank yet. I was hoping to have the block work done, then down the road bring the rotating mass to be balanced.

I should mention my machinist is an hour away. One of the downfalls of living in a small town in the boonies.

Hmmm... ironically trying to spread costs out by machining in sections would cost more in the long run.

Most shops don't like engine builds to become projects, they like to keep the ball rolling and get it done once they start.

Mitch, for what it's worth my 1980 stock truck engine has a chamfer also. Please correct me if I am wrong but won't the increased compression height and lack of dish help some with compression? Mitch were the pistons that came out of the engine chamferd? Another question that hopefully someone with more knoledge than me can answer, does a chamfered edge kill off quench even at 0 deck height? Jay 6155

As I understand quench, the chamfer will kill it just like a dish since the air/fuel mixture will squeeze out between the flat top and head, but at the chamfer the lack of squeeze will counteract it. So you can't get the extra dynamic compression out of it, most sources estimate that you can go between 8.25:1 and 8.5:1 with proper quench.

True, you can have ideal quench and higher compression, but poor piston design and still not have as good of an engine output compared to an engine with less compression, but good quench and good piston design. All the pieces of the puzzle have to work together to be beneficial or you end up with more equally less.

I think that I get it. When the piston gets to the top of the stroke with a chamfer the "squish" goes 2 ways, part of the charge goes the right way toward the combustion chamber to create turbulance and part goes the wrong way towards the chamfer. This (if I understand it corectly) would have the same effect as not as much quench with a straight edged piston. Like .040 with a chamfer is maybe the same as .060 without one even if the 2 compression ratios were the same. Am I on the right track? Jay 6155

I managed to get a little time on the computer during my training. I received an email from Federal Mogul, the valve dishes and chamfer take up a total of 2.76 CC's. Should help me being able to calculate things, however I'm out of commission on this project until the beginning of June, so for now it's just info.

Thanks a lot Mitch. Can someone tell me how much loss 2.76cc would be or please point me to a forumla to figure it? Thanks Jay 6155

I just use this one to plug numbers into and get static and dynamic:

https://www.uempistons.com/index.php?main_page=calculators&type=comp

If I am doing this correctly, with a 0 deck height the difference would be about .25 of a point of compression loss compaired to a flat piston. Please feel free to corect me if I am wrong. Jay 6155

Just picked up a complete 250 engine to steal the crank out of. I took out the crank and will be bringing it to my machinist soon. I have lots of spare parts now!

I just used Summit Racing's compression calculator. Going with a cylinder head volume of 72 CC's (I won't know for sure until I see which head I'm going to use and if it needs planing), bore of 3.935, stroke of 3.53125, 2.76 cc dish, .041 compressed head gasket size, and 0 deck clearance I come up with 9.49:1 compression ratio, which is exactly where I want to be. Can someone verify my findings, or am I missing something again?

Now that I have my 250 crank, I should be able to bring it and my rods up to be balanced. I can also have the deck height set accurately since I will have my rotating assembly there. I may ask the machinist to check the deck measurements out of my own curiosity to see how close it is to the "assumed" measurements I had earlier.

I would again like to thank all who have helped thus far. This project is years away from being anything, but it's fun to plug away at and since I'm not in a rush I have time to figure out more details.

All the parts are at the machinist except for the rod bearings and main bearings. I'm waiting to hear if the crank or rods need machining before I order them. Starting to get slightly excited, although it's a little bit away yet.

Alrighty, in what little free time I have lately I've been browsing the web dreaming of parts. I found a great deal on a cam, the Clevite MPG/Performance (.273/.282 @.004, .461/.491, 194/204 #.050, 110 LS) and was curious on everyones opinion on this.

My engine is going to as close to 9.5 as I can get it, I plan on doing a lump port on the head, as well as a performance valve job and possible larger valves. I have a Clifford intake, although I may consider switching to a Offy, and I plan on running a TBI injection. I may run roller rockers, but that's somewhat undecided, and I will run headers, although I don't know if I'm going cast or tube.

It's going to be a summer driver, I'm not concerned with huge amounts of power. Honestly, a lot of what I'm doing is for fun/experience more than anything. I'm trying to create an efficient setup that has a little more power than stock. It'll be run through a T-5 and 3.4ish rear end, tire height around 28-29.

Do you guys believe this would be a good cam for my goals/set up?

Not a bad choice. I got one of those for a near-stock 250 build I was planning just in case, still sitting new in the box.

I used that same grind on our 289 Mustang, still gets 20mpg highway thru an auto trans and runs over 87mph in the 1/4, very happy with it.

Of course, given your engine that little cam will leave a lot on the table but as an everyday cruiser car, better to undercam than overcam.

Thanks for the reply Deuce! I will definitely not be using the most of the engine with that cam. Most of the mods I'm doing are in the name of efficiency. The compression is about as high as I can go while staying on pump gas (91 octane locally), so I'm getting the most "bang for the buck" there. The lump port and valve job will help make the air flow "easier" which should increase volumetric efficiency, although I might have too much volume and be sacrificing a bit of intake velocity. The roller rockers will be reducing friction, which is increasing horsepower without adding additional fuel making it even more efficient. Although I could go strictly for efficiency with an even smaller cam, I do want it to have a little snort and growl to it! The fact that it's on a very good sale doesn't hurt either!

If anyone else has helpful hints or critiques, throw them in! I'm trying to learn as much about these sixes as possible.

Just got a call from the machinist. He said it would take .175 to zero deck it, which is too much. I told him to take off whatever he is comfortable with. We shall see!

Are you sure he's measuring it with the new (250) crank?