Inliners International Forums General Discussion Hi Performance 307 piston pin height

Please forgive me if this is in the wrong place. I am going to buy a set of 307 pistons for my upcoming build. All the ones that I see on ebay say the pin height is 1.655. If you look at GM archives they say the 307 has a pin height of 1.675 and the 250 height of 1.655. There are some NOS 307 pistons for sale part number 6271087. Does anyone know the actual pin height of these pistons? I would love to buy the Ross pistons that Tlowe sells but the budget says no. Thanks in advance for any info/comments. Jay 6155

Most aftermarket pistons are slightly shorter on the compression height to compensate for the rebuild processes such as overbore, head and block decking, etc...as to not let the compression or piston height to get out of control when its rebuilt.

I thought so. Enginetech even says that the sometimes take .010 off the top to make up for overbore ect. This would mean that NOS pistons would have a taller pin height. To me this means less or no decking of the block to get to "0" for quench and higher compression. Maybe I can get by with a small "clean up" cut on the block. Thanks CNC-Dude. Jay 6155

Sure! Anytime.

I see two types of cast replacement flat top pistons on Summit Racing, but they don't list the CC size for the valve reliefs. Anyone know what the CCs should be for 307 pistons?

Most SBC 4 valve relief pistons have a 4cc combined volume. Make sure that the pistons you select do not have the chamfer around the top edge of the pistons either.

Summit Racing has four 307cid pistons on its site, but the information on the listing is pretty spare. It doesn't mention any of them being chamfered.

http://www.summitracing.com/parts/slp-235np/overview/

http://www.summitracing.com/parts/slp-w295ap/overview/make/chevrolet

http://www.summitracing.com/parts/uem-1412-std/overview/make/chevrolet

http://www.summitracing.com/parts/uem-1438-std/overview/make/chevrolet

The W295 and 1438 are for the Chevy 307 or 250, with a compression height of 1.655". The photo of each shows a chamfer.

The other two are for the Chevy 283 or 230, with a compression height of 1.780".

Oh, I just searched by bore diameter. I wasn't looking at the other specs. So Summit doesn't offer anything piston-wise for 250cid owners. Anyone have an inexpensive source for replacement flat top 307 pistons?

I would suggest either Summit or Jeg's for the 307 pistons. I think that most of the 307 pistons sold in the last 10-20 years have probably been for 250's, I just can't see someone actually rebuilding a 307 on purpose.

Most of the guys I knew in high school with 307s just did top end rebuilds, anyone with a 307 block that was out to lunch would just replace it with the plentiful 350s.

Well Summit Racing only had those two Sealed Power pistons in their catalog which were chamfered. Jegs just has the same Sealed Power piston as Summit:

http://www.jegs.com/i/Sealed+Power/844/295P20/10002/-1

I might be missing some in my search parameters, if anyone is able to find them please post a link.

What's the issue with the chamfer? Does it interfere with proper quench?

No, the chamfer is just really huge and kills a lot of compression off. It almost defeats the purpose of the flat top.

Ah, so it would be more than the 4cc for the valve reliefs. It's possible that the photo is not the actual 307 piston and may not have the chamfer.

I haven't had my head off yet to do some measurements on the factory head gasket, deck clearance or piston cc. When I play around with the compression calculator to try and achieve 8.1:1 SCR I assume 76cc chambers, .015" x3.875" gasket, .005" deck clearance and that comes up with a 16cc dished piston. Is that about right for the stock piston?

I have looked and looked and I can't find any 307 pistons that do not have the chamfer.(Unless you go to custom pistons like those made by Ross and others). From digging through GM archives it looks like the chamfer costs about .25 in lost compression in a 307. Has anyone actually measured in some way how many cc's that the chamfer adds.(Don't know how it would be done). This was done to reduce HC emisions in the early 70's. By the way,looking inside my 1980 truck 250 at the pistons they are also chamferd also. I have also seen 250 pistons advertised with .85 and.115 dish. How many cc,s are those dishes in the top of 250 pistons? Jay 6155

If you had an actual piston you could calculate the volume the chamfer was. I knew it was pretty substantial above and beyond the valve reliefs. The dish volume can also be calculated if you have some basic dimensions.

I am going to buy a cc test kit and check and post what the actual cc's of the chamber, stock piston ect really are. With a little math we should be able to figure out what volume the chamfer is and what it costs in compression. Even though my 1980 truck engine has a advertised compression of a blistering 8.25 to 1 I bet reality will be lower. Probably be lucky to break 8 to 1 in the real world. Jay 6155

Yeah, that's my plan too. CC the original head and pistons, measure deck clearance, and try to measure the compressed thickness of the factory head gasket, then share my baseline findings. That won't be until I get a new head, valve job, and clean up the casting a bit. As soon as it warms up, I'm going to check the factory cam and see what the ABDC and the rest of the opening closing points are. Then I'll not what the real starting point is on compression.

I got into this mess while trying to find pistons for my 153 4 cylinder build. Same combo as a 230. When I threw a 181 Mercruiser crank in the mix the 307 pistons came into play. The stock pistons were .050" down the bore. I got a few sets that should have worked but all came up shorter than expected. That could be the .010 discussed earlier as a rebuilder cushion. The chamfer on some are way more than the valve cuts and are more equal to the dishes. I go a set of tlowe's 250 pistons but still need to get them on the 181 rods so I can measure. Any way I have a couple of sets of rebuilder 307s for cheap.

I got a lot of people on the NastyZ28.com forum asking me about my L6 Camaro, thinking of getting or doing something similar for an economical daily driver car and saving their V8 car for weekends or racing. But the 153cid sounds like it might be a good alternative for a daily driver. You can hook it up to a TH350 and I presume it fits on the same motor mounts as the V8/L6 with just a lot of space between the radiator and the pulleys. I should make that suggestion to them.

What are the specs for those pistons? Pin height,chamfer,make,ect Jay 6155

They came in some Chevy IIs. I wouldn't want one in a car heaver than that but a little '62 two door post with a built 4 cylinder would be fun. Probably not much savings in the build but different. Start with a 181 Mercruiser and you are already at 140 HP.

So something under 2800 lbs. Not even the early '70 to '74 2nd gens fall into that range. The early 3rd gen Camaros had a 4cyl option, but I'm not sure what engine that was, but I know they were lighter than the later 2nd gen Camaros.

Mine is going in a '26 Chevy roadster and my goal is as close to 1500 lbs as I can get.
Here are some pictures comparing 250 (dish) and 307 (valve cuts) pistons. LINK
250 and stock pistons with 181 crank (3.60" stroke) LINK 2
Ross 250 from tlowe & stock 153

Wow! I have never seen even a picture of a 307 piston that only has 2 valve reliefs and a dish. I am talking about the first link. Did you measure how deep the dish was on both pistons? How far down the hole did each one sit? Are these the ones for sale? Very interesting. Jay 6155

I can try to measure it again when I get home. I in Hank's area right now. Those are in he 153 block with the 181 crank. the 181 has a 3.60 stroke which puts them .035 higher than a 250 would. The stock pistons with the 153 crank were .050 down the hole. It is the same stroke as the 230 at 3.25".

Thanks for checking when you get time. Please tell me if I am figuring the math correctly. The stock 153 piston sits down in the hole .050 with a 3.25 stroke. You swap in a 3.60 stroke 181 crank all other things being the same. 3.60-3.25=.350 increase in stroke. .350 divided by 2 (half of the stroke increase is at the top half at the bottom)gives a upward movement of the stock piston of .175. .175-.050 leaves the stock 153 piston sticking out of the hole .125. Just a tiny bit too much positive deck IMHO. So the next thing tried is a stock 250 piston. 3.6 -3.53=.070 half of which is .035. Do I understand this correctly? If I have the math correctly and if you can tell me where both the 250 and 307 pistons come from and take measurements I can tell where your 307 pistons will sit in my 250 block. Are the pistons oversize? I realize that this is old hat to a lot of people here but some of us (me) are trying to learn. Jay 6155

I think you have it. In the second group of pictures the one sticking out is the stock 153/230. The dished one is a 250. All my pistons are .040 over. What messed with me is that none of the ones I bought had the compression height advertised. I think now it is what CNC was saying about lowering the top .010" to allow for truing up the deck or head. I haven't been able to try the Ross flat top 250 because the pins have to be fitted. They are a little taller and I have high hopes for them. This project has been sitting too long. My goal is to have this engine running on a stand by the end of May. (20140) Headed home tomorrow.

Looking at the first set of pictures, it seems that the 307 piston sits down in the bore more than the 250 piston. I guess we will know when they get measured. Do all of the 307 pistons that you want to get rid of look like the one in the picture? If they are like the pistons that I see around here, the 307 will have a pin height of 1.655 and the 250 will have a pin height of 1.640. That should make a .015 difference.(A little hard to see in a picture). To me at least, this is very interesting because I have heard as much about 307 pistons as I have the 194 head over the years. The wonderful thing about this site is sooner or later the facts come out and myths are dispelled. Jay 6155

The tough part of this site is that a lot of us are friends. Some of us actually know each other. Even the ones we disagree with are not our enemies and the groups don't stay the same. Facts are plentiful but truth is elusive.

I thought the 307s were the only stock replacement flat top pistons for the 250cid? Does anyone else make a flat top cast replacement? I've only been able to find the stock dished pistons on Summit's site:

http://www.summitracing.com/search/Part-...ine-6-cylinder/

Beater,
Just put your nose to the grindstone and get to work. We all know your not getting any younger or will have more energy. Heck, that describes me!

The 307 pistons are not direct replacements. 250 pistons are made and sold.
They have just been a reliable choice of a cheap alternative piston choice.

I've only been able to find cast flat tops for the 307cid V8. The cast pistons I've found for the 250cid L6 are all dished about 0.085" and chamfered. Are there flat top 250 replacement pistons?

No, just custom is the only way to get a flat top for the 250. That's why the 307 piston is such a popular swap, its cheap and not expensive like a custom piston is.

Yeah, that's what I thought. It should still be an improvement in compression for the 307 flat top over the 250 dished, since both types of cast replacement pistons seem to be chamfered. It might be my only option when I decide to go the final step on my project and rebuild the short block and shoot for improved lower end durability and maximizing compression.

I put the 153 in my 47 CJ2A Jeep. The engine was carefully rebuilt ;.023 shim gasket to get quench at .043....steel tube header, modified distributor advance and rejetted stock carb.
The Jeep weighs about 2200 pounds, stock 5.13 gears, 4x4 and 3 speed. The engine runs great and the performance is better than the original 60 HP Red Devil flattie but just still adequate at best.
Maybe in a 2000 pound car with a 5 speed , hotter cam, more compression and carb you might have something until the next punk in a old 4 cylinder Interga blows ya off

Beater, I am very glad that people on this site are friends. People have disagreements all the time, After all there is just more than one way of doing things. I am sure that no one wants to willingly run an intergrated head but me. That just proves that I am crazy not unfreindly. If I have offended anyone please forgive me. Please tell me what brand those 307 pistons are.

intergrated j 78

You are not the only that has or does use that head but many cut both intake and exhaust off of it. And to the best of my understanding the factory built a small lump into the head. I never got my hands on one yet! to cut the intake & exhaust off one to see for sure. But anyway you can still clean the bowl and Some vary little but a little of the port over the short turn area.

Wow! I never thought of cutting a head like that. If I ever end up with a cracked one I will saw it and "see whats inside" Do you know which head was cut, 1 or 2bbl? Jay 6155

Wait, you literally mean there were people that cut the integrated intake manifold off the side of the head and machine the intake/exhaust face for use as a non integrated head?