[sledge hammer]

I'm helping a friend with his '69 250ci that he is going to install in his 1940 Pontiac Coupe and we need to find pistons for it.
I have been reading how the Chev 250-6 and 307ci pistons will work with a little modification but I can't seem to find any info on what modification is required.
Can anyone help us out and post exactly what modification is required to these pistons so we can do it right the first time...
Thanks allot! Dave

[PMD1969]

I am also researching pistons for my 250. I have found that EGGE has cast pistons available. I have yet to find any forged pistons at a reasonable price.

[hpp]

Im prob going EGGE pistons heres the link.

http://www.egge.com/site/?d=48&dt=1&...PONTIAC&rpp=10

[Cammer-6]

Again I would ask alot of questions on using Egge,havent heard anything(good or bad) and I believe they make their own.I think Quenton Olson was having some hypers made from someone,at least we talked about doing it,so check with him and see.That would make more sense than a forged or cast piston for the street.

[PMD1969]

Speaking of Hyper pistons I looked around ebay and found some 307's that are hyper. I think the problem with 307's is that they are very low compression.

[GTOWAGON]

How low compression if you are using a sprint head you have around 10.5 stock that is a bit much for today's fuels somewhere in the 9.0 9.5 range will still give you great performance and run well with 93 octane

[sledge hammer]

I wonder what the compression would be with the 307's on a .030 over 250ci and what modification to the piston would be required?
We want to make this a 94 octane engine with the compression ratio correct for this fuel so we were thinking forged pistons for the extra security incase we run into a bad batch of fuel.
So far the prices I have seen to make custom pistons is a little high so we would like to use the Chev piston if possible.

[sledge hammer]

I agree, 9.5 is what we will be shooting for. Will have to figure out deck height, valve reliefs and also can modify the combustion chamber to zero in on the cr.
In my 455 pump gas race motor I grooved the combustion chamber squish pad which I believe makes the motor more octane tolerant as I am running 9.2 compression and can burn 87 octane with best power at only 29 degrees total mechanical timing so this can be an option for reducing compression.

[PMD1969]

Originally Posted by GTOWAGON

How low compression if you are using a sprint head you have around 10.5 stock that is a bit much for today's fuels somewhere in the 9.0 9.5 range will still give you great performance and run well with 93 octane

Mine is a Sprint that is stock. So far, it is set at factory settings and is running fine with 93 Octane and not pinging under load. I have added (to the last tank) some Torco Octane which claims it raises the octane to 97 and if the entire can is added it could be at 100+.

[Cammer-6]

I think the 307s are .004 shorter in deck height
This is going off memory and that can be tricky

[sledge hammer]

I do know what going by memory is like some times! LOL!

.004 shorter deck height is workable.
Are there any other mods you know of that are required to use these pistons? Is the pin the same size, do you know if they need skirt trimming to clear the crank counterweights?
Thanks, Dave

[TheSilverBuick]

You know when you shave the deck on these engines it retards the cam timing as the cam sprocket and crank sprocket get closer to each other.


Also I don't understand the desire for hyper pistons, they are shatter prone if you do encounter detonation. With iron head(s) they make me nervous anyways.

[sledge hammer]

I think I read some where, can't remember right now but someone was offering an adjustable camshaft gear. With it you can degree your cam to the proper position.

[hpp]

Originally Posted by sledge hammer

I think I read some where, can't remember right now but someone was offering an adjustable camshaft gear. With it you can degree your cam to the proper position.

Plan on spending $250 and up for one.

[TheSilverBuick]

Sprintbird adjustable sprocket.

And actually as I was thinking about it, if you run a standard Felpro headgasket instead of the steel shim gasket, that will re-raise the head somewhere around .040", and probably more than offset any decking. Possibly even advance the cam. More food for thought

[Cammer-6]

theres alot of problems that a piston lower in the hole,due to thicker head gaskets
causes too,with pre detonation. Look up squish area and the effects it causes.
Cam timing is easy to fix with an offset bushing or just redrilling the cam gear and
remarking the timing marks on it.Factory gasket was .018 compressed I think.

[sledge hammer]

Yep, a while ago I was reading some papers that the Chief Engineer in charge of the combustion chamber development on the Cadillac Northstar engine wrote up.
He said that detonation is the result of the "end gas" which is the fuel on the far side of the combustion chamber "squish pads" from where combustion starts, becoming super heated and changing molecular structure and becoming highly volatile. This new composition ignites violently by its self before the flame front reaches it causing pinging or knocking and engine damage.
Pre ignition is when the fuel charge ignites on it's own before the correct time, "spark advance point". As compression increases the fuel charge becomes harder to ignite so he says pre ignition usually happens when the piston is at the bottom of the cylinder and starting to rise on the compression stroke. Glowing carbon or sharp metal edge etc ignites the fuel causing a huge rise in pressure working against the rising piston. He says there is no warning of this, it just happens and you loose the engine, blow head gaskets, hole pistons etc.
There is a guy on one of these forums that goes by the name "Autobreath" and he swears by the procedure of machining groves in the head squish pad. This not only creates turbulence to fracture the flame front for better combustion and reduced timing , but also speeds the flame front to the end gas before it super heats and changes molecular structure thus reducing ping/knock. I have tried this and can say I have had success with it. It cut my 60 foot times down, increased my mph, it has reduced my engines total timing requirement and also my engine is more octane tolerant. Why I mention this is that he recommends .060 clearance on the squish pad to piston for best results so the thicker head gasket may match up with this idea. I think Somender Singh has the original patent on this idea.

[BMFH]

Hello All

When Primatech was going I ordered a set of Forged pistons from Q, He got them made at Diamond Pistons, I think they are in Detroit.

I think he also took a head to them so that they could get an accurate shape for the Combustion Chamber.

Mine are 12.5 CR for a 250 engine, I am using a 67 Sprint 230 Head that has been professionally ported and had the combustion chambers opened up allot. They look like good pistons too, however they are Ruff Finished on the top with the 12.5 Kickers.

I am doing clearancing with them and the head without using any head gasket to make sure I will have adequate Interfiting clearances, I plan on using a self made .020” copper head gasket when I put it together.

Very slow progress so far though.

Take Care

Dusty

[Cammer-6]

Hummm
I have a set of JE that are also custom made,going with a 67 230 head thats ported but CC isnt changed that much. Block has been O ringed and I have plenty of stock steel single layer head gaskets.Have thought many times of sending them out and having domes cut off but 13:1 engine sure is alot of fun, and thats why I have stock forged TRWs,so I wont have to.John,on the head gasket Ive been seriously thinking of converting one of my graphics CAMM plotters into a cutter for gaskets.
This may be the makings of an E-85 motor.