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VetteInOz 01-20-2008 01:04 AM

How to get a Chevy 302 under 10:1 CR
Firstly a couple of apologies/Disclaimers
I am located in Australia so my applogy for not being able to interact directly because of the time difference. I guys gotta sleep!

Secondly I wont pretend that I know a lot about engines.

I recently bought a 57 Vette in from the US with what I was told was a rebuilt stock 283 engine.

After giving it to my uncle (who is a retired mechanic and a chev nut) we discovered that it was actually a 302 with 12.5:1 CR TRW pistons, 5.7" Carillo rods, roller rockers etc etc etc. My Uncle was very excited and couldn't understand why I was told it was a stock 283 when it was so much better!!. I believe it is a 283 block bored out to 4" but has a genuine 302 Crank ( i think they had large journals or something). The head is 64 cc

Of course the problem we have is that the engine will not run on the available pump gas. The max octane we have is Australia is 98 and I am told we need to get the compression under 10:1 to get it to run.

I have scoured the internet but cannot find any flat top pistons for a 302 chev that I can get. The closest I came wa a set of KB with 11:1 with a 6cc head.

So without getting a set of custom pistons made which I have been told will cost me over $1000 I am looking for some other option

Any suggestions

junior stocker 01-20-2008 01:26 AM

Larger combustion chamber heads.Or, if the block is truly a 283 then it's SMALL JOURNAL, unless some NUT had the main journals cut down to 283 specs on the crank. You could use the L-99 rods with the Lunati bearings used to put large journal rods on small journal cranks, #CR 867 HP (thanks Machine Shop Tom), and then use flat top 350 pistons.Puts the pistons .025 in the hole on a stock 9.025 deck height. If the pistons are solid dome you have now you could just mill the domes off.

VetteInOz 01-20-2008 05:13 AM

Thanks Junior Stocker

I am pretty sure it has the crank with the larger journals. I will check it out.

Milling the pistons could be a good answer, Thanks for you help

y112 01-20-2008 06:39 AM

The small journal crank flywheel flange is completely round, where as the large journal flanges are made with a counter weight.

Be careful with milling the pistons: Factory 302's had 11-1 pistons that could be milled flat. You must check dome thickness before milling, because different production runs of the same piston are not alike. You might have to mill 1/2 of the dome and install 76 cc or larger heads.

Good luck Mate!

speedydeedy 01-20-2008 06:55 AM

Everybody here gave good advice. The only thing I would like to add is Head gasket thickness.I don't claim to know it all but I do know this changes Compression ratio. Of course the easiest thing to do is put the larger combustion chamber heads and the thicker head gaskets then you don't have to disassemble the bottom end.If you already have it disassembled then I would check into milling the domes off.Hope this helps and good luck down under. :thumbup:

woodz428 01-20-2008 07:21 AM

If it is a 283 block it will be 2 bolt mains and small journal, you can verify with casting #'s what the block is. For compression I second all the comments about heads, it also gives you much better porting which could offset any loss of power from the cmpression drop. By the way , punching 283s to 4 inch bore was a pretty common trick, they were called 301s at the time. We used to do them regularly when I worked as a machinist. The 302 crank is also forged, not cast, the 2 have entirely different seam lines.

302 Z28 01-20-2008 07:42 AM

What you probably have is a 327/350 block with a 4" bore. If you really have a 302 crank you have something super special, those cranks are nearly impossible to find. The crank alone is worth a ton of money. You can confirm the block casting numbers and even the crank number at Mortec.

The small journal 265/283/302 cranks were 3815822 (302 was 1967-68)
The medium journal (400 is large journal) 302 cranks were 3941178 and 3923279 (302 was 1969)


onebadmerc 01-20-2008 07:48 AM

If your pistons will permit it, mill the domes down as far as they will go. Then install a thicker head gasket. I wouldn't bolt on a set of larger cc heads with domed pistons. Yes, it would lower the compression ratio but it would also increase the surface area being exposed to more heat. Dome pistons are not the best pistons to use on pump gas, they have lots of spots on them that that get hot. A flat top piston will disperse the heat more evenly, being less prone to hot spot and detonation. You can also install aluminum heads which will allow you to run more compression, but keep them at 64cc's. Even if you install aluminum heads you will still need to get the pistons domes down or find some flat tops.

Double_v23 01-20-2008 11:00 AM

I think you need to find out more about your pistons. Then you can find a piston with the appropriate compression height, wrist pin diameter, and piston diameter and use it. It doesn't have to be for a 302 or 283 or 327 it just has to have the right specs...with all the pistons out there I am sure one will work!!!

MI2600 01-20-2008 11:31 AM

If you can get 98 octane gas then you don't have to lower the CR much. When the 302 was new, the best pump gas we could get was 100 octane.

If it was my engine, I might take a little off the piston tops. Putting thicker head gaskets on is going to mess with your chamber quench.

VetteInOz 01-21-2008 04:55 AM

How to get a Chevy 302 under 10:1 CR
Thanks all for your comments.

I haven't been able to check the crank castings, will probably be tomorrow and I will let you know 302 Z28

Unfortunately my pistons are hollow domes so milling is not an option.

Following up on Double V23 suggestion I have again had a look for some 4 inch pistons and came across some Ford 351 $" pistons
So could someone let me know if these might work and again my apologies for my lack of mechanical knowledge

From Kb Pistons Website

Chevy 302 Spec
Bore 4"
Pin Dia 0.927"
Comp Ht 1.825"

Ford 351
Bore 4"
Pin Dia 0.9.12" maybe could be bored out to 0.927"????
Comp Ht 1.774"

the 351 pistons are flat tops and they quote CR of 10.5:1 with 64cc heads.

the reduced Comp height would reduces the CR some but by how much I have no idea??

Someone told my mechanic Uncle that there was a ford 302 piston available with Chevy pin diameter. Anyone ever heard of this??


Double_v23 01-21-2008 07:21 AM

So here is a comparison of the two pistons,

The compression height on the Ford piston is almost .300 less than the 302 chevy, this means that the piston would be .300 down in the Cylinder head. This would be very bad.

You could try to compensate with additional rod length but that would get expensive. Have you tried actually calling wiseco or JE and asking if they have a flat top that meets those specs?

Double_v23 01-21-2008 07:30 AM


The above piston from Wiseco, is for 4.030 but there may be stock diameters available

It is a dome but the lower comp height gives it 10.5 to 1 with 67cc chambers.

You could have the top milled slightly to further decrease compression as well as remove material from the combustion chamber on the head to get it right where you want it.

I think any way you slice it, it will be a lot of work.

I have seen advertisements for a fuel treatment that increases your octane significantly, it is a cannister that you put inline with your fuel system and could bump the 98 to over 104 or something like that. You should check that out as well.

marks914 01-21-2008 02:03 PM

You can get a set of pistons made for around $600.
I am running SRP 10.4:1 pistons that have been radiused, and a little clean up on the heads. Did it get it below 10:1? I do not know but it should be close.
I have been running 93 with no problems, never tried anything lower. Aluminum heads help too.

Since you have dished pistons, you might be OK. Do the math to find the real CR.

The 302s are really fun! I am running a large journal DZ in my 914! Spins up like a Porsche.

Cool 59 01-21-2008 07:06 PM

too much compression
If 302 cranks are rare, sell it and buy a 327 or 350 crank. The piston selections for these cubic inches are almost endless and reasonably priced.

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