I im assembling a 400 sbc with std bore, and just cracked a piston during installation, and can't find a matching replacement. I did find a good deal on a set of pistons designed for a longer rod and stroke, pistons seen here http://m.summitracing.com/parts/jep-182038-1. I would like to use them on a 5.65" rod to lower the compression. I know the obvious issue would be "will there be clearance at bdc?" Just wondering if anyone has done it or has any experience. Thanks!

 

The more obvious question will be "does it have enough compression to even run"?

 

6 - 5.65 = .35

That's almost 3/8".

Considering that good-quality engine builds look for no more than a few .001" in that place, I'd recommend against this (lack of a) plan.

To Apeiron's point, this will result in a static compression ratio of around 6:1. How much belt-drive blower are you planning on using just to get the engine to produce enough power at low RPMs to be able to cycle all by itself?

If you want to use those pistons, get a set of the rods they were designed for. If you want to use the rods you have, get the right piston(s) to fit them.

 

Ok, it says they'll be about 15:1 with my milled dart heads. Just wondered if it could get to around 10:1, I've got a set of 5.7 rods too. But you're right, doesn't sound like a good idea, would be sweet if they'd work tho, bc they're forged and cheap.

 

Sounds like domed pistons for racing.

This kinda belongs in the "if you have to ask because you don't already know, then it's not for you" category.

 

You do realize that's for a single piston - almost $125 for a single piston isn't what I'd consider cheap at all....

That said -no you really can't use a piston designed for a 6" rod on a 5.7 or 5.565 (stock 400 rod) unless you're adjusting the crankshaft's stroke for the difference in compression height.

Just order a single replacement piston from the same MFGR and same part number and have it re-balanced to match the one that got cracked. If it's a factory piston just let everyone know as I'm pretty sure someone has a couple laying around somewhere they'll let you have (free or cheap)

 

I could have gotten a whole set of those forged trw's for <$100 shipped. the piston im looking for is part number h400p. The reason ive considered switching is because I cannot find a matcher... I believe its a speedpro hypereutectic. std bore. anybody got a set under the work bench?

 

The link you posted was for a JE piston and not a TRW... The H400P piston is a speed pro part number and I'd bet if you call summit they'll sell singles - probably $40-ish for the single piston

Heck - here's one - it's skirt coated (C suffix) but would be fine to match the rest. http://www.summitracing.com/parts/sl...make/chevrolet

 

JE was what I meant but that's great, I was looking at the h400CP piston but wasn't sure if they were the same piston with a coated skirt or not. I haven't been able to find literature on the H400P's that I have to confirm a comp ratio. that's good news though. Im still going to unpress my pins in order to weigh my piston and compare to the CP but sounds like my best bet.

 

This cracked piston is likely a blessing that given what I am about to tell you,you will see what I mean.

To find a good 400 is rare and to find,well for that matter any used first gen SBC that you can run as standard bore almost never happens.So when you tear it down dial bore measure it in multi locations in each bore.I suspect the broken piston is a result of trying to put a round peg in a oval hole.The other thing is bore to pistons clearances are much different on aftermarket pistons and sometimes is by manufacture. The forged pistons require bigger bore to piston clearances than the OEM cast pistons.That is why the machine shop needs to have the pistons in hand before any work can be done.Any bore work on 400's needs torque plates because the 400 bores move around alot.Being 400's castings vary a ton,they need to be mag'ed and sonic checked for wall thickness.

To your original question,no you can't use that piston.You should build based on the stacked parts method trying to target quench measurement of .040.

 

thanks 1gary, I actually just got it all back from the shop and the cracked piston was a result of my chincy autozone ring compressor rocking slightly sideways and allowing a ring end to catch the deck as I was tapping it it.