Good afternoon. I'm new here. My name is Tinman and need help selecting pistons for my sbc. It started life as a 400 and was bored to a 406 by a previous owner. I tore it down and noticed some pitting and ring marks in the cylinder walls. To clear that up, I'll need to bore it to .060 over and install new pistons. It currently has double hump 461 heads on it that have been decked and ported, running hydraulic flat tappets and a more-than-mild thumper cam (not sure of the specs). The pistons in it are 4 dimple flat tops. The previous owner told me it was at 10.5:1 compression before I took the heads to have them resurfaced. Do I need to run 4 relief pistons like it has or can I run 2 dimples? I don't know enough about these to make a decision like that. I like the high compression although it does worry me a bit now that I have to have it bored to .060 over. I had originally planned on running aluminum AFR heads and a blower in the future but, with the cylinder walls being so thin after the .060 bore, I've decided to stick with the double humps and keep it NA. Especially with the small Model A radiator.
That's what I thought when I pulled it apart. I've been running the hell out of this motor for the last month or so. It already had 1000 miles on it when I bought it. I messed up and forgot to plug my electric water pump back in after doing some work on the alternator bracket and drove it a few miles down the road before realizing it. It was overheating at around 265 degrees when I realized what happened. I plugged the pump back in and drove it home but, the head gasket was already blown by then. I lost compression in 2 cylinders and pulled it apart. That's why I'm having the heads surfaced. This damage to the cylinder was already present and the engine was running great with 215psi compression and no smoke. I haven't pulled the pistons out yet to see what kind they are or what rings are in it. I was going to do it all at once when I order new pistons.
I have been a big fan of the 400 sbc for years, I have built probably 20 or more. I only stopped because the availability of core's have dried up and people think they are gold now. That being said the max overbore I have ever done is .040 . I know guys have done .060 on race engines but you would really need to sonic check it. also since your engine got so hot they tend to crack between the upper steam holes and the adjacent head bolt holes. IMHO I would find a good 350 and maybe go with your blower Plan.
If it was me I would disassemble the motor and take it to a machine shop. Have them clean it and check it all the way around to see if 40 will clean it up. I'm currently building a 40 SBC with a balanced rotating assembly from Motorsports Unlimited. I had the block originally worked over for boost for my Chevelle. But the wife's roadster engine was worn out and got hot a few too many times. So I opted to use my block as it is fresh and ready to go. The pistons that I'm running in the new motor only have the 2 upper valve relief's which are fine. Those 4 valve relief pistons are run of the mill stuff.
You need to piece engine, put sleeve to one cylinder. Bore bigger, byu new cylindersleeve (tractor....) which gives You outer dimension. zero fitting (freeze sleeve and drop in). I use 2/100mm press (2-3/100mm bigger sleeve than hole. After that You bore and hone cylinder for old piston. works Your lifetime. Aulis
Well, I think if it were mine, it would have its cylinders treated to a rigid hone to remove the surface discoloration. Then measure the bore to see if it would up at .040. Being the cheapskate I am, I'd probably go to really coarse stones and remove the added .010, or close before switching to finish stones. If it doesn't look reasonable after the first hone, it would get a 350.
First thing : CHECK FOR CRACKS !! YOU CAN DO THIS !! Go to your local welding supply & buy a DYE-CHECK KIT, Disassemble the engine & thoroughly pressure wash the block CLEAN, then use the dye-Check kit! If you've gotten lucky it won't have any & all you will be out is the cost of the kit! Then lightly hone the block & reassemble(suggest new rings & oil pump)), unless you discover damaged pistons(bummer!). & of course the new gasket set, but that expense was a "given" from the beginning! NOW : if you found cracks in critical areas you're going to need a new block anyway & can choose to build what you're able to find. Now that you've learned to use that Dye-check kit you will be able to determine quickly if the block(s?)) you find are worth buying or not quickly, & if the seller won't let you check it, move on to the next block immediately!
As to piston selection, by your description it appears that it got to 406 by the +0.030" over bore and kept the stock rod length. So when looking for pistons keep that in mind and be aware that the popular 406 recipes use a 5.7" or 6" rod. Those pistons won't work for you. As to piston configuration and compression ratio. The wrist pin location in the piston (compression height) has more to do with compression then what the piston top looks like. That determines where the piston sits in relation to the deck. The piston top configuration does affect the plus or minus volume it takes in the combustion space and you'll need that info when you crunch the numbers to calculate actual CR. You will also need to know the combustion chamber volume on your cylinder heads after being milled. There are several on line calculators that will do the math ounce you have all of the info.
Those pistons are the factory style "dished" pistons. If you can buy+ .040 replacement pistons and there are no cracks in the block, I'd say have it honed and buy a new set of pistons like you have and use the 461 heads.
Your best bet is to move on and get another motor or block. Even if you get it to clean up at .060, you will most likely have problems and ruin the new parts you put into it. If you want a 400, look at the aftermarket blocks, or buy/build a 383. Don't talk yourself into doing something you will regret later, it gets expensive.
If it was my engine, it would come completely apart, thoroughly cleaned, then taken to a competent machine shop for a mag particle and sonic check.
So, is that pitting in the cylinder or just a stain? I had a 350 race engine that had one cylinder that looked similar after sitting over the winter in an unheated garage. It looked awful, but it was really a stain on top of the cylinder wall. I might have taken some emery to it, but I don't remember. Whatever I did was minimal. I checked bearing clearance and put it back together. It ran fine in the race car for the next season.
I would roll-the-dice and hone it and run with it ! Considering what you have the motor in.....pulling the engine won't be a big deal. RUN IT ! 6sally6
If that pitting is only in the one cylinder, I would have your machinist measure the rest of the bores and if they are still in spec and not in need of an overbore I would have the one hole sleeved and stay at .030 over. As for piston choice, you need to cc the combustion chamber on the heads after they were milled and then check the deck height after that you can choose a piston that will give the compression you want. What you have now are stock replacement style dished pistons. Also check the rod length, most people building a 406 will switch to a 5.7 or 6" connecting rod and you will need to know which rods you have in order to get the right pistons
Looks to me that it’s not going to clean up with a simple honing. plan on installing a sleeve on at least the cylinder shown.
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