Engine build question

[XCELLR8]

I'll make this short. I have a 400 block that has a 4.375" bore. I have been looking at the Keith Black 240 pistons in a .030" overbore, so 4.372". Keith Black has the piston to wall clearance at .0025" min in a street motor, and .005" in a drag motor. So my question is this the piston I need to buy? 4.375"- 4.372"=.003" clearance, is my thinking right or am I missing something. I am a novice at mopar motors and this will be a street/cruiser motor.

 

[Garys1969RR]

Yes, I think .003" is a good piston to wall clearance for a High Perf street motor. Take your pistons and block to the machine shop to have your machinist set the final clearance.

 

[DLevel]

You better check your piston to wall clearance a bit more. Most pistons that you purchase for a specific bore will have the clearance on the pistons built into the bore size. Those Keith Blacks will actually measure smaller than 4.372, probably by .0025 or so. I would measure your cylinders good with a dial bore gauge or some inside mics if that's all you have, then start doing some math. I have skirts built up by Calico on jobs like this by a couple thousands to save buying new pistons. A heavy build by them will get you an extra .0025-.0027 of piston skirt size for around 200 $ when you figure the freight. Not cheap but a big savings over buying custom pistons or when you can't make a change to a shelf piece. The calico coating has held up very well on the jobs I've used it on.

 

[69Bee]

The clearance that the manufacturer wants is built into the piston. A Std 400 has a 4.340" bore, so a 0.030" oversize bore will be 4.370". It is already too big for a normal clearance piston. If your Keith Black piston wants 0.0025", and add that to the 0.005" overbore for a total clearance of 0.0075". If your application requires more clearance than the piston has built in, then the bore is made larger to suit, but normally, the stock bore plus the oversize is how it is done. There are situations that require a deviation from this which is why I measure the piston, and hone to the desired clearance.

 

[XCELLR8]

The clearance that the manufacturer wants is built into the piston.

Thanks! That is the info I have been trying to find.

 

[747mopar]

How are you measuring the bore? You really need to check the bore top to bottom and from multiple locations. You'll measure the least at the top but could be sloppy in the hole. I wouldn't purchase piston until you have your shop check it with a dial bore gauge.

 

[lewtot184]

those pistons will be very noisey when cold at those clearances. any of those KB pistons at or near .004" clearance cold really rattle until warmed up.

 

[XCELLR8]

Yeah I'm not going with the KB pistons as they won't fit. From what I have been seeing, it looks like I'm going with a stroker. Summit, Jegs, and 440 source have them in my bore size.

 

[bm02tj]

Clean it up at .040 to fit the pistons you should buy not try to fit to what you have
Cheap man pay twice or more

 

[XCELLR8]

Clean it up at .040 to fit the pistons you should buy not try to fit to what you have
Cheap man pay twice or more

I made this post to find out if I needed to factor in the piston to wall clearance (what I thought) or if the clearance was built in to the piston (which it is), as I could not find this info. I did not go into detail about my motor. So here is the scoop, I pick up this motor for cheap (ie a rebuilding price). The motor has been finished machined, decked, has cam bearings and block has been painted. I am just looking at all options.

 

[moper]

Generally speaking, you need to order pistons after the block has been properly measured for wear and taper. YOu do that with a dial bore gage, and in the areas wear occurs. Thne you order pistons. When they arrive, the shop measured them and bores and fianl hones the bores to size to achieve proper piston to wall clearance. Modern alloys (hyper and forged) all are more thermally stable - so they run tighter piston to wall clearances. That makes more power than loose pistons of old.

 

[miller]

Yeah I'm not going with the KB pistons as they won't fit.

They won't fit? Lol.
moper is on the mark. The right method is measuring the bores, and buy the pistons for what the final cut will be on the cylinders, for the clearance 'you' want!
Machine shop shouldn't cut any on the bores, without pistons in hand, so they can be fit. I've always prefer .002 - .0025 piston clearance, though .003 is also fine.

 

[69Bee]

Myth, Urban Legend, Wives Tales, and just plain old misinformation... After cleaning the block, and even measuring it if you want, you need to bore the hole 0.005" shy of the finished size in order to know what piston size to get. I start out centering the boring bar just under the unworn lip at the top of the cylinder instead of the unworn portion at the bottom. Why you say, because centering at the bottom will ensure you have to cut one or two sizes larger to get a good cylinder. Cylinders will wear more to one side usually due to the thrust angle of the piston/rod combo in the cylinder and rotation of the crank. Any cylinder alignment parallel to the crank is not important for a street driven car. If you are not using a bore alignment plate when the cylinders are bored, they are not correct anyway. To minimize the amount of material removed from the cylinder, I bore the worst hole 0.015" (for a final bore of 0.020"), and if it cleans up, I order pistons 0.020" oversize. If I need to go 0.025" to clean up the cylinders, then I get 0.030" oversize pistons. I once had a 225 Chrysler that was std bore, and measured to clean up at 0.020", but the block and cylinders were so out of whack, I had to go to 0.040" to clean them up. Just because they measure a certain amount does not mean they are aligned well enough with the deck to clean up at your measured oversize. ALWAYS bore first to 0.005" smaller than the final size, and you won't have to return those pistons to get the next larger size...

On another note, if your machine shop "Plateau Hones" cylinders, I suggest you have that done also. After a cylinder is honed, there are peaks and valleys where the metal has been ripped from the surface. The plateau hone removes the valleys which hold the oil for the rings. It basically prepares the cylinders for a faster break-in of the rings, and keeps your break-in oil just that much cleaner. I do this as a last step to the bore & hone process, and don't charge for it. All it needs is a few strokes of the plateau brush ( I do 10 strokes), and you can even purchase your own hone brush to use in your drill.

 

[moper]

Sort of true... Least in my opinion...lol.
The CNC my shop uses will correct for bore placement and angles and indexes them off the cam centerline and crank centerlines. If need be the bore can also be moved slightly to center it within a core shifted cylinder or center off any spot along the bore in question. If the shop is still using an oldschool bar that indexes off the individual bores themselves the shop's already hobbled and that might be best to bore first. Because at that point if the bores were REALLY bad then yes, boring first will definately tell you for certain. In some cases rust pits or wrist pin gauges may mandate that too. I've known guys that had seen enough of them that they could eyeball what should clean up and what won't and they were wrong maybe 1 out of 30.

 

[69Bee]

I agree with you Moper, but I don't think the average customer would be inclined to pay for that. I assume your shop charge for a blueprint operation like that wouldn't be that same as a regular "Bore & Hone" job (I could be wrong though). A lot of times, my "calibrated" fingers can tell if a bore will clean up at 0.020", or if it warrants a 0.030" bore, but sometimes, I have to go larger all over...

ps.. I use an "Oldschool" bar, and it works quite well.

 

[moper]

Nope. All the block work goes through their Rottler cnc center. The basic operations all cost the same, the equipment makes blueprinting simply a by-product. And yes, this shop gets more than a shop using a bar. But there's no up charge for blueprinting.