I am in the process of building a 406ci motor and was looking at a a certain SpeedPro hyper piston with the coated skirts. The block is going in to get machined and there is a good deal on these pistons right now on ebay. The question is should I wait and get the pistons first and measure them before getting the block machined (bored) ?

The pistons are 30 over and the block is going to be 30 over so is this OK for piston to wall clearance or should I make more accurate measurements. It is for street use no nitrous.

 

Wait until you have the pistons in hand.

 

:iagree:

 

Piston side clearance is extremely important. You need to final hone the block to provide the clearance recommended by the piston manufacturer.

I had too much side clearance once and the piston slap was enough to break the skirts off of the TRW forgings. For that reason I would never buy a piston with the oil slot again. The stronger pistons have oil holes only.

 

How do you measure this clearence and what should it be?

 

Actually the shop boring my block did all the measuring while i watched. They used a inside caliper to measure the bore and a regular caliper to measure halfway down on the piston (it tapers) and subtracted the piston measurement from the bore measurement. If i recall correctly about 4 thousands clearance on a forged piston and 2.5 thousands on a cast. I would guess the hyper pistons would be somewhere in the middle, but check with the piston manufacturer.


I had over 6 thousands clearance with the TRW forgings and the shop thought it would just be noisy with piston slap, but after about 20k miles the piston skirt separated and it really got loud!

 

If your machinist will finish hone a block without measuring the pistons first I'd quietly pick up my block and leave immediately.Mine wasn't sure what to do so he even called the piston mfg.and got the recommended specs straight from the horses mouth.Every piston expansion rate is different and I'd hate to seize one in the bore turning 6500 rpms!

 

My speed pro hypereutectic pistons (H426CP) are made for a block .030 oversized. Minimum clearance is 0.0015, bore range 4.2800---4.2810. My machine shop says this piston uses a very tight clearance.


(these are for my 454, just posted sizes as an example)


[Modified by greg454, 5:55 PM 12/9/2003]

 

The machinist told me that the piston clearance is built in to the piston therefore if the manufacturer says the piston needs 0.0015 clearance then the standard size piston will measure 0.0015 less than standard bore or 0.0015 less than 30 over for a 30 over bore etc.

 

Back in the mid 80's I used to put together small blocks all the time. Then I was 16 and the little insignificant details like checking clearences wasn't necessary. If the piston fit in the bore it was fine. If it was a little tight just coax it a little. :smash: :yesnod: Those motors ran great with regular trips to 7k.
Now that I am older and know the value of a dollar these insignificant details are making more since to me. :yesnod: However I haven't learned all of my leasons yet :eek: I had the block machined before I bought my pistons. A mistake I want make next time around.

Greg I have almost the same pistons that you do only mine are the H693CP Hyper's Just checked the box and it to says .0015

So let me see if I have this right. Measure the inside of the bore then measure the diameter of the piston. Subtract piston diameter from bore and I have my clearence correct?

Thanks
Wade

 

Is the clearance .0015 per side? If so, then the piston would be .003 smaller than the bore?

 

Box just says .0015 minimum. I would assume that that would be per side.

Wade

 

It seems that even piston diameter is not a easy measurement...
From Lingenfelter:
"Because pistons are eccentric, measuring the piston diameter in the proper place is critical to establishing the correct piston-to-wall clearance...piston manufacturers and where they specify piston diameter should be measured...varies dramatically...check with the manufacturer of the pistons you're using before measuring your pistons. All measurements must be taken on the piston skirt."

 

If your machinist will finish hone a block without measuring the pistons first I'd quietly pick up my block and leave immediately.

Good words of advice from Money Pit. No machine shop should touch the block without the pistons before hand.

 

When piston clearances are given it is not per side. If the spec calls for .0015 clearance that is the size of the hole over the diameter of the piston measure in the location the manufacturer specifies.
Do not double any clearnaces given on the motor, NEVER. Clearances are given as total over the diameter and not per side.

 

Ok I agree with this to a point and I definately don't want to get a mud slinging contest going here. The machinist that I am working with has had a impeckable reputation here in the Charlotte NC area. He has put together and machined many dirt track, grand National and local drag strip motors. When I took my block to him he did exactly what I asked of him which was check the busted bore out and see if it is worth sleeving if it is go for it and bore the other 7 +.030. I am not saying anyone is wrong here I am just saying that I know of not only this machinist but also others that would bore a block without piston in hand. Do to thier experience and not lack there of. In fact as I stated in an earlier post on this topic in the mid '80's most if not all of the Hot Rodders here would buy pistons after the block work was done. I think by following the advice of picking up and leaving based on that in some instances may be flawed. I seriously doubt if a majority of budget Hot Rodders or newbie Hot Rodders have done every last thing by the book in terms of measurements, machine work etc and still have good running dependable engines. Again I am not saying that fitting the block to the pistons isn't the way to go. In fact I agree 100% after doing some reading that it is a valuable step for added insurance. But so is a lot of things. I bored my block before buying pistons as I have always done. Next time around I will go the other way. But I do know from a lot of experience with SB chevy's and Fords they will run just fine with out bore matching and a lot of other tricks that over years have been tossed in the category of must does. Just my 2c :seeya

 

I just took a new Mitatoya Bore gage out of the package and had a good look at it. It is just a very high priced precision dial indicator. You mike the piston with you micrometer and regardless of the reading you then calibrate your bore gage, You take the bore gage, insert it in the micrometer and bring it to Zero. Regardless of the accuracy of the micrometer the bore gage reads the same with a very precise movement plus and minus of the set reading. No chance for inaccuracys. The mic can be off by .001 or .1 or 1 inch and the bore gage is still precisely calibrated.
I always use torque plates, torque up the main caps and supply the pistons after personally checking with another gage far more accurate then a mic to make sure they are all exactly the same. If one is out even .0005 I mark it as special.
I also always check the shops work. They know I will check and I get what I want exactly. If I ask for .0045 I don't mean .004 or .005 I mean .0045. I don't hesitate to pay for the extra work and tell them money is not an issue but precision is.

 

amen to the torq plates!
I wondered when someone would bring that up

 

I will have the pistons first but thinking about this and talking to someone who makes a living doing this I get this impression. For a stock street motor with a 30 over bore and stock ( well flat tops ) 30 over pistons I don't think there is too much to worry about, this I feel is due to the precision machining processes they have these days ( CNC I'm refering to the pistons ). Now of course you should mic every bore and every piston before you assmble the motor but the chances you are going to have to make any adjustments are slim. Flames away :lol:

I was also told that for full racing applications, nitrous use etc. you can adjust the piston wall clearance but for most street/strip applications you can bore it 30 over and the 30 over piston will be just fine.


[Modified by MotorHead, 9:26 PM 12/10/2003]


[Modified by MotorHead, 10:50 PM 12/10/2003]

 

All great poiints guys.

Norval
Your attention to detail and knowledge is awesome and I appreciate having you on this forum as a knowledge resource. I have read many posts from you and know that you take every precaution to ensure a great running high output motor.


:iagree: I live in the heart of racing country here in NC and growing up in the late 70's and 80's I have worked for several small budget race teams as well as having a HS job at Super Shops when they were in buisness. Back then on a budget I have seen many circle track motors put together where the block was bored first then the pistons were purchased. Again I am not arguing with the importance or advantage of piston fitment. I am just saying that walking out of a machine shop because the machinist would bore the block without piston in hand makes no sense for your average stock\street\moderate hi po build up.

One last comparison that I want to make to support my position is cam installation. There have been many posts on this forum over the years regarding degreeing a cam. Some are adament about the importance and some are not. I venture to say that the vast majority of home hobbyists install the cam straight up w\out degreeing. I personnaly have done both methods and have never had either a cam failure nor have I found that the cam did not meet the spec card. That being said I am sure it happens and people in manufacturing make mistakes. If you do install the cam straight up you are trusting that the machine work and the cam manufacturing are correct much the same way as you would be trusting the fitment of pistons purchased after the block was bored.

In conclusion anyone building a motor should check clearences, keep everything clean and be as precise as posible. But there are some things that just because somebody said something sometime ago doesn't make it GOSPLE. You are not going to engine builders hell if you don't follow every practice\method out there. And your machinist is not incompetent or a crook if he bores your block with out the pistons in hand. Some will agree some want but that is what makes this country and this hobby fun.

Wade :thumbs: