Don Terrill’s Speed-Talk

I'm assembling a 540 BBC for a customer: street driven 4X4 and probably a little mudbogging thrown in . Running the pins through the pistons I found some to have a more resistance compared to others so I decided to measure the clearance using the sunnen dial comparator on our rod honing machine. I found that all the pistons appear to have the same clearance, whether they feel tight or not, and that this clearance is .0017". According to my research this seems large...is it anything to worry about? I had been thinking of giving the tight piston's a few strokes on the hone...but haven't really seen any recommendations for having the piston to pin clearance greater than .0013". SRP 140329 Pistons, 2618 material, piston weight 625g, 4.5" bore. JE Piston Pins 990-2930-15-51S...nothing special. The entire rotating assembly is one kit from SCAT, so im not mixing and matching parts and pieces. Thanks for your replies.

that clearance is loose I find that at .001" the pins fall through, and you shouldn't feel any resistance. clearance over .0015" will need some good thick oil. I've seen them run at .003" no problems but take that clearance in to account,I would confirm your clearance use an inside bore gauge other than the ag 300 .and how did you set it up not to be a smart a-s.

I would suggest using a clearance of .00050-.0007 pin clearance but I think your gauge may need to be reset and checked.
Mic the 2 pins you use to set the Ag300 and then use a small bore gauge if you can to check the pin boss ID.
Or use the pins to set the Ag300 then use a pin as a standard and mic it and compare the mic'd size to the "0" on the Ag300 they should be the same. Checking pin bores may require you to use a different set of gauge point locations depending on the piston design. Which set of threaded holes do you have the points in?

apprentice1 wrote:I'm assembling a 540 BBC for a customer: street driven 4X4 and probably a little mudbogging thrown in . Running the pins through the pistons I found some to have a more resistance compared to others so I decided to measure the clearance using the sunnen dial comparator on our rod honing machine. I found that all the pistons appear to have the same clearance, whether they feel tight or not, and that this clearance is .0017". According to my research this seems large...is it anything to worry about? I had been thinking of giving the tight piston's a few strokes on the hone...but haven't really seen any recommendations for having the piston to pin clearance greater than .0013". SRP 140329 Pistons, 2618 material, piston weight 625g, 4.5" bore. JE Piston Pins 990-2930-15-51S...nothing special. The entire rotating assembly is one kit from SCAT, so im not mixing and matching parts and pieces. Thanks for your replies.

You shouldn't feel any resistance with .0017 clearance, On pistons with a .990 pin we run .001 to .0013 clearance.

We bore gauge all our pin bores and most pistons need honing to acheive the .001 except the Mahle pistons those seem to fall right at .001 to .0012 and so far have never had to hone a set of those pistons before.

We just had a set of Diamond turbo pistons come in the measured .0012 which is rare. Most Dimaond pistons need honing!!

It hard to measure pin bores with an AG-300 gauge.

i think big joe would have something to say about running .0005-.0007 pin clearance.too tight in my opinion.especially with something that makes crankcase vacuum.(in drag race situations,of course)

.0005"-.0007" is fine for that application, I would go ~ .0008" on the rod to pin....assuming it's not dry sumped or running a vac pump.

Another thing to check on your setup is the centralizer pin, you should have a setting ring with a slot in the bottom for setting up the gauge. Are you using the setting fixture that holds 2 pins, or are you just setting with a hand mic?

I have the points in the holes that are the farthest out. I had set the ag300 with a mic set to the wrist pin size...I don't think we have a way to set it over 2 pins, ill check tomorrow. But look at my name...i'm learning...and thanks for bearing with guys like me and answering all our questions. It is much appreciated. I will have to check with a dial bore guage tomorrow. Thanks again for the replys.

yes I had checked the centralizer pin with the setting ring...but of course if I'm off because a hand mic isn't good enough...

[quote="needforspeed66gt"].0005"-.0007" is fine for that application, I would go ~ .0008" on the rod to pin....assuming it's not dry sumped or running a vac pump.quote]

Yup that was my thought as well for a street truck. No mention of oil system or vac. pump.

Carl
Are you seeing the heat in street stuff for that much clearance? I would have thought a bit tighter I have a hard time seeing a pin grow that much over the boss.

it is not dry sump and it will not have a vac pump

The setting fixture is explained in this set of instructions.

http://www.sunnen.com/graphics/assets/d ... be80c6.pdf

ProPower engines wrote:

needforspeed66gt wrote: Carl
Are you seeing the heat in street stuff for that much clearance? I would have thought a bit tighter I have a hard time seeing a pin grow that much over the boss.

Most sreet engines are press fit and I do run that much clearance, We have seen the GM Crate engines 602 that make 355 horse and we see that with .0006 pin clearance from the factory that the pins will gaul.

Again look at the Mahle pistons as we use a lot of them for street performance and circle track engines those run right at .001 to .0012 clearance and zero issues.

Most pistons come with pins that are .9272 to .9275 but will the same size per set!! And if the pins were .927 I don't think I would have to hone the pin bores.

We took apart an engine from another shop and the Manley pistons the pin bores were to tight and the pin bores were black, I have not check the clearance yet but I would say they are to tight.

needforspeed66gt wrote:


The setting fixture is explained in this set of instructions.

http://www.sunnen.com/graphics/assets/d ... be80c6.pdf

The sunnen comparator has brass shims on back that can wear sunnen acn send you instructions how to adjust, furtermore using a good mic and setting the ag300 properly or a bore gauge seems to be more accurate than the comparator.

needforspeed66gt wrote:


The setting fixture is explained in this set of instructions.

http://www.sunnen.com/graphics/assets/d ... be80c6.pdf

That gauge is for checking the pin end of the rod and seems to be pretty accurarte as we check with our bore gauge when we are done.

Its hard to get a piston square to the face of the AG-300 and we have found a sunnen bore gauge works the best.

Its hard to get a piston square to the face of the AG-300 and we have found a sunnen bore gauge works the best.[/quote]


Yes it is....it takes some time to get the "feel" for it...more of a pivot and sweep across the fingers..
The AG 300 is the only way I have ever check piston pin bores...20 plus years