piston clearance with lots of NOS

[qkdragster]

freshening up 632 BBC, in the T/D and taking measurements .... this 18* Big Chief engine has Diamond pistons -- hard anodized pistons with ceramic coated domes/and skirt coating from Diamond .... they were made for a 4.617 bore ... my question is , this is an ALUMINUM Dart Big M block, "and" will be sprayed with 2 kits of NOS , 300 and 250-300 ...looking for where the piston to wall clearance should be ?

lots of things coming into play ....

anodized pistons require less than normal clearance ?
aluminum block expands more so requires less than normal clearance?
5-600 HP of nitrous makes alot of heat ? requires more than normal clearance ??

i'm kind of lost on all of this .... need help from engine builders in the know on this one ....

[GARY C]

I would consult the piston and block mnf as well a known nitrous engine builder before asking a bunch of people on an internet forum.

[qkdragster]

there are some intelligent engine builders on this forum , .... I'm sure there will be some who would like to help ...

[GARY C]

All I have ever run in street and race form are nitrous small blocks but I would not even pretend to tell you a clearance because of what I have learned, my guess is the good engine builders may give you a rough idea but each combo is different.

All of the years I have followed this forum the nitrous threads seem to be short and die fast.

[piston guy]

Talk to Diamond FIRST. They know what they make and how it reacts.

[Newold1]

I agree with talking to Diamond as they should have done similar piston sets for other NOS users. I would also recommend talking with some engine builders with direct experience working with racers who have similar big inch aluminum engine blocks on NOS. Obviously some clearances are set differently in BBC aluminum blocks but remember the blocks are aluminum but the cylinder sleeves are iron not aluminum and as such their properties will need clearances for their changes with engine heat and loads but they will not be necessarily the same as those for other aluminum engine block areas.

[cjperformance]

freshening up 632 BBC, in the T/D and taking measurements .... this 18* Big Chief engine has Diamond pistons -- hard anodized pistons with ceramic coated domes/and skirt coating from Diamond .... they were made for a 4.617 bore ... my question is , this is an ALUMINUM Dart Big M block, "and" will be sprayed with 2 kits of NOS , 300 and 250-300 ...looking for where the piston to wall clearance should be ?

lots of things coming into play ....

anodized pistons require less than normal clearance ?
aluminum block expands more so requires less than normal clearance?
5-600 HP of nitrous makes alot of heat ? requires more than normal clearance ??

i'm kind of lost on all of this .... need help from engine builders in the know on this one ....

Yes always check with the maker.
Do you know part number or at least the material grade of the pistons?
Freshen, sounds like it has already been running? If so how does it all look now, what clearance is it at now?

[GARY C]

I agree with talking to Diamond as they should have done similar piston sets for other NOS users. I would also recommend talking with some engine builders with direct experience working with racers who have similar big inch aluminum engine blocks on NOS. Obviously some clearances are set differently in BBC aluminum blocks but remember the blocks are aluminum but the cylinder sleeves are iron not aluminum and as such their properties will need clearances for their changes with engine heat and loads but they will not be necessarily the same as those for other aluminum engine block areas.

Yes, I would want to know from Dart what casting block I had in case they have changed for structural integrity.
Same with the piston plus I would want to consult piston mnf because ceramic surface coat is added, have they seen any adverse effects clearance wise.
Brand and thickness of the rings will make a difference, mnf thought with nitrous on a coated piston.
Brand and thickness of the sleeves.

[qkdragster]

already talked to Diamond , couldnt get a solid answer , got a quick reply of .."the pistons should be set for your bore " ... thats not an acceptable answer ...that doesnt help much when you have rehoned ... or what if you are spraying vs not spraying .. aluminum block or iron ..etc etc ... I have used Diamond for my last 6 sets of pistons , but havent had much luck at all with customer service ....


yes engine has been run before -- last 2 seasons but N/A ... no NOS .... sitting at .0075 clearance now ... ....

[mag2555]

I am sure that placing a call to Pat Musi racing would be worth while as they know what needs to be done!

[user-17438]

I am sure that placing a call to Pat Musi racing would be worth while as they know what needs to be done!

Yes I agree, when I'm doubt, call any of us professionals and waste our shop time so we can hold your hand on your own personal builds.

[Old School]

freshening up 632 BBC, in the T/D and taking measurements .... this 18* Big Chief engine has Diamond pistons -- hard anodized pistons with ceramic coated domes/and skirt coating from Diamond .... they were made for a 4.617 bore ... my question is , this is an ALUMINUM Dart Big M block, "and" will be sprayed with 2 kits of NOS , 300 and 250-300 ...looking for where the piston to wall clearance should be ?

lots of things coming into play ....

anodized pistons require less than normal clearance ?
aluminum block expands more so requires less than normal clearance?
5-600 HP of nitrous makes alot of heat ? requires more than normal clearance ??

i'm kind of lost on all of this .... need help from engine builders in the know on this one ....

I have sprayed 400-500 in a Donovan block. We set the first clearances up at .0075 like yours. Pistons had the tops coated but skirts were standard. After a few seasons and hone jobs the clearance is up to .009. I can tell no difference in performance, piston wear, ring life, or anything else. This is 4.375 stroke engine so my piston is not coming as far out of the bottom of the cylinder as yours will be. I would rather have the 632 a little tighter in that aspect, but by no means would I want less clearance than you have. Those skirts rocking out of the bore at the bottom is not good, but they keep on running.

The key to managing the heat is the exhaust opening. The more nitrous leads to a little earlier exhaust valve opening. I have found exhaust durations should begin around 310@.050 maybe up to as much as 316@.050 with lobe separations in the 116-118 area. That and a safe tune should be ok. This is what worked for me, others experience may vary.

I, like you, find most tech broad and vague. The folks that really know have a lot of time and money invested in their knowledge and aren't too interested in giving it away.
We could argue that pistons and blocks have different expansion rates, this and that, but experience will let you know what is right or wrong. Gets expensive sometimes though.

Best of luck with your engine. Billy

[rebelrouser]

When deciding your clearances for a nitrous motor, call the company that makes the coatings for the pistons as well. An engine I help with uses big three stage shot of nitrous, burnt several pistons after the only change was coated pistons on a freshen. The coating kept more heat in the chamber, without the coating heat could migrate down the rod and was cooled by oil splash. With the coating heat stayed in the chamber. Owner went back to non coated pistons and same tuneup, engine lasted the season no issues. I came to this conclusion after poking around in the speed talk archives.

[Frankshaft]

I would say your in the ball park now. Rehone to .008 and run it. The hard anodizing usually requires less clearance. Aluminum block, same thing. I would say you were on the loose side na for sure. The trouble is, no one really wants to commit to a number, because they have no idea how you will run it. Meaning, there are guys getting away with stuff others can't, simply because of their set ups. I work with guys I have 100% confidence in, because they are meticulous, and are very aware of the requirements. I have others, that I sweat bullets, because in their minds, it don't matter, you can't tell me 2 degrees of timing will make a difference. So, I would say .008 is fine.

[qkdragster]

I just rehoned , so .0075 is where its at.... I will go ahead and put it back together ... i would have thought anodized and N/A I would have had Calico add .001 to each skirt for .0055 total clearance.... but with the spray , I think I’ll stay at .0075 ...... a little to loose is better than a little to tight .. thanks all for the help...