Bracket Talk
Piston to Deck Clearance
October 18, 2021, 09:06 PM
Jimmy FranciscoPiston to Deck Clearance
Mocking up short block and pistons are sticking out of the hole .011. Is this something you would be comfortable with running by going with a .053 head gasket rather than a .041? What's the most you'd be comfortable with the piston out of the hole?
548 BBC (4.530 x 4.250), big m cast iron block, steel rods, forged aluminum pistons, afr 335 heads (aluminum).
October 18, 2021, 09:59 PM
Mike RietowLay the head on it no head gasket, spin it over.
What did you find?
October 18, 2021, 10:20 PM
Mike RietowIt's rudimentary but if it don't clear, you have no other option but a thicker head gasket. 053 minimum if you have a good machinist, excellent piston to cylinder clearances.
October 18, 2021, 11:11 PM
imakehpThe pistons will clear with either head gasket.
You probably just won't build up any carbon on the pistons with the .041" gasket..Normal with a max effort engine.
If the pistons are crazy loose in the bores or you don't mind losing a little compression, play it safe and use the .053" gasket.
p.s. I wouldn't bolt the head on without a gasket and try to turn it over. You MAY be able to turn it over with a big enough breaker bar but you risk closing up the ring grooves and pinching the rings.
.
Dave
F J B
October 19, 2021, 12:10 AM
TOP38The 41 will hit, the 53 will knock the carbon off and may also hit, use a 60 and move on.
October 19, 2021, 12:55 AM
imakehpWith steel rods I disagree. But it's not my engine or decision.
.
Dave
F J B
October 19, 2021, 01:22 AM
M120Im not an engine builder but I agree with Top38.
It's a bracket engine. You're not giving up much.
If you was racing comp elim it would be a different story.
Be safer than sorry. You're not going to lose a tenth from the gasket.
October 19, 2021, 01:31 AM
imakehpquote:
Originally posted by M120:
Im not an engine builder but I agree with Top38.
It's a bracket engine. You're not giving up much.
If you was racing comp elim it would be a different story.
Be safer than sorry. You're not going to lose a tenth from the gasket.
I agree. That's why I said "If the pistons are crazy loose in the bores or you don't mind losing a little compression, play it safe and use the .053" gasket."
I coudn't just assume it was a bracket engine, so I gave a couple of qualified opinions/choices.
.
Dave
F J B
October 19, 2021, 05:55 AM
Busted KnucklesMost LS engines end up with proud pistons once they've been decked/trued and it seems to be a non-issue.
Organized people are just too lazy to look for their $h!t.
October 19, 2021, 06:14 AM
adv ET 266It’s not good and will be an issue in time. Any piston to head contact is bad, even enough to clean carbon off.
Many ways to resolve. If the piston top is thick, mill 15 off the tops. Off set rod bushings is an option.
2005 2000lb 4 link dragster
home brew 582 BBC Dart 355
1.058
2.98
4.629@149
6.094
7.310@185
October 19, 2021, 06:36 AM
"The Bender"You say the pistons are .011 out of the hole, but what you don't know is how much piston
dome to head clearance there will be. Don't assume that the flat portion of the piston will be the closest part of the piston to the head.
You could put a .060 gasket on it and have .049 clearance on the flat portion of the piston, but .000 (or less) at some point of the dome. Lot of variables there with piston dome/chamber design.
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October 19, 2021, 07:24 AM
Jimmy Franciscoquote:
Originally posted by "The Bender":
You say the pistons are .011 out of the hole, but what you don't know is how much piston dome to head clearance there will be. Don't assume that the flat portion of the piston will be the closest part of the piston to the head.
You could put a .060 gasket on it and have .049 clearance on the flat portion of the piston, but .000 (or less) at some point of the dome. Lot of variables there with piston dome/chamber design.
All other clearances are good.
October 19, 2021, 07:24 AM
Jimmy Franciscoquote:
Originally posted by imakehp:
With steel rods I disagree. But it's not my engine or decision.
So you'd prefer it be an aluminum rod... that doesn't make sense to me
October 19, 2021, 08:38 AM
wideopen231Do not think he is saying it should be aluminum rod.Just that with steel rod less chance of rod stretch causing issue.
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October 19, 2021, 09:08 AM
TOP38quote:
Originally posted by imakehp:
With steel rods I disagree. But it's not my engine or decision.
It's a free world, you can disagree!
Should clarify a few things here, first just talking about the piston flats - no dome, and there's touching and there's hitting, big difference.
By touching I mean the piston contacts the head due to piston rock at TDC and this usually cleans of the carbon. Hitting means there's no where else to go and you are compressing the piston/rod between the head and crank, definitely not good.
As bore diameter increases, so does piston rock therefore more piston to head clearance is needed, just as it would when the piston gets looser in the bore. Piston design also plays a role here too...
RPM plays a role here also.
No engine builder out there I know builds a motor for bracket racing that wants hitting or touching to occur!
For the 4.5" to 4.600" BBC's, .050 to .055 total deck clearance is safe, meaning the piston won't touch or hit.
October 19, 2021, 09:27 AM
Jimmy Franciscoquote:
Originally posted by TOP38:
quote:
Originally posted by imakehp:
With steel rods I disagree. But it's not my engine or decision.
It's a free world, you can disagree!
Should clarify a few things here, first just talking about the piston flats - no dome, and there's touching and there's hitting, big difference.
By touching I mean the piston contacts the head due to piston rock at TDC and this usually cleans of the carbon. Hitting means there's no where else to go and you are compressing the piston/rod between the head and crank, definitely not good.
As bore diameter increases, so does piston rock therefore more piston to head clearance is needed, just as it would when the piston gets looser in the bore. Piston design also plays a role here too...
RPM plays a role here also.
No engine builder out there I know builds a motor for bracket racing that wants hitting or touching to occur!
For the 4.5" to 4.600" BBC's, .050 to .055 total deck clearance is safe, meaning the piston won't touch or hit.
I don't necessarily disagree with what you're saying but with what you're saying here you would be uncomfortable running 0 deck and a standard .041 gasket? I have to imagine there's a ton of people out there running that setup on a steel rod bracket bbc (540-632).
October 19, 2021, 09:51 AM
Jerry KatheFWIW, my reference - preference is a working clearance of no less than .025 (good) steel rod and .05 aluminum as a rule of thumb.
I kind of feel what is being communicated is why push the boundaries for bracket racing @ minimum?
The amount of power gain between being safe and safe enough to get by isn’t going to be worth the effort without closing the deal on the rest of the piston to chamber profiling. If these are off the shelf, out of the box pistons, I wouldn’t waste another minute on pinching it tight, I would make sure it is adequately safe and run it.
Jerry Kathe
October 19, 2021, 12:18 PM
TOP38quote:
Originally posted by Jimmy Francisco:
quote:
Originally posted by TOP38:
quote:
Originally posted by imakehp:
With steel rods I disagree. But it's not my engine or decision.
It's a free world, you can disagree!
Should clarify a few things here, first just talking about the piston flats - no dome, and there's touching and there's hitting, big difference.
By touching I mean the piston contacts the head due to piston rock at TDC and this usually cleans of the carbon. Hitting means there's no where else to go and you are compressing the piston/rod between the head and crank, definitely not good.
As bore diameter increases, so does piston rock therefore more piston to head clearance is needed, just as it would when the piston gets looser in the bore. Piston design also plays a role here too...
RPM plays a role here also.
No engine builder out there I know builds a motor for bracket racing that wants hitting or touching to occur!
For the 4.5" to 4.600" BBC's, .050 to .055 total deck clearance is safe, meaning the piston won't touch or hit.
I don't necessarily disagree with what you're saying but with what you're saying here you would be uncomfortable running 0 deck and a standard .041 gasket? I have to imagine there's a ton of people out there running that setup on a steel rod bracket bbc (540-632).
I know my 632 that sees a max rpm of 7800 plus touches at .050",, can't argue with the results, this was a new motor, new block. Like I said, bore diameter, rpm, piston design etc all factor into what's really needed. I will say moving done in cubes to a 555 or so does allow for a longer skirt piston which helps.
October 19, 2021, 02:49 PM
JPoseyMy engine builder installed offset rod pin bushings 0.015 to cure simular issue in my 505 BBC.
October 19, 2021, 04:03 PM
imakehpI think everyone agrees, in this case, just use a thicker head gasket and live happily ever after.
Nobody will know and nobody should care.
.
Dave
F J B
