Quench safety distance for a 496 MAG gen 7 BBC

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koutroul
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Quench safety distance for a 496 MAG gen 7 BBC

Post by koutroul » Sun Apr 14, 2019 3:01 pm

Hello I have a Mercruiser 496 Mag 375hp that has been bored over 030 inch to 4.280 and decked. The resulting quench varies and wanted to verify that using the original gaskets  will be ok and what is the minimal quench that I can have. I am using the Wiseco PTS524A3 pistons and K1 Technologies rods CH6700APRB8-A.
All 8 pistons are proud (above the deck) with following measurements:
Side A:
0.10mm
0.18mm
0,16mm
0.20mm
Side B:
0.18mm
0.26mm
0.25mm
0.27mm


Turner said he decked one side 010 mm and other side 020mm.

I think the genuine gasket is 0.051inches so if from 1.2954mm I subtract 0,27mm (max) I am left with 0.040 inches quench. The gasket is made to be used both with the standard pistons and the 030 over pistons that the manufacturer offers (however I am using Wiseco PTS524A3 pistons and K1 Technologies rods CH6700APRB8-A).

Is that safe or I need to buy thicker gaskets (I am trying to avoid if not necessary because will cost additional 350EUR with delivery and taxes).

On the following website I can see that for under 6000 RPM engines the valid range is 0.35-0.45 but not sure exactly for my engine:
https://help.summitracing.com/app/answe ... /a_id/4941
​​​​​​​
Thanks

In-Tech
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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by In-Tech » Sun Apr 14, 2019 5:23 pm

If you are at stock hp I don't think you will have a problem with stock ecu and non supercharged. I never liked the stock graphite head gaskets much though.

Personally, if I found that and had the ability( guess you are out of country, where I live it is a no brainer) to have the block truly squared decked I would buy whatever thickness Cometic I needed to get the quench I wanted.
Heat is energy, energy is horsepower...but you gotta control the heat.
-Carl

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by Bazman » Sun Apr 14, 2019 5:35 pm

Would have been helpful to have given all measurements in inches
Last edited by Bazman on Sun Apr 14, 2019 5:41 pm, edited 2 times in total.

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by rebelrouser » Sun Apr 14, 2019 5:37 pm

I have used .040 for steel rods and .060 for aluminum rods for 40 years, never had an issue. Steel rod engines, have seen them as tight as .035 and you could read the part number of the piston in the carbon on the head, but it did not seem to hurt anything. And just to back those numbers up, my race car engine has an aluminum block, aluminum rods and an aluminum head, my son was going to school to be a mechanical engineer and I got him all the info on what alloys my components were rod length etc. and he calculated the quench clearance for several temperature ranges. 300 degrees for rod temperature came up as .058, so I think the numbers most use are spot on.

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by koutroul » Sun Apr 14, 2019 5:47 pm

Bazman wrote:
Sun Apr 14, 2019 5:35 pm
Would have been helpful to have given all measurements in inches
the quench is 0.040 inch at the most proud piston.
regarding variation of quench between the 8 cylinders the tuner said to trust him and that he decked many blocks up to know and that it is ok to have that variations.

some of his tools are in inches and some are in milimeters so need to convert

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by koutroul » Sun Apr 14, 2019 5:51 pm

In-Tech wrote:
Sun Apr 14, 2019 5:23 pm
If you are at stock hp I don't think you will have a problem with stock ecu and non supercharged. I never liked the stock graphite head gaskets much though.

Personally, if I found that and had the ability( guess you are out of country, where I live it is a no brainer) to have the block truly squared decked I would buy whatever thickness Cometic I needed to get the quench I wanted.
what is square decked?

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by In-Tech » Sun Apr 14, 2019 6:04 pm

koutroul wrote:
Sun Apr 14, 2019 5:51 pm
what is square decked?
Google "square deck block" for plenty of reading. Here's one that describes it using a CNC machine. http://www.performanceracingengine.com/ ... aring.html
A CNC machine is not required. Most machine shops have a fixture to square 90 degree of cam/crank journal. Here is a link that shows a fixture. http://www.bhjproducts.com/bhj_content/ ... _intro.php

The reason your heights are all over the place(assuming correct stroke, rod length and compression height of piston, since you have aftermarket rods and pistons they are probably pretty good) is they surfaced based off of the original surface and/or are careless and just surface so the gasket doesn't leak.
Heat is energy, energy is horsepower...but you gotta control the heat.
-Carl

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by Schurkey » Sun Apr 14, 2019 10:41 pm

koutroul wrote:
Sun Apr 14, 2019 3:01 pm
Side A:
0.10mm
0.18mm
0,16mm
0.20mm
Side B:
0.18mm
0.26mm
0.25mm
0.27mm

Turner said he decked one side 010 mm and other side 020mm.
In-Tech wrote:
Sun Apr 14, 2019 6:04 pm
The reason your heights are all over the place(assuming correct stroke, rod length and compression height of piston, since you have aftermarket rods and pistons they are probably pretty good) is they surfaced based off of the original surface and/or are careless and just surface so the gasket doesn't leak.
If it was only the block needing to be square-decked, it wouldn't have sags in the middle. (16 mm in between 18 and 20, 25mm in between 26 and 27)

To me, the block may need square-decking, but there's ALSO problems with compression height, rod C-to-C, or (most likely) equalizing the crank throws (stroke).

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by MadBill » Mon Apr 15, 2019 12:55 am

My 4.0" stroke BBC with loose-fitting TRW anvils and 0.040" squish turns 7,000 RPM with never a sign of contact; the short answer is per some replies you'll be fine at 0.040"/1.0 mm.

After that the thread drifts into the bottomless realm of blueprinting...

Also, fun fact: USA, Liberia and Myanmar use all imperial measures. No one else in the world does.

PS: did you leave out a decimal in your decking numbers? 020 mm is less than 0.001"
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by In-Tech » Mon Apr 15, 2019 1:25 am

I did say correct stroke didn't I? :wink:
Heat is energy, energy is horsepower...but you gotta control the heat.
-Carl

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Re: Quench safety distance for a 496 MAG gen 7 BBC

Post by Newold1 » Mon Apr 15, 2019 10:11 am

It is a little obvious the deck is not exactly square but in the 8.1L 496 that's not uncommon as the decks are not real thick to begin with. Redecking the block now to square up that minor irregularity on this engine now will just create less quench distance and less clearance piston to head distance. If this engine ever had a serious overheat it might be wise to check the mainline crankshaft bore and hone it ever so slightly if needed. If the original main bearings looked as good as most in these 8.1L's do then it should also be fine. With the bore size and the fact you are now using forged pistons now versus the original hyper-eutectic pistons and the larger piston to bore clearance will allow some more piston rock in this stroke so the distance becomes a more critical thing. Most stock 8.1L blocks measure about +.001"-.003" above the deck and the .051" compressed stock gasket keeps the quench distance at about .045"-.048" which is safe even with tight piston to bore clearance of the stock block. If it were me I think I would shave each head approx. -.010" and run the .060" Cometic MLS head gaskets and end up in that same .045-.050" quench. The heads probably can benefit from a nice smooth cut. The compression ratio change is almost nothing and in a marine engine is ideal for extended runs. JMHO :wink:
The Older I Get, The Dumber I Get :wink:

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