Request Competent Mechanics
Thread Starter
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Cruiser
Joined: Feb 2014
Posts: 107
Likes: 10
From: texas
Request Competent Mechanics
so, made a post awhile back about rebuilding my top end. leaking from a base gasket anyway, decided to rebuild whole damn thing since im getting at it. by recommendations here, and from google, i ordered the Cometic gasket set. now im running into procedural problems, but first, a reason why im paranoid-
i am a former helicopter mechanic for the Marine Corps. maintenance will be done by the book, properly, step by step, or people will die and you get a one way ticket to Ft. Leavenworth (in this case, i loose my bike, equal importance). So when something is out of spec, (bike, car, washing machine) i do my best to find answers, this is one of my resources.
The gasket for my head was of graphite construction. maybe 1/32 thick. the gasket for my jug appeared to be of asbestos construction, or a derivative of. Both my gasket surfaces on my jugs are concentric serrated, probably because of the construction of the gaskets.
the cometic gaskets are MLS: multi-layered stainless coated in a thin film of Viton. the half sheet of paper it came with states that the surfaces must be finished to a 50RA.....what the **** is that? and why is it that when i google it, i get a load of sites using it but not explaining what it is or what it compares to. To obtain this feeling, try asking your doctor when he tells you your blood pressure. "120/80" ...ok, 120/80- what. SCFH? PSI? in/WC? ...its actually mm/Hg but more than likely they will look at you like a deer in head lights.
the best i can come up with is that 50RA "compares" to is 50 grit....in which case im pretty sure serrations will be good enough. IF NOT I AM DEAD WRONG THERE....any one know how much i can shave off my jug safely in order to obtain that 50RA surface? i have a machine shop, all i need is a ballpark safe number to give to my machinist.
as always, thank you for any and all help.
i am a former helicopter mechanic for the Marine Corps. maintenance will be done by the book, properly, step by step, or people will die and you get a one way ticket to Ft. Leavenworth (in this case, i loose my bike, equal importance). So when something is out of spec, (bike, car, washing machine) i do my best to find answers, this is one of my resources.
The gasket for my head was of graphite construction. maybe 1/32 thick. the gasket for my jug appeared to be of asbestos construction, or a derivative of. Both my gasket surfaces on my jugs are concentric serrated, probably because of the construction of the gaskets.
the cometic gaskets are MLS: multi-layered stainless coated in a thin film of Viton. the half sheet of paper it came with states that the surfaces must be finished to a 50RA.....what the **** is that? and why is it that when i google it, i get a load of sites using it but not explaining what it is or what it compares to. To obtain this feeling, try asking your doctor when he tells you your blood pressure. "120/80" ...ok, 120/80- what. SCFH? PSI? in/WC? ...its actually mm/Hg but more than likely they will look at you like a deer in head lights.
the best i can come up with is that 50RA "compares" to is 50 grit....in which case im pretty sure serrations will be good enough. IF NOT I AM DEAD WRONG THERE....any one know how much i can shave off my jug safely in order to obtain that 50RA surface? i have a machine shop, all i need is a ballpark safe number to give to my machinist.
as always, thank you for any and all help.
HDF Community Team
Joined: Dec 2011
Posts: 8,591
Likes: 4,021
From: San Diego, CA
Have you considered contacting Cometic directly and ask them what they mean by 50RA?
Have you contacted Hammer Performance or NRHS, both of whom sell the gaket kits, and ask them directly about installation?
Help is out there, just need to ask the right people.
I can't see any reason you would have (or want) to do anything to the mating surfaces,
John
Have you contacted Hammer Performance or NRHS, both of whom sell the gaket kits, and ask them directly about installation?
Help is out there, just need to ask the right people.
I can't see any reason you would have (or want) to do anything to the mating surfaces,
John
Last edited by John Harper; Dec 2, 2014 at 02:26 PM.
Thread Starter
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Cruiser
Joined: Feb 2014
Posts: 107
Likes: 10
From: texas
Have you considered contacting Cometic directly and ask them what they mean by 50RA?
Have you contacted Hammer Performance or NRHS, both of whom sell the gaket kits, and ask them directly about installation?
Help is out there, just need to ask the right people.
I can't see any reason you would have (or want) to do anything to the mating surfaces,
John
Have you contacted Hammer Performance or NRHS, both of whom sell the gaket kits, and ask them directly about installation?
Help is out there, just need to ask the right people.
I can't see any reason you would have (or want) to do anything to the mating surfaces,
John
And i havent done anything to the mating surfaces as of yet. But the instructions in the kit say it must be a "finish" so im thinking flat where as i found my jug surfaces serrated
Seasoned HDF Member
Joined: Jul 2011
Posts: 27,097
Likes: 4,676
From: Frozelandia, Minnysota
Don't think it has anything to do with "grit".
"Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surfaces peaks and valleys."
I think, in perhaps oversimplified terms, this means that you measure the highest points of the surface in a given distance, and the lowest points, average the highs and lows, and take the average of those two sums (probably measuring in micrometers). Not gonna do that at home with any accuracy. That should be a function of the machining process; ask your machinist, he should know. Lot of folks put new gaskets on without machining the heads; if they aren't damaged and you're not doing mods to the motor, should be able to just put it back together with the new gaskets. Cometic gaskets have a good rep, heard of people using them as direct replacements; if you're really nervous about that, might be best to call Cometic and ask.
Some mating surfaces have a serrated appearance; a tiny bit of unevenness will grip the gasket with more force and resist pressure leakage better.
Know what you mean about copters, any aircraft; was a Huey mechanic in the Army, over 30 years as an airline mechanic. Have an 04 1200C, too.
"Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surfaces peaks and valleys."
I think, in perhaps oversimplified terms, this means that you measure the highest points of the surface in a given distance, and the lowest points, average the highs and lows, and take the average of those two sums (probably measuring in micrometers). Not gonna do that at home with any accuracy. That should be a function of the machining process; ask your machinist, he should know. Lot of folks put new gaskets on without machining the heads; if they aren't damaged and you're not doing mods to the motor, should be able to just put it back together with the new gaskets. Cometic gaskets have a good rep, heard of people using them as direct replacements; if you're really nervous about that, might be best to call Cometic and ask.
Some mating surfaces have a serrated appearance; a tiny bit of unevenness will grip the gasket with more force and resist pressure leakage better.
Know what you mean about copters, any aircraft; was a Huey mechanic in the Army, over 30 years as an airline mechanic. Have an 04 1200C, too.
Advanced
Joined: Apr 2014
Posts: 64
Likes: 0
From: Louisburg, KS
Simplest explanation is that it is an identifier for a machinist to tell how fine a final cut has to be on the material being mated.
On car head gaskets, some people run rougher levels thinking it bonds better or smoother, thinking that a rougher finish well tear the gasket.
On car head gaskets, some people run rougher levels thinking it bonds better or smoother, thinking that a rougher finish well tear the gasket.
Thread Starter
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Cruiser
Joined: Feb 2014
Posts: 107
Likes: 10
From: texas
Don't think it has anything to do with "grit".
"Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surfaces peaks and valleys."
I think, in perhaps oversimplified terms, this means that you measure the highest points of the surface in a given distance, and the lowest points, average the highs and lows, and take the average of those two sums (probably measuring in micrometers). Not gonna do that at home with any accuracy. That should be a function of the machining process; ask your machinist, he should know. Lot of folks put new gaskets on without machining the heads; if they aren't damaged and you're not doing mods to the motor, should be able to just put it back together with the new gaskets. Cometic gaskets have a good rep, heard of people using them as direct replacements; if you're really nervous about that, might be best to call Cometic and ask.
Some mating surfaces have a serrated appearance; a tiny bit of unevenness will grip the gasket with more force and resist pressure leakage better.
Know what you mean about copters, any aircraft; was a Huey mechanic in the Army, over 30 years as an airline mechanic. Have an 04 1200C, too.
"Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surfaces peaks and valleys."
I think, in perhaps oversimplified terms, this means that you measure the highest points of the surface in a given distance, and the lowest points, average the highs and lows, and take the average of those two sums (probably measuring in micrometers). Not gonna do that at home with any accuracy. That should be a function of the machining process; ask your machinist, he should know. Lot of folks put new gaskets on without machining the heads; if they aren't damaged and you're not doing mods to the motor, should be able to just put it back together with the new gaskets. Cometic gaskets have a good rep, heard of people using them as direct replacements; if you're really nervous about that, might be best to call Cometic and ask.
Some mating surfaces have a serrated appearance; a tiny bit of unevenness will grip the gasket with more force and resist pressure leakage better.
Know what you mean about copters, any aircraft; was a Huey mechanic in the Army, over 30 years as an airline mechanic. Have an 04 1200C, too.
thank you. thats the explanation i was searching for, but not the answer i wanted, would like something quantifiable....well that is but you know what i mean. thanks man i appreciate it. As a fellow airwinger, as im sure you know, ive tried to explain to my fellow mechanics here in the oil and gas industry that the world i (we) come from, when you put your name on an action item (QA step) we are basically saying "i have inspected this item and deem it capable of operation to specifications and if it fails, send me to prison"...they think im full of it. such a small small view of the world they have.
When you say "serrated" are you referring to the machining marks left on the surfaces?
John
John
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Road Warrior
Joined: Jul 2006
Posts: 1,532
Likes: 7
From: Rochester N.Y.
Here's a chart I found
Surface Finish Comparison
RMS
(Micro-inch) RMS
(Micron) Ra
(Micro-inch) Ra
(Micron) Grit Finish
160 4.06 142 3.61 36
98 2.49 87 2.21 60
80 2.03 71 1.80 80
58 1.47 52 1.32 120
47 1.20 42 1.06 150
47 1.20 42 Before
Bead Blast 1.06 USDA
Bead Blast
34 0.86 30 0.76 180
21 0.53 19 0.48 220
17 0.43 15 0.38 240
14 0.36 12 0.30 320
10 0.25 9 0.23 400
5 0.13 4 (+/-) 0.10 Mirror
50Ra (52) would roughly equal the finish of 120 grit sandpaper <>
Mick
Surface Finish Comparison
RMS
(Micro-inch) RMS
(Micron) Ra
(Micro-inch) Ra
(Micron) Grit Finish
160 4.06 142 3.61 36
98 2.49 87 2.21 60
80 2.03 71 1.80 80
58 1.47 52 1.32 120
47 1.20 42 1.06 150
47 1.20 42 Before
Bead Blast 1.06 USDA
Bead Blast
34 0.86 30 0.76 180
21 0.53 19 0.48 220
17 0.43 15 0.38 240
14 0.36 12 0.30 320
10 0.25 9 0.23 400
5 0.13 4 (+/-) 0.10 Mirror
50Ra (52) would roughly equal the finish of 120 grit sandpaper <>
Mick
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Ra (Roughness Average)
has traditionally been used to describe surface finish, most gasket engineers today say a more accurate perimeter is Rz, which is the average difference between the peak height and valley depth. Ra can have a wide variance across a given surface profile, so Rz gives a better indication of the actual texture across the surface.
To measure Ra or Rz, you need a profilometer. If you dont have one, you are shooting in the dark and assuming the surface finish youre getting is in the ball park. Maybe it is and maybe it isnt. The only way to know for sure is to actually measure it. Most dry milling machines with the proper CBN or PCD cutting bits can achieve an extremely smooth surface finish.
has traditionally been used to describe surface finish, most gasket engineers today say a more accurate perimeter is Rz, which is the average difference between the peak height and valley depth. Ra can have a wide variance across a given surface profile, so Rz gives a better indication of the actual texture across the surface.
To measure Ra or Rz, you need a profilometer. If you dont have one, you are shooting in the dark and assuming the surface finish youre getting is in the ball park. Maybe it is and maybe it isnt. The only way to know for sure is to actually measure it. Most dry milling machines with the proper CBN or PCD cutting bits can achieve an extremely smooth surface finish.
Thread Starter
|
Cruiser
Joined: Feb 2014
Posts: 107
Likes: 10
From: texas
Here's a chart I found
Surface Finish Comparison
RMS
(Micro-inch) RMS
(Micron) Ra
(Micro-inch) Ra
(Micron) Grit Finish
160 4.06 142 3.61 36
98 2.49 87 2.21 60
80 2.03 71 1.80 80
58 1.47 52 1.32 120
47 1.20 42 1.06 150
47 1.20 42 Before
Bead Blast 1.06 USDA
Bead Blast
34 0.86 30 0.76 180
21 0.53 19 0.48 220
17 0.43 15 0.38 240
14 0.36 12 0.30 320
10 0.25 9 0.23 400
5 0.13 4 (+/-) 0.10 Mirror
50Ra (52) would roughly equal the finish of 120 grit sandpaper <>
Mick
Surface Finish Comparison
RMS
(Micro-inch) RMS
(Micron) Ra
(Micro-inch) Ra
(Micron) Grit Finish
160 4.06 142 3.61 36
98 2.49 87 2.21 60
80 2.03 71 1.80 80
58 1.47 52 1.32 120
47 1.20 42 1.06 150
47 1.20 42 Before
Bead Blast 1.06 USDA
Bead Blast
34 0.86 30 0.76 180
21 0.53 19 0.48 220
17 0.43 15 0.38 240
14 0.36 12 0.30 320
10 0.25 9 0.23 400
5 0.13 4 (+/-) 0.10 Mirror
50Ra (52) would roughly equal the finish of 120 grit sandpaper <>
Mick
thank you sir, thats more like it. but just using that as a reference, im well within what i need to be
thank you too elfnyc, ill bother my machinist for a profilometer. if not, ill convince marketing that we need to expense one for the shop...for reasons.
on another side completely unrelated note...anyone know where i can get a more illustrated parts breakdown for my model carb. i took out the shutter, and now i forgot what placement these thin washers and felt washers go in. and the manual doesnt show it broke down that far. woops.