You need mirror finish surface because of different expansion of metal. Head is aluminium and block is iron. They need to slide not rub.
Question about mirror finish (head) & I'm lookin for a reputable machine shop in WA
- Thread starter 87mgte
- Start date
lol @ the mirror finish argument. it will go on forever. I know someone personally who's been boosting his 7M for years with a metal HG and ARP studs that had the surfaces just cleaned very well and it's not BHG. The engine puts out 420+ ponies and has been flogged down the track numerous times.
At any rate, I just had some stuff done at Bud's Machine Shop in Tacoma/Lakewood, they're pretty decent.
At any rate, I just had some stuff done at Bud's Machine Shop in Tacoma/Lakewood, they're pretty decent.
Tanya;945756 said:
Thanks for the input. Gorgeous Mkii you've got there, btw. I think I took a gander at your build thread a while back... not sure though.
I can't exactly ship my head anywhere, so I'd like a shop a little closer to Sequim. I found one in Port Angeles called Tri-Star, so I think I'll check that out.
It's fairly simple. You don't want a mirror finish. Some texture will help mate the surfaces to the HG, and create resistance to cylinder pressure. It's the same principle that your hand texture (fingerprints) use to attain grip.
If all surfaces were polished to an atomically perfect smoothness, then the only resistance to cylinder pressure would be provided by the torque of your head fasteners. However, if there is some residual texture from the machining process, then you have the fastener torque in addition to the structure provided by the "ridges" of the texture left behind by the mill. It's not a matter of whether or not the HG will skate around. It's a matter of texture which provides structural resistance to cylinder pressure. Residual texture from the mill = good.
If all surfaces were polished to an atomically perfect smoothness, then the only resistance to cylinder pressure would be provided by the torque of your head fasteners. However, if there is some residual texture from the machining process, then you have the fastener torque in addition to the structure provided by the "ridges" of the texture left behind by the mill. It's not a matter of whether or not the HG will skate around. It's a matter of texture which provides structural resistance to cylinder pressure. Residual texture from the mill = good.
zurud;945569 said:
They aren't supposed to move.
As for the topic, since you posted I did some reading and talking to some people I trust and the bottom line is, forget the mirror finish. Get the head lapped. If you're planning on mirror finishing on the head do it only for the looks, which won't matter on the mating surface.
Guyana00;946174 said:
psst
metal expansion gives you or anyone else ZERO choice!
They will move as they expand at different rates! First the Aluminum head and then the iron block playing catchup. A less "rough" surface lets the headgasket do it's job. Hell if you get it flat enough there is no need for a headgasket. Ask any race engine builder on that one!
Just read through this thread and it looks like there's a few things that need made to be clear:
- The most important thing is both the block/head decks must be flat (Brent you are correct)
- For a MHG, you have to meet the RA (smoothness) spec of the MHG used...that varies between 30-50 RA, depending on who made it. A mirror finish for a MHG would be very expensive to do and is not necessary...a final lap using diamond paste is a good idea to get the block/head decks as smooth as reasonably possible
- For a OEM composite HG, it needs to be finished smooth, but can tolerate a higher RA
- You DO NOT want to use copper spray on a MHG...the chemicals react with the factory Viton coating
- I would use copper spray on an OEM composite HG...it helps to fill in the imperfections after a standard machine decking
- Proper torque values for the head is just as important...70-72 ft/lbs is good for stock head bolts. If using ARP hardware, follow the spec sheet and lube recommendations for proper torque. You DO NOT want to over torque and the lube used changes that value. ARP head BOLTS are limited to 75 ft/lbs on an aluminum head. This should be required reading..."Proper Fastener Retention" on the left:
http://www.arp-bolts.com/Tech/TechInstall.html
- An OEM HG is fine for stock boost...the problem has always been inadequate head bolt torque from the factory. I would take a stock HG up to fuel cut (~14 psi of boost) with no worries whatsoever torqued down to 70 ft/lbs on stock bolts
- The most important thing is both the block/head decks must be flat (Brent you are correct)
- For a MHG, you have to meet the RA (smoothness) spec of the MHG used...that varies between 30-50 RA, depending on who made it. A mirror finish for a MHG would be very expensive to do and is not necessary...a final lap using diamond paste is a good idea to get the block/head decks as smooth as reasonably possible
- For a OEM composite HG, it needs to be finished smooth, but can tolerate a higher RA
- You DO NOT want to use copper spray on a MHG...the chemicals react with the factory Viton coating
- I would use copper spray on an OEM composite HG...it helps to fill in the imperfections after a standard machine decking
- Proper torque values for the head is just as important...70-72 ft/lbs is good for stock head bolts. If using ARP hardware, follow the spec sheet and lube recommendations for proper torque. You DO NOT want to over torque and the lube used changes that value. ARP head BOLTS are limited to 75 ft/lbs on an aluminum head. This should be required reading..."Proper Fastener Retention" on the left:
http://www.arp-bolts.com/Tech/TechInstall.html
- An OEM HG is fine for stock boost...the problem has always been inadequate head bolt torque from the factory. I would take a stock HG up to fuel cut (~14 psi of boost) with no worries whatsoever torqued down to 70 ft/lbs on stock bolts
figgie;946182 said:
Agreed, but if you read the post I replied to, it implies that it would be desirable to have a setup where they slide. Movement is inevitable, but keeping it minimalized would be best.
an actual machinist gives you the advice your looking for...then you proceed to say later on "finally someone who knows what they are talking about"...as you can see the machinist was right and will not help you anymore...
And no you do not want the mateing surfaces to slide that is by farther dumbiest thing I've heard.....it is unavoidable yes but you don't want to make it easier....make sure you know what your talking about before you give advice.
And no you do not want the mateing surfaces to slide that is by farther dumbiest thing I've heard.....it is unavoidable yes but you don't want to make it easier....make sure you know what your talking about before you give advice.
gaboonviper85;946242 said:
Cracks knuckles.......
the mating surface WILL "slide". Simple physics and metallurgy. The sliding will occur as the metal expands. The head will not be sliding all willy-neely as that is what the STUDS are there for but it will slide none the less. The headgasket is there to keep the surfaces sealed through those expansion and cooldown periods along with helping keep the gasses where they belong.
My advise is check your ego at the door.
Just because we are here does not mean we do not understand the physics of it. Most of us "old timers" do.
Its going to slide the same amount reguardless of the finish, those little ridges wont do anything when it comes to the forces of metals expanding and contracting. The tooling marks have a tendancy to tear a comp head gasket as they move across it. Its science :sarcasm: Ill trust the "old timers" on this one
gaboonviper85;946242 said:
Make sure you know what you're reading before you freak out on the OP. I took your advice. You're not the only one on SM. There are other opinions to be gotten and I wanted em. Mk3brent chimed in and I was glad, cause from all the other posts I've read from him, I've gathered that he's one of the most knowledgeable guys on this forum. I never said you didn't know what you're talking about.:3d_frown:
jdub;946206 said:
Awesome, I was waitin for you or another expert, jdub. I'll be using an OEM gasket with ARP studs. No point in going metal, as I'll be going 1JZ over the summer. Copper spray sealant? I'll get on that then. RA for an OEM gasket? Sorry if I didn't search hard enough for that one. Like I said, I'll be at 10-12 psi daily driven, so I guess I'll be fine at those boost levels.
Since you're an engine oil expert and whatnot, how many times would you suggest flushing my oil system with cheap-ish stuff before I put in my Royal Purple? I was gonna flush it twice with Valvoline (new filters each time) and then throw in the full synthetic...? I'd really like to minimize my chance of rod knock or anything else that could occur if I didn't completely flush the old chocolate milkshake shit out of there.
gaboonviper85;946297 said:
You can see yourself on a 50 RA finish. 30 RA finish it is better and still see yourself clearer. In essence mirror finish. If you want to talk about surface flatness..... at an RA of 50 that is equivalent to 1.4 - .9 microns. Still rough.