Measuring how much a head has been skimmed

dj_efk

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Hi guys

I understand you can’t skim our heads very much before it starts to affect the cam timing, is this correct?

I have a Spanish copy / replacement head, wherw do I measure it to see how much meat has been taken off it, and what is acceptable here?

Any informed thoughts would be appreciated. The car is an early 2800cs.

Dave
 

tochi

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From manual (11 12 719 - Flat facing cylinder head mating surface )

When flat facing the cylinder head mating surface, the total thickness of the cylinder head in its standard [fORM]
A = 129.0 ± 0.1 mm (5.0787 ± 0.0039 in) must not be reduced by more than 0.5 mm (0.0197 in).


12719001_z.jpg
 

rsporsche

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as i remember from a post by our resident expert SFDon, there are recesses on the head surface. if those recesses are gone that means the head has been decked and is at the minimum.
 

HB Chris

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as i remember from a post by our resident expert SFDon, there are recesses on the head surface. if those recesses are gone that means the head has been decked and is at the minimum.
Yep, called dimples a bit smaller than a dime near intake ports
 

m5bb

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as i remember from a post by our resident expert SFDon, there are recesses on the head surface. if those recesses are gone that means the head has been decked and is at the minimum.
Scott is correct. They are like a round recess about the size of a dime.
BTW this is so little change in the chain length that I doubt you would ever know the cam timing changed.
Also the front timing chain cover will have to be milled the same amount or you will have oil leaks.
Tried to find a photo but couldn't find one showing the recess in the head.
 

dj_efk

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Scott is correct. They are like a round recess about the size of a dime.
BTW this is so little change in the chain length that I doubt you would ever know the cam timing changed.
Also the front timing chain cover will have to be milled the same amount or you will have oil leaks.
Tried to find a photo but couldn't find one showing the recess in the head.

Thanks all for your input.

@m5bb - what about a very light skim of a few thou, rather than a “serious” / compression raising 10 or 15? Will I get away without the work to the cover?

My head was a new casting 40k ago according to the car’s history, so I’m not expecting it to have been skimmed previously, but the original post was prompted by the need to assume nothing!
 

mulberryworks

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I think early heads don't have the dimples, and I have no idea if the Spanish heads have them either. But they should adhere to the measurement shown above.
 

m5bb

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Thanks all for your input.

@m5bb - what about a very light skim of a few thou, rather than a “serious” / compression raising 10 or 15? Will I get away without the work to the cover?

My head was a new casting 40k ago according to the car’s history, so I’m not expecting it to have been skimmed previously, but the original post was prompted by the need to assume nothing!
dj,
As you may or may not know BMW engines of this era are somewhat prone to leaking oil. Timing chain covers are usually the worst but oil pans and the covers on the back of the heads are suspects as well. So I would be very careful not to match what ever you remove from the head and also remove the same from the timing chain cover. Could be you are trying not to have to remove it but I would not take a chance.
As stated above I believe the A measurement as the actual thickness of the head is probably correct and personally feel that the suggestion to not remove more than .0197 is reasonable but I do know of many heads that have had .030 removed without problems. ( BTW I read about a head that had .030 removed and it changed the timing by about 1/2 of a tooth on the timing gear).
A good machinist would certainly be able to guide you in this process and from my experience please check the qualifications from referrals of a potential machinist. So many cylinder heads have been ruined by unqualified machinist.
Since you have a head from Spain I'm really not sure what that means? Was it manufactured in Spain for BMW or what?
Gary
 

dj_efk

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Since you have a head from Spain I'm really not sure what that means? Was it manufactured in Spain for BMW or what?
Gary

See here: http://www.firstfives.org/faq/cylinder_head/cylinder_head_faq.htm

"There is also a Spanish made head, which has AMC cast into the side of the head instead of the year. These don't look as nice as factory heads, the casting is noticeably rougher, but they have a very good reputation and I wouldn't hesitate to use one. The casting is similar to the late one having holes instead of slots for water passages."

These heads don't have the dimples so I'll get my guys to measure it as per @tochi 's post. I'll also alert them to take the timing cover to the machine shop at the same time as the head.

Thanks all.
 

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