head rebuild questions

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Danny Wilde
Posts: 83
Joined: Wed Apr 18, 2012 8:44 pm
Location: Germany

head rebuild questions

Post by Danny Wilde »

Hi to all! I’m beginning to think about head rebuild, chain replacement etc..
Considering I’m pretty unprepared on the mechanical stuff here are a few questions if I may:


1) At what mileage one should rebuild the head/replace the chain?

2) Should one go for a preventive rebuild once reached a certain mileage or should one wait for some failing symptoms before rebuilding?

3) Without tearing the engine apart, is there a proved empirical method or some kind of diagnostic tool that can be used to determine when the head needs to be rebuilt and the chain and tensioner to be replaced?

4) Once one rebuilds the head, is there any other fix that is advisable to do now that the head is open?

5) Does anyone have a vague idea of the average cost of the whole operation in the EU?


Thanks for you kind help guys
S
635csi manual euro 1986
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Ralph in Socal Online
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Re: head rebuild questions

Post by Ralph in Socal »

M30 is akin to a tank motor and is very stout. Head rebuild is on a case by case basis but typical motor seals degrade after years of use. Oil consumption, whether blue smoke out of the tail pipe or leaks on the ground is the usual call to action on head rebuild

Burning blue smoke is usually the valve seals (old style O-ring design are terrible) but in rare cases piston rings are the culprit. A Leak-Down Test will determine if the rings are shot. There's no test for valve seals but after 30 years I would expect them to be in pieces.

A Compression Test will tell you if a specific cylinder is not functioning properly. The Leak Down can pinpoint where the problem exists in that cylinder.

Pricing for rebuilding the head is a crap shoot. I have a trusted shop for my work and I don't question what the machinist requires. Bronze Valve Guides may need replacing. There are 6 Intake and 6 Exhaust guides. They're $20 each to replace with my machinist. Make sure to use Umbrella type Valve Seals as they are far superior to O-ring type.

The water fitting on the back of the head is NLA but should be replaced. The aluminum degrades and corrodes over years of use. The position of the water fitting makes for in-situ replacement problematic

My cost for a "stock" head rebuild years ago was over $600 due to replacing 6 Exhaust Guides.

Modifications will increase your cost. Later B35 heads have dual valve springs for better revving and is a very good upgrade. Porting the head for increased performance will make you smile but will hurt the wallet. Lots of options are available. Good luck and ask more if you need more guidance

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)
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