After careful disassembly, the problem has become more obvious (if possible). The valves themselves, have their stems in tolerance (Inlet: 6.960 – 6.975 mm ;Exhaust: 6.945 – 6.960mm), but the problem relies with the mating surface of the seats. I used the optical measuring machine at ICAI to take a few reference points to compare, but just at a glance, they are not in a good condition. Again, as I want to do a fine job, it makes sense to replace all valves.
Old valve 
New valve
I’ve taken the chance to take the valve head to the metrology lab at ICAI to measure the valve guides and valve heads to check if they are in spec, in case I can save some money. Long story short; according to the manual, the valve guides should measure between 7.000 mm and 7.015mm when new and the service limit is in 7.100mm. My valve guides? On average around 7.110mm. It seems i’m in for a valve guide change as well…
I’ve found a guy that can provide brand new original replacement parts for less than a third of the price. Wanting to avoid a shady transaction, I inquired about the price of his parts. Apparently he bought all the warehouse of a BMW dealership that had gone bust. For a while he sold replacement parts, but now he is not in this business anymore and he doesn’t have time to sell the remaining stock. He is willing to sell me parts well below their price just for his peace of mind. If for any chance you are reading this, get in touch with me and I will try to talk to him.
The guy even let me borrow the official valve installation tool, and valve guide pliers. I bothered to take measurements of the tool as I couldn’t find them online,in case I to remove valves again. Get in touch if you want CAD drawings.
CAD Drawing of Official K-bike spring compressor tool 
Installing Valves with the official BMW valve tool
I took the head to a machine shop were I had new valve guides installed (and re-drilled, as BMW sells the replacement guides 0.2mm smaller), valve seats re-cut and had the head resurfaced for just under 200€.
When re-assembling an engine head, typically, you should ensure that every valve should go in the same port it was taken out of. In my case this does not make any sense as I will have a new set of valves and seats have been re-cut. However, as the guides were reamed, it will be best to match the guide with the largest ID (Internal diameter) with the valve with the largest OD (Outer diameter). For that, I took a total of 20 measurements per valve and valve guide (2 per cylinder for a total of 3 cylinders that makes a total of 240 measurements) to have any statistical significance. My results were as follows (all measurements in mm, rounded to 5 decimals):
| Inlet Valve #1 | Inlet Valve #2 | Inlet Valve #3 | |
| Measurement | μ=6,96621 σ=0,00137 | μ=6,96650 σ= 0,00109 | μ=6,96490 σ= 0,00122 |
| Exhaust Valve #1 | Exhaust Valve #2 | Exhaust Valve #3 | |
| Measurement | μ=6,95533 σ= 0,00114 | μ=6,95300 σ=0,00080 | μ=6,94900 σ=0,00065 |
After measuring the valve guides, I technically I could’ve gone back to the machine shop and asked for the guides to be reamed to the specified 7 H7 tolerance(7mm +0, +15mm), but the difference is minimal and eventually valve guides will wear to this value. Therefore, I matched the valves with their guides as follows:
| Cylinder 1 | Cylinder 2 | Cylinder 3 | ||||
| Inlet | Exhaust | Inlet | Exhaust | Inlet | Exhaust | |
| Valve Guide Measurement | μ=6,97840 σ= 0,00750 | μ=6,99220 σ= 0,00110 | μ= 6,98980 σ= 0,01402 | μ=7,00040 σ= 0,00378 | μ= 6,99920 σ= 0,00432 | μ=6,99180 σ= 0,00110 |
| Matched Valve | Inlet #3 | Exhaust #2 | Inlet #1 | Exhaust #1 | Inlet #2 | Exhaust # 3 |
| Valve Stem Measurement | μ=6,96490 σ= 0,00122 | μ=6,95300 σ=0,00080 | μ=6,96621 σ=0,00137 | μ=6,95533 σ= 0,00114 | μ=6,96650 σ= 0,00109 | μ=6,94900 σ=0,00065 |
| Valve Stem/guide Clearance | μ= 0.0135 σ= 0,00760 | μ= 0.0392 σ=0,00136 | μ= 0.02359 σ=0,01409 | μ= 0.04507 σ=0,00395 | μ= 0.0327 σ=0,00446 | μ= 0.0428 σ= 0,00128 |
| Valve Stem/guide Tolerance | 0.025-0.055 | 0.040-0.070 | 0.025-0.055 | 0.040-0.070 | 0.025-0.055 | 0.040-0.070 |
| IN/OUT of tolerance | OUT by 0.0115 | OUT by 0.0008 | OUT by 0.00141 | IN | IN | IN |
From the results, theoretically 3 of my clearances are out of spec, however, its on the low-side, meaning that wear will bring them up to specification. Also, down to this level of precision, probably my measuring had something to do.Even the temperature . I’m not making excuses, If I was a BMW quality engineer in 1990 i would’ve pushed this engine head back, but I’ve done better than any mechanic would’ve.
With the machine shop work done and inspected it has just been a matter of installing back the valves, springs and tappets, which is a breeze with the tool. After a short explanation Alvaro Perez installed all the valves with little to no help at all. I’m proud to say he would be a good mechanic. If you ever need to install valves into a head, he is the guy to call!