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Today we’ve organised a guest post from Dirk Lohmann, more commonly known as the operator of the website www.senator-monza.de. This expert on technology has kindly agreed to give us a lesson on the cylinder head screws for Opel cih & 24V.
Myths & secrets accompany the cylinder head screw from Opel cih and DOHC – R6 engines
The fact is, that there are actually three different sorts of cylinder head screws for Opel cih-engines:
The oldest version with the 12mm triple square bit:
M12 x 60mm with catalogue number: 2048276 and GM 11062601 (10.9)
M12 x 92mm with catalogue number: 2048277 and GM 11062611 (10.9)
The middle version with 12mm triple square bit:
M12 x 65 mm with catalogue number: 2048283 and GM 11083852 (10.9)
M12 x 98 mm with catalogue number: 2048284 and GM 11083842 (10.9)
The newest version with 10mm Allen key:
M12 x 65 mm with catalogue number: 2024153 and GM 11088461 (8.8)
M12 x 98 mm with catalogue number: 2024152 and GM 11088201 (8.8)
Which means it is important to note, that when making changes or repairs on an old cih- engine, none of the round 5-6mm too long screws can be used. Otherwise the cylinder head can’t be correctly tightened up! For this reason, it makes sense to measure the length of the screws removed during the disassembly of an old cih-engine, for later reference.
Especially the older triple square screws tend to turn around the drive profile, if they aren’t perfectly cleaned. Unfortunately the immersion depth of the screws is much less than seen in the newer hexagon socket (or Allen) screws.
And what about in the 3 litre 24V?
The DOHC-6-cylinder engine types C30SE / C30SEJ / C30XEi / C40SE with TORX T55 profile:
M12 x 89 (85) mm with catalogue number: 607941 and GM 90322507 (8.8)
(14 head screws of the same length are used here.)
Now we come to the topic of “stress bolts” or “no stress bolts”
In principle, all head screws from the cih-engines are not stress bolts !
Some manufacturers deliver these bolts with torque – & angular degree specifications. These doesn’t mean, even for a moment, that these are therefore stress bolts.
The background of the angular degree is the better tolerance compliance to the torque during tightening. Through the minor deviation of the breakaway torque with many, staggered re-torques of the screw, the correct torque will be reached.
The screws must be tightened in a certain order in multiple rounds. This counteracts the setting properties of the cylinder head materials and any subsequent warpage of the cylinder head. This is why every repeated rotation of every single screw results in a change to the torque. This is the fault of a bolted joint’s breakaway torque. After mounting with a certain torque, a much higher amount of force must be applied in order to further rotate the screw head than was previously required. This breakaway torque can by all means exceed the maximum mounting strength given for the screws in the last mounting stage.
The revolution angle mounting method is vastly more precise: all screws are fastened in the first stage of the torque specifications, as before. Once this has been completed, they can be further tightened in stages 2 and 3 with even angle °, without uneven breakaway torque tolerances. Thus they will be tightened with approximately uniform levels of strength. Possible tolerances result from the material of the screw and the condition of the thread in the engine block.
The screws of the cih-engines have no washers underneath the screw heads. This is because the casting material of the cylinder head is hard enough and the tolerance-compliance is guaranteed, even by staged torque mounting.
Washers in the C30SE
As the cylinder head in the six series C30SE is cast in aluminium, you will find thick, captive steel washers under the screw heads. This helps to distribute the load more equally over the surface of the cylinder head, as the contact surface is made a bit bigger. Consequently, the breakaway torque and its evenness during staged rounds of screw tightening is much improved, regardless of revolution angle technique. Furthermore, due to the higher thermal expansion of the aluminium cylinder head, the undesirable tilt under the screw head from the increased surface area is minimised and the narrowing of the screw hole in the upper area is counteracted.
Does the DOHC type C30SE have stress bolts?
According to Opel, these screws can be used twice. On these screws, you will find the specification 8.8, which speaks against a stress bolt. Of course, these screws classified as stress bolts by the accessory industry, in order to increase their use.
Is it sensible to reuse old screws?
In cih engines, depending on the thread condition, previously used Opel cih cylinder head screws can be reused. However, in the DOHC, due to the higher thermal expansion of the cylinder head and the resulting expansion forces, you probably shouldn’t shy away from the cost of sourcing some new screws. The expense of leaks in the C30SE is much higher than in the simpler cih-engines.
A big thank you to Dirk for this excellent post!
You can, of course, find suitable cylinder head screws for Opel cih & 24V engines in the edelschmiede web – shop!
– Set of 10 Reinz cylinder head screws, Opel 4- cylinder cih
– Set of 14 Reinz cylinder head screws, Opel 6- cylinder cih
– Set of 14 cylinder head screws, Opel 3.0i – 24V / C30SE