Moss Engineering

33 Kings Lane
South Croxton
Leics. LE7 3RE
UK

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    Cylinder head fitting procedure                   

 

 We received a cylinder head from a Scott owner with the request that we inspect it for flatness and re machine if necessary. 

We checked the head and found it to be well within acceptable limits of flatness.  It had a maximum deviation of 0.002”

And as the 16 studs combined have great clamping power, we would not consider it to be necessary to skim the head unless flatness deviations exceeded 0.004”.

We phoned the owner, who told us that when he bolted up the head, he could see the water in the radiator going up and down as he turned the engine over.  I explained that it will take us between 15 and 20 minutes to secure a head as we must progressively tighten at least four times.  He asked how hard he should tighten down the head and explained that he had asked this question several times and received the answer “tight enough”.  As he had not spent his working life dealing with bolted up assemblies like this, then “tight enough” did not help.

I have progressively tightened a head as I would normally do and recorded the torque figures shown on the torque wrench.  Please note that because the softer filling of the gasket will compress as intended, according to the load imposed, then it is progressively becoming thinner as the tightening sequence progresses.  For this reason, if you start with the middle nuts and work outwards, then if you test the inner nuts where you started again, you will find they are now loose. For this reason, it is absolutely essential that the tightening sequence is carried out at least four times, or better still, till none of the nuts will go up further.

Personally, I am a great lover of using a thin smear of silicone RTV on the faces of the block and the head as an extra safety precaution.  This practice is advisable where there the head is not very flat and / or there are scours in the faces that might act as a duct for the passage of water. If RTV is used, it should only be an even thin smear as excessive amounts can restrict the head to block small water passage holes and impair cooling.

 

Fixing and tightening a Scott detachable Cylinder head.

We start with a dry trial assembly to be sure everything will fit properly

1)                  Barrel.  Ensure head sealing face and tapped holes are clean and degreased. 

2)                  Loosely fit new or serviceable degreased studs and be sure they are well engaged in holes with good threads.

( The SOC Spares Scheme have good sets in stainless steel)

3)                  Slide head gasket over studs with cylinder bore folded lip side to the iron block.

(The iron is more capable to resist indentation caused by local pressure.)

4)                  If gasket does not fit properly, open out mismatched holes with a fine round file until it does.

5)                  Loosely fit the cylinder head and file out any holes that the studs will not pass easily.

(As head studs often corrode into the head, you might find a build up of corrosion in the bolt holes and this should be cleaned out with a drill of file before the trial assembly

6)                  Check the condition of the bosses that the washers below the nuts will seat on.  These often get much abuse and can be badly distorted.  If bolted to such faces, the bolt can be veered over or the partial seating  can be too small and lack stiffness.  In such cases the seating can “sink” in use and relax the clamping pressure. Re face if necessary removing minimum metal to achieve at least 85% cleanup.

7)                  Check that the thread on the studs extends to at least 1/8” below the surface of the head bolt clamping bosses. There are several different head patterns and we have seen some where the bolts needed to be shortened.

8)                  Fit thick washers and nuts finger tight only.

9)                  Now we are sure that when we do final assembly ass will fit.

10)              Take everything off but mark gasket to be sure that if you have had to file out any holes, you will know which way to assemble it.

                     

The well known problem of corrosion of the studs in the holes is caused by capillary seepage of water between the thread of the stud and the tapped hole in the iron block passing through into the clearance hole for the stud in the aluminium head.  There is also a problem where the water passes from the block to the head via holes punched through the gasket. The gasket is made from two outer layers of thin copper sheet with a softer filling.  Depending on the soft filling used, some are able to allow the seepage of water along to adjacent holes.

To try and avoid these problems the following steps can be taken;

11)              Fit studs into block using Loctite “Stud Lock” or similar to impede water seepage past the thread.

12)              Coat the inside of the water transfer holes in the gasket with a thin coating of silicone RTV

13)              Fit the gasket over the studs

14)              Smear the stud plain diameters and thread with a thin coating of Rocol “Dry Moly Paste”    

15)              Fit head, washers and nuts.

16)              Use torque wrench and socket spanner to tighten to 10 lbs ft in the sequence shown.

17)              Repeat tightening sequence to 16 lbs ft

18)              Repeat tightening sequence to 20 lbs ft

19)              Repeat tightening sequence to 20 lbs ft

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email roger@mossengineering.co.uk or richard@mossengineering.co.uk