Monday, 10 October 2011

11. Cylinders, Pistons and Heads

Pistons


It does not really matter whether you are building a stock engine or using a big bore kit as the rebuild details are the same. I was using new Shasta design 83.5 pistons and have had the cylinders re bored to match.

http://shastadesign.net/pistonandcylinders.htm

Normally the piston rings are fitted to the pistons and ready to install with the correct gap which should be between 0.004" to 0.016"
A good way of checking if the gap is correct is to place the ring into the cylinder near the top and measure the gap with feeler blades to ensure it is correct. if the gap is too small then the ring end must be gently ground down. Porsche made a special tool to grind the rings. When fitting the rings to the pistons look for a dot or sometimes the word top on the ring. This should point to the head of the piston.


Ring placed in cylinder. Gap at top of picture can be measured.


Original cylinders and new Shasta pistons.


Piston, wrist pin and retaining circlip which fits into a groove in the piston. 

Pistons will be marked on their top surface with a small arrow. This should always point towards the flywheel.
They are held onto the rods with a wrist pin that is held in place by 2 snap rings. These have a sharp edge and a bevelled edge, the sharp edge faces outwards. The snap ring should be positioned as in the photo below and when properly in its seating can be rotated quite easily.


Retaining snap ring in position using snap ring pliers (shown left)  to fit. Sharp edge should face outwards.

Clean the pistons carefully with WD 40 and wipe clean. Make sure the piston wrist pins slide in and out easily. lubricate with some oil. Take a piston and MAKE SURE THE ARROW ON THE HEAD IS POINTING TOWARDS THE FLYWHEEL.

For pistons 1 & 3 it is easiest to install the wrist pin from the front (flywheel end) to the rear of the engine. For cylinders 2 & 4 it is easiest to fit from the the pulley end. Sometimes you will need to heat the piston with a hot light bulb for a few minutes to allow some expansion for the wrist pin to go in easily. The wrist pin is fed through the bushing on the connecting rod.

You can fit one of the retaining snap rings before attaching the piston to the rods, and then drive the wrist pin in until it touches the retaining snap ring you have already fitted and then fit the second retaining snap ring which holds the wrist pin in place.

Repeat until you have all four pistons fitted.

Now you need to arrange the piston rings. Starting with the the oil control ring (the lowest one on the piston) turn it so the gap faces upwards towards the top of the engine.
Now arrange the top ring with its gap facing towards the intake valve, the second ring gap towards the exhaust valve and the third ring (if present) towards the bottom of the engine.

Give all the pistons a final clean with WD 40 and paper towel.

We will be coating them with stp/engine oil mix before fitting the cyclinder heads.

Clean the insides of the cylinders using WD 40 and paper towel until they are spotless.
Fit the shim to the cyclinder. Line up the cut-outs in the gasket with the cylinder cut-outs as shown in the picture below.


Cylinder gasket in place with cut outs lined up correctly. I use a little of a product called triple QX eazigasket under the gasket. This is optional.


Now take your piston ring compressor. I use a VW 123b special tool as in the pictures below.


Piston ring compressor in place.


 VW / special tool 123b


Coat the inside of the cylinders with engine oil. Take one cylinder and making sure the squared off side faces the correct way (the two squared off  faces go side by side facing each other) line up the cylinder on the studs and tap hard with your palm to drive the cylinder head over the piston. The ring compressor should slide down the piston as this happens. It is not as easy as it looks but persevere and it will work. On some pistons, the ring compressor is not wide enough to compress all the rings in one go and you need to compress the top 2 or 3 rings first, get the cylinder over these and then compress the oil ring on its own and tap the cylinder over this.


Tapping the cylinder into place. Note the squared off side of the cylinder facing towards us.

Once the cylinder has just covered the rings, remove the piston ring compressor and push the cylinder home against the case. Porsche made special nuts which you can see in the photo below to hold the cylinder in place until you are ready to fit the cylinder head. 


Cylinders held in place by the special bolts made by porsche.

These are hard to find but can be easily re created by cutting a piece of 15mm copper pipe to the correct length and using this as a spacer between the normal cylinder head bolt and the cylinder. Porsche also made these spacer tubes as a special tool. 


Cylinder heads

My cylinder heads have been rebuilt professionally with new valve guides and valves. This is beyond the scope of my blog as it is specialised work.

If you are reusing your cylinder heads get a magnetic probe and check every one of the 8 holes in your head in case there is an old washer still in place. This is a very common problem. if you feel resistance with the magnet check carefully and prise out any old washers. Do the same for the other side.

Take your 8 push rod tubes and fit the seals to the ends as in the photo below. use a little STP/oil mix or some assembly grease on the seals and on the push rod tube ends.


Push rod seal in place, Bevelled edge outwards.

Having fitted all the seals to the push rod tubes, find the 2 air deflector plates as shown below. Note that one has a square cut out and one has a round cut-out. The square one goes on the head of the through bolt side and the round one on the acorn nut side of the case.


They are held in place by a spring that fits over one of  the push rod tubes as shown in the photo below.
When fitting the new (don't reuse the old ones) push rod tubes to  the engine, first clean the case area around where each push rod will seal. Then make sure the welded seam (most likely point to leak) on the tube points upwards or vertically and that the longer end of the push rod tube goes into the case and not the cylinder head.


Spring fitted to push rod tube and supporting heat deflector which sits under the cylinders


Another view of the spring and heat deflector plate.

We are now ready to fit the cylinder head. Take the best cylinder head and use this for cylinders 3&4 as this is the hot side of the engine. (left side of engine looking from pulley end) 

Clean the cylinder head mating surface with WD 40 and paper towel and do the same for the cylinder mating surface. Lubricate the push rod holes in the cylinder head with STP/oil mix. 

Check your long cylinder head bolts have had the old "O" ring washers removed and fit the new ones from the Gasket Kit. They should sit in the groove under the head of the bolt. The head bolts should have a groove machined into them at the mid point of their shanks. This indicates that they are late head bolts and use washers. Early ones did not have this groove and did not use washers.

You will be using some blue loctite on all the head bolts when fitting them.

Now line up the cylinder head with the engine studs and carefully push it over the studs and down towards the cylinders. You will need to line up the push rod tubes and make sure they fit into their holes before the cylinder head will sit properly. It should now look like the photo below.



Now check each of the 8  bolt holes on the cylinder head to check the studs are visible and long enough to drop a washer over. Take a cylinder head bolt washer and by placing it over a screw driver blade drop it over the engine stud deep in each hole. Repeat until all 8 washers are in place. Check with a screw driver that they are sitting flat by tapping the washer around its circumference. 

The head is then retained by the cylinder head bolts. You will notice that there are 2 different head sizes on the long bolts.


Wide brimmed bolt to left with narrow brimmed to right.

Depending on whether you are using an aluminium rocker stand or a cast iron one the arrangement of these bolts will be different. Have a look at the picture below. this arrangement is for the Aluminium Rocker stand.


We have a (from left to right) a wide brimmed, a narrow brimmed, and then 2 further wide brimmed bolts. 

The arrangement for an cast iron stand would be (left to right) wide brimmed, narrow brimmed, narrow brimmed and then wide brimmed.

Apply some blue loctite to the bolts.

Take a 10mm Allen wrench and snug down the 8 bolts using the following pattern. (10-15lbs max)

6  2  4  8
7  3  1  5

When they are snug, leave this side of the engine and repeat the process to fit the other cylinder head.

We are now ready to torque the bolts to their correct level. Starting on one side and following pattern above torque to 15 ft lbs. Repeat for the other cylinder head. 

Now go back to the original side and torque to 20 ft lbs. Repeat for the other cylinder head.

The factory suggests a final torque to 22 ft lbs although some prefer to go a little higher (25ft lbs)

Finally check the torque of each nut 3 times. 

You have installed the cylinder heads.


5 comments:

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  5. your blog is an obvious labor of love.
    i am a non-mechanic whose 1963 356 b was diagnosed with
    zero compression in cylinder 3. engine starts and runs. am i up against it as far as a complete rebuild or might it be less work or cheaper (10,000) than i imagine? thanks for any comments.

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