February 19th 2019
The car seemed OK though whilst running had lost some of it's 'get up and go' over the past year, with low compression on cylinder 3. A dry and wet compression test confirmed there was a problem with the cylinder head area and not the pistons, but as it was running with no noises and would reach 65 mph it was felt best to leave well alone, mistake!
We weren't sure what the problem was until today following an even poorer compression test when the cylinder head was finally removed. The images below just show how rugged the Perfo engine is to run with some heavy damage to number 3 cylinder exhaust valve.
The head had been rebuilt before I got the car with unleaded exhaust seats fitted, but it looks like a lot of parts had been reused rather than replaced, so it's going to be a full head rebuild this time as the valve guides are worn, some of the valve heads are 'thin' (looks like they have been cut back and reused more than once) and the coolant water tube is badly corroded, possibly causing a hotspot at cylinder 3.
Parts bill alone is £442 (just ordered everything....) New Valves, Valve Guides, Valve Springs, Coolant Water Tube, Cylinder Head Core Plugs, Rocker Shaft, Head Gasket, Manifold and Water Pump Gaskets, Cylinder Head Nuts etc. etc.
The head gasket hadn't leaked and the cylinder walls look OK as well as the heights, there is minimal carbon on the piston tops so hopefully the only work required is to the cylinder head itself.
Once the new valves and other parts arrive from Belgium and the Netherlands the head will go off to the machine shop to get the guides fitted and the valve seats 'recut' to match the new valves.
23rd February 2019
New parts have arrived and the difference in the valve head thickness is visible in the image on the left of the screen. The water tube in the cylinder head disintegrated when it was removed due to corrosion so may have led to a 'hot spot' at no 3 cylinder causing the valve to burn out, but looking at how thin the heads are it's possible it was simply that they were too weak after never have being replaced and reused for 67 years.
The old springs after years of use had shortened as well at 44.3mm, down from 48mm (new spring size) and whilst circa 4mm average amongst them all doesn't seem a lot, at -9% would all add to the general weakness of the valve train.
5th March 2019
Cylinder head came back from the machine shop today. The new valve guides are in place, the valve seats cut and the valves lapped in and the new water tube is fitted along with the core plugs. The inlet and exhaust manifolds have also been refaced. Now it needs the valve springs fitting, the head painting before it goes back on the car to have the head torqued down and the valve clearances set once the rocker shaft is rebuilt and fitted.
7th March 2019
James has got everything together and the car runs sweetly but (why is there always a but...) I had a phone call from the parts supplier in Belgium today who says that the rocker shaft that was supplied wasn't made correctly and he is going to send a replacement next week. Now on running the engine, the oil is flowing and reaching the rocker shaft ok so it's difficult to say what the problem actually is. However erring on the side of caution, we are going to wait until the replacement arrives.
Now the new shaft is fitted it is now apparent that 4 of the actual rockers themselves are worn in the centre bush, 1 in particular has oil pouring out at the bottom in a stream, not a simple drip, so a replacement set has been ordered to come with the new shaft putting the parts bill up to £540.00 plus the machine shop charged £158.00 to recut the valve seats, fit the new guides and hone the valves in, plus machine the inlet/exhaust manifolds face level.
19th March 2019
Well the best laid plans are on hold as the replacement parts are stuck somewhere in France due to 'industrial action' so until they arrive the car remains stuck in the workshop.
21st March 2019
Parts have arrived and now we know what was wrong with the rocker shaft, the rocker oil feed holes were in the wrong location!!
Everything is back together and the car runs well again. Just need to put a few miles on it then get the cylinder head nuts re-torqued down and the tappets adjusted when hot.
25th March 2019
Cylinder head has now been re-torqued down after 50 miles. Tappet settings are ok so we are all set for the summer.
Additional Information:
It's worth adding a bit of context to the potential issues from my own research found on-line and in back issues of the UK Traction Owners Club magazine and I have summarised the main findings below:
The Perfo engine (and the later 11D) was designed back when petrol was circa 70 Ron and it's a low compression engine:
Fuel History:
Before World War 2, leaded fuel was unknown. The maximum octane rating was about 92 but normal petrol had around 70. After the war, octane rating rose to 75 for normal petrol in 1956. At the end of the forties, lead was added.
Traction Avant engine development:
The first 7's with a 1600cc engine had a compression ratio of 5. 7:1. Later, with the increase of piston displacement, the compression ratio also rose to 5.9:1 and then to 6.2: 1. The first 11 had a compression ratio of 5.9: 1, increasing to 6.2:1 with the introduction of the ‘Perfo’ engines. From 1950 until May 1955 it was at 6.5: 1.
The 11 D engine had, from 1955 to the end of production, a compression ratio of 6.8: 1. The 15/6 had a ratio of 6.2: 1 until 1949, and later 6.5:1 until the end of production.
Now Lead additives in petrol not only avoid ‘knocking' but also decreases the valve seat wear. In addition, the lead helps to improve heat transfer. In the case of leaded petrol, the closing valves ‘beat’ onto a relatively soft, damping layer which is not built-up with unleaded fuel. Consequently the valve clearances decrease more rapidly, and burnt valves can be the consequence of neglecting regularly to check the valve clearances.
Until the end of the 1940's, petrol was unleaded and cylinder head reconditioning was practiced every 20,000 to 50,000 km (12,500 - 31,000 miles) and was considered to be a regular expected repair.
Back in 1998 FBHVC (Federation of British Historic Vehicle Clubs) has a further item basically confirming the 1st article but leaning towards any ‘additive's' available only being partially effective, however technology has improved now.
A further article I have read concludes that cast iron engines were more at risk from unleaded fuel and that whilst wear rates were unpredictable and could vary widely, in a 4 cylinder configuration, No 3 cylinder frequently displayed the highest rate of exhaust valve seat recession. This mirrors my personal experience from the cylinder head rebuild required detailed above.
Last Update: 25th May 2019
