B12 engine issues

[williamcorke]

My B10 has a B12 engine installed, the running issues with which I decided to attack this week. The engine has 78mm liners and pistons fitted (B20).

The 1-3-5 bank is, for whatever reason, where the problem lies:
- exhaust exit on this bank runs at a lower temperature
- 3 and 5 are reluctant to fire at idle, though sometimes OK and working when on the road operating at higher revs

The first thing I tried was to rebuild my spare Solex 40 PAAI carb and fit it.

The produced no change or improvement in the issues.

Strong spark on 3 and 5, so it seems fuel and spark are not the cause. Deeper investigation within the engine itself required...

When removing the head, I found that I couldn't pull out the most of the pushrods until the head itself was off.

If you look at the photos below, the reason why becomes clear... the head gasket is fouling the 'rods.

1st photo - shows the head gasket openings for the pushrods are not aligned with the holes in the head and block. You can also see the worn section of a rod where it has rubbed against the head gasket.

2nd photo - the gasket is distorted by the rods' interference, or when the rods were perhaps hammered through on installation.

3rd photo - photo of B12 head gaskets that I found online with a single oval hole for each pair of pushrods (top), not two individual ones as on my gasket.

I am wondering if I need to get hold of a new head gasket with the design seen in the 3rd photo, or if I could perhaps modify the one I have, as indicated in the 4th photo below. There is a copper sealing ring around each of the two holes which it would be hard to reproduce (perhaps with gasket material?) but is this an area where sealing under pressure is critical?

5th photo - the head.

I'm not sure if the diameter of the ring / aperture (firewall) for the bores could be an issue in my modified engine. On the head gasket fitted this is 80.5mm, but what is the measurement on a standard B12 head gasket? If one of you has one you can measure for me, that would be helpful, before I order a replacement.

I'm hopeful that the pushrod interference was causing the running problems, perhaps because valves were not fully closing at low revs, hence lack of compression causing a lack of detonation.

[Sliding Pillar]

I'll measure my spare head gasket for you in the morning.
Ade

[williamcorke]

Thanks Ade,
A photo of it would be useful too, if you could also provide that.

[Sliding Pillar]

These are a pair of B12 head gaskets I got from Cavalitto in 2010
(The measurement is the correct diameter, couldn't photograph it with the vernier caliper in situ)

[williamcorke]

Thanks Ade,

Cylinder firewall diameter / aperture on the gasket looks like 80.5mm, same as mine, but with the single oval hole for each pair of pushrods.

EDIT: I think (thanks Chris) that I misread the calliper vernier and the actual reading is 76mm, so the same size as the piston & bore.

William

[Parisien]

As installed on my B12 rebuild, hope of use

P

[williamcorke]

Where did you get your gaskets, Frank?

[Parisien]

I'm almost certain they came from Cavalitto William, bought as part of a job lot of B12 parts.

P

[chriswgawne]

I am away for a few weeks William and have only Just seen this thread. I will email you in the morning. Chris