Javelin Engine - Piston Protrusion At T.D.C.

[Mike Allfrey]

We are still working on the Javelin engine that had the head gasket failures. The crankcase being used this time has undergone the indignity of having the cylinder head gasket surfaces machined after the welding of splits at the rear coolant jackets. Have just installed one piston/rod assembly and found 0.027-in. piston crown protrusion when it is at top dead centre position. This concerns me as being a mite excessive.

In my past work, we used to check 'bump clearance' on tractor diesel engines, this was done by placing a suitable piece of lead (or solder) on the crown and installing the cylinder head with a slave head gasket, rotating the crankshaft one revolution, removing the cylinder head and accurately measuring the piece of lead after its thump. Then the specifications were referred to so that the piston could be machined to achieve the correct height tolerance. Of course, with diesel engines of the time, this was a critical requirement.

Has anyone carried out such a task on a Javelin engine?

We will be carrying out a 'bump test' prior to proceeding further.

The pistons being used are Australian made JP pistons, manufactured to Jupiter specification.

I have read about Jowett cylinder head gaskets with larger diameters to accommodate larger over-sizes of pistons, but not read about thicker head gaskets to accommodate excessive piston protrusion - yet.

Here in Melbourne, we have been under strict lock-down again and I have not been able to go shopping for Loctite products and oil seals - quite frustrating!

All the best,

Mike A.

[Keith Clements]

I had a similar problem when experimenting with high lift cams on my Jup.
You can also have an issue with worn bearings as my Subaru proved dramatically on the M4 when the piston slap bent a con rod.

[Mike Allfrey]

Thanks Keith,

Our local COVID-19 lock-down situations are making this engine an even slower process.

I have now accurately measured the piston protrusion and found it to be 0.032-in. at numbers one and three cylinders. Numbers two and four will be installed this afternoon and their protrusion at T.D.C. measured. We carried out a solder 'bump test' and the result makes matters a bit close at the cylinder head faces. Therefore, we are now considering cutting from copper sheet, a pair of shim plates to insert between the head gaskets and the cylinder heads, to compensate for the protrusion. This bears in mind, that numbers two and four pistons could have a different protrusion dimension from the other side. Copper sheet, semi-hard, is available from a local supplier in 0.7 mm and 0.8 mm thicknesses, so we have a bit of scope.

I intend to clamp the copper sheet pieces to a piece of dressed hardwood, with a cylinder head and then drill through the head stud holes. After that operation, it will be a case of cutting out the apertures after marking out from a new head gasket, for coolant, balance pipe, combustion chamber and push rod apertures. I have done this before, but with air-cooled diesel engines to achieve the correct 'bump clearance' which was very critical on direct injection engines, thus I think it should work on the Javelin engine.

Has anyone out there tried this recovery method on a Jowett engine?

I am loathe to machine the outer faces of the pistons, due to balance and piston durability concerns.

All comments will be welcome.

Keep well,

Mike A.

[Keith Clements]

What is your liner protrusion and head gasket thickness?
Both will affect the clearance between head and piston.
The compressability of the gasket will also play a big part.
Are these Jup pistons longer?

[Mike Allfrey]

Hi Keith,

We have painstakingly set the cylinder liner protrusion to 0.010-in. and propose to use the NZ gaskets, which had let go at 0.008-in. protrusion. With these gaskets being slightly thicker than the previous NZ gaskets, I feel that 0.010-in. should be OK. We shall see!

The pistons were made by an Adelaide company who made them at the request of the JCCA, before I arrived here in 1968, to the same specification as the original Jupiter pistons. Currently, the JCCA tends to use modified Mazda pistons. For me, the JPs have worked well over the years.

Dinner is on the table,

Mike A.

[Keith Clements]

Piston Comparison:
. Hepolite Triumph VW
Diameter (mm) 72.5 73.647 77.00
Compression Height 30.50 38.50 38.00

Above is taken from your notes Appendix 1 of Part 2-15-05
So what is the compression height of the pistons you are using?
It is not clear in the above which Hepolite piston is being compared, but presumably the original standard Javelin and not those referred to in the Competition tuning notes Part 2-15-35 section h on page 4.

[Mike Allfrey]

Hi Keith,

Continuing, the cylinder liner protrusion was set with just a couple of shims each, no copper spacer washers, so that just about equates to the 0.032-in. piston protrusion proud of the liner lips.

The JP pistons are the same, with regard to dimensions, as the original Hepworth & Grandage pistons Jowett Cars Limited used for Jupiter engines. the club here was after more performance for Javelins. The only difference has been that there is no split skirt. They do seem to have a problem with their crowns separating. That had not happened with the JP pistons in the Javelin I had, nor with my Jupiter - both had covered many miles with complete reliability. The Javelin covered well in excess of 100,000 miles on JP pistons.

I will have a look at the notes you mention, I think I did not actually write them, just placed them in the system I created. Will get back on that, and the gasket thickness. It is too cold, damp and dark to go to my workshop at present!

All the best,

Mike A.

[Mike Allfrey]

Back again, Keith,

Part 2-15-05 was a direct re-typing of Dr. Harry Brierley's screed on the Javelin. My intention was to put it into a format that younger enthusiasts could easily share. The same applies to the Competition Tuning Notes, re-typed and illustrated from an original.

I note that in Appendix I one of the alternate pistons has 'smaller' gudgeon pin size!

Will come back with gasket thickness tomorrow.

Mike A.

[Keith Clements]

Do you know that the blocks have not been resurfaced and are thus smaller?

[Mike Allfrey]

Hi Keith,

The crankcase halves were both welded where they had cracked inboard of cylinder head studs '7' and '10'. Having had a similar repair done on Phil's crankcase, right hand rear, and found that head gasket surface had sunk about 0.012-in. in that area, I can well understand why the alternative crankcase had been machined after the weld repairs. Both halves have definitely been machined in that area, and the balance pipes have had their flared ends machined off - to me, a definite non-event on a Javelin engine.

Those head studs have been extended through the coolant inlets.

I am convinced that the crankcase joint faces have not been machined, thank goodness!

We are about to make the shim plates. I am using a dressed piece of mahogany 40 mm thick (left over from my hi-fi turntable platform!) and will clamp a spare cylinder head over the copper plates. Then I will drill four holes through the end head stud holes to firmly clamp the assembly, while the other six stud holes are drilled through copper and timber. With that task completed, the timber will be turned over and the holes countersunk for 10 mm Allen screws. Why 10 mm? Well, the stud holes in the cylinder head are a neat fit for a 10 mm drill and bolts.

Once the countersinking is done, the copper plates will be clamped to the timber with the 10 mm screws, one of Bill Lock's head gaskets and flat washers so that the plates are firmly held while the other apertures (push rod openings, coolant openings, oil drain holes and the combustion chamber openings). Should suffice to achieve desired result.

Will send photos as job is done.

Stay well,

Mike A.

[Mike Allfrey]

Hello Folk,

We are yet again in strict COVID-19 lock-down, so, not much has been happening. The spacer plates have been made and polished with Brasso!

The engine is now ready for the cylinder heads to be fitted. My back will not let me do that on my own. Will take my camera to my workshop and hopefully get some photos as we proceed with the job.

Stay well,

Mike A.