VBB/Sportique Low Compression After Rebuild
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Joined: 21 Jul 2020
Posts: 5
Location: Huntingdon UK
Thu Jul 30, 2020 7:34 am quote

My VBB (64 Sportique) has had its engine rebuilt but it won't fire/start.

The bore was in very good condition but I fitted a new, std, piston and rings and had the cylinder honed by an engineering company.

The compression test only showed 50psi/4bar....less than half what I'd hoped for.

There's a good spark and fuel is getting to the carb. I tried spraying "Easy Start" into the carb to eliminate fuel starvation.

I have removed the head and will try polishing it on a flat surface.....any advice on how to do this would be appreciated.

Also, is it best to put some sort of sealant on the head/cylinder faces. I know it shouldn't have a gasket but see some postings saying it's best to use copper spray, copper grease, or the stuff detailed below.

Any advice would be appreciated......Thanks

WELLSEALJOINTING COMPOUND-55C to +200C100ml Tubestag wellseal jointing compoundStag Wellseal jointing compound (100ml tube)Wellseal was developed by Rolls Royce Ltd and manufactured under license This compound is the complete answer to sealing problems on machined faces.Formulated to provide the best possible sealing performance under a wide variety of conditions.Non-Flammable - Non -Partability- Non -Hardening. Wellseal Jointing Compound is formulated to give the best possible sealing performance under a wide variety of conditions.Commonly used to seal Aluminum cylinder heads and transmission boxes. It is used to seal threaded connections as well as flat-faced joints and can be used with or without a gasket, depending upon the design of the assembly.Wellseal Jointing Compound is highly resistant to the majority of commonly used field, lubricants and coolants and is non-corrosive and free from abrasive fillers.If you are building vintage engines with a bad cylinder block or cylinder head face or fitting compression plates, spacers or saver shims then this is the perfect product to use.
2007 Stella 150
Joined: 02 Nov 2019
Posts: 761
Location: Rochester, Minnesota
Thu Jul 30, 2020 7:50 am quote
I used a flat bed belt sander with a 600 grit belt. If you use this approach, sand something else with a new belt, preferably metal to dull the sharp edges of the new paper. I rotated the head constantly and picked up when stopping, letting the weight of the head do the work and didn't press down to speed up the process, while sanding it down. Took a few minutes but with the lower grit paper, it wasn't aggressive allowed me to not take off to much at once. I needed to take off 2.5 mm so I would have been there a long time by hand.

Style Maven
74 50s x3 78 P200 84 Cosa PK50XL2 58 AllState 68 Sprint 80 50special '66(?) Super125
Joined: 22 Nov 2010
Posts: 7799
Location: seattle/athens
Thu Jul 30, 2020 9:57 am quote
I use a full sheet of 500 to 800 grit wet or dry with the corners taped to a clean sheet of glass. Get it wet and trace a figure eight as you slowly rotate the head. You can easily check to see when the mating surface is 100% cleaned up and should be flat if you use even strokes and rotate as described.
Joined: 21 Jul 2020
Posts: 5
Location: Huntingdon UK
Thu Jul 30, 2020 10:19 am quote
Many thanks to you both. This is really useful information.

What do you suggest about the sealant?

I have more experience with 4 stroke car restorations....How hot does a VBB cylinder head get?

Thanks again.....
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