Bump clearance

[Kieron G]

Hi, does anyone have an idea where I might be able to get hold of some 1/8" dia lead rod to feed into the injector holes in the cylinder heads on my flat twin to measure the bump clearances. I haven't tried the vintage engine specialists yet but not sure if they'll stock lead rod in lengths.Thanks in advance for any ideas / suggestions. Kieron.

[bizzard]

Hi, does anyone have an idea where I might be able to get hold of some 1/8" dia lead rod to feed into the injector holes in the cylinder heads on my flat twin to measure the bump clearances. I haven't tried the vintage engine specialists yet but not sure if they'll stock lead rod in lengths.Thanks in advance for any ideas / suggestions. Kieron.

.177 air gun pellets might do it.

[Kieron G]

Hi Blizzard, I think i'll need a length of lead to feed in and remove after the engine's over TDC with the cylinders being horizontal. This is how the manual instructs to do it. Unless I glue a pellet to the piston crown and scrape it off to measure it.

[bizzard]

Hi Blizzard, I think i'll need a length of lead to feed in and remove after the engine's over TDC with the cylinders being horizontal. This is how the manual instructs to do it. Unless I glue a pellet to the piston crown and scrape it off to measure it.

You could solder the pellets to bits of thin wire.

[bizzard]

You could solder the pellets to bits of thin wire.

Airgun pellets have a little hollow at the tail end, below the waist. A bit of wire poked in there and the pellet skirt squeezed over it with pliers should trap and hold it without soldering.

[Kieron G]

Problem solved. Thanks for that.

[bizzard]

Problem solved. Thanks for that.

Or a tiny fishing weight, a little ball which has a slot for trapping the line in, they are soft because teeth are used to squeeze the slot closed over the line.

[Arthur Brown]

Must it be lead? Solder comes in reels and shorter lengths and can be bought at moderate cost in reasonable diameters,

[Kieron G]

Yes , I didn't think of solder . I've got a few possible solutions there to get me moving when I get my gaskets delivered now. Thanks.

[RLWP]

Really, you need to do this in three places around the piston at the same time

 

I twist thin solder together to make thicker test pieces

 

Richard

[bizzard]

I have a reel of plumbers solder here.Fry's powerflow 3.25mm dia, but it does seem a bit harder than ordinary lead.

[zenataomm]

I bend a piece of sheet metal at right angles and melt solder into the V formed by the bend.

That gives me a stick of the stuff.

[Kieron G]

Have just been looking at different diameters of solder on ebay myself . Re the three positions on the piston crown, do you take the average reading of the three and is the reason to spread the load evenly and give more accurate readings ? thanks.

[bizzard]

Have just been looking at different diameters of solder on ebay myself . Re the three positions on the piston crown, do you take the average reading of the three and is the reason to spread the load evenly and give more accurate readings ? thanks.

To spread the load as the pistons can cant slightly, springy piston rings.

[Alan de Enfield]

There is a special kind of compressive filament that is used to headspace rifles - it fits between the bolt head and the cartridge / chamber to measure the gap.

 

Its a bit like 'high-tech' play-doh, but retains its 'squashed' shape and thickness when removed. It certainly works down to gaps of 10s of thou ( ie 60 thou) without any problems.

If your other options don't work I can get the details for you.

[RLWP]

There is a special kind of compressive filament that is used to headspace rifles - it fits between the bolt head and the cartridge / chamber to measure the gap.

 

Its a bit like 'high-tech' play-doh, but retains its 'squashed' shape and thickness when removed. It certainly works down to gaps of 10s of thou ( ie 60 thou) without any problems.

If your other options don't work I can get the details for you.

 

Plastigauge: http://plastigauge.co.uk/

 

Richard

[BMC problems]

Hi, does anyone have an idea where I might be able to get hold of some 1/8" dia lead rod to feed into the injector holes in the cylinder heads on my flat twin to measure the bump clearances. I haven't tried the vintage engine specialists yet but not sure if they'll stock lead rod in lengths.Thanks in advance for any ideas / suggestions. Kieron.

lead solder

[Alan de Enfield]

 

Plastigauge: http://plastigauge.co.uk/

 

Richard

That's the sort of stuff - I was thinking of a different brand name, but yes - that's it.

[RLWP]

That's the sort of stuff - I was thinking of a different brand name, but yes - that's it.

 

Bump clearance is usually around 0.035" to 0.045", it varies from engine to engine

 

Richard

[Kieron G]

That's great, thanks for link for plastigauge. Have ordered solder so should be ok. I've got bump clearance figures of .060"-.065" provided by Surgeneris from the Coventry victor manual. Piston crown isn't flat though so I think that's for the low area.

[Neil Smith]

Ask an old fisherman, my dad used to buy lead wire from tackle shops.

 

Neil

[Tony Dunkley]

Hi, does anyone have an idea where I might be able to get hold of some 1/8" dia lead rod to feed into the injector holes in the cylinder heads on my flat twin to measure the bump clearances. I haven't tried the vintage engine specialists yet but not sure if they'll stock lead rod in lengths.Thanks in advance for any ideas / suggestions. Kieron.

 

Lead wire poked in through the injector holes will only enter the combustion chamber, and won't be flattened against the head by the flat part of the piston crown to give you a reading for the bumping clearance.

With the heads off you need to put two suitably sized bits of lead, about three quarters of an inch long and bent into a shallow v-shape, on the piston crown at right angles to the line of the gudgeon pin and the valves, and no more than half an inch from the cylinder wall. Position the pistons about one quarter to one third of the stroke down the bore from TDC with the bits of lead in place, re-fit the head with a new gasket, just nipped down, not fully tightened, and turn the engine over TDC once only.

Suitable strips of lead can be obtained from the roof of the nearest church, or by trimming bits off the lead flashing that builders and roofers use.

[BEngo]

Tony,

 

Read the OP. It's a flat twin so there is not much hope of balancing the lead on top of the pistons without rotating the whole engine, and then you could only do one pot at a time.

 

 

Your point about the wire only entering the combustion chamber, is valid, unless of course it is a DI engine. The OP doesn't tell us.

 

N

[RLWP]

Tony,

 

Read the OP. It's a flat twin so there is not much hope of balancing the lead on top of the pistons

 

Stick them on with grease

 

Richard

[Tony Dunkley]

Tony,

 

Read the OP. It's a flat twin so there is not much hope of balancing the lead on top of the pistons without rotating the whole engine, and then you could only do one pot at a time.

 

 

Your point about the wire only entering the combustion chamber, is valid, unless of course it is a DI engine. The OP doesn't tell us.

 

N

 

I see that RIchard has already dealt with your first point, . . . Thank you Richard.

 

Poking lengths of lead wire through injector holes won't work on a direct injection engine either, . . . the wire will just protrude into the bowl shaped combustion chamber in the piston crown, and in any case, you really should have two bits of lead in diametrically opposed positions on the piston crown for an accurate result.

 

The Coventry Victor Vixen, which I believe the OP has in his boat [see Posts #1 and #20, . . and his boat is called 'Vixen'] is indirect injection, as were the engines it evolved from.

They were developed from the pre-war horizontally opposed flat twin 'Cub' engines which had flat topped pistons and a spherical combustion chamber in the head, very similar to the Ricardo Comet indirect injection design later used in the BMC 1.5 and 1.8.

The Cub was made under licence by Coventry Victor, Enfield and Petters, produced 9.6 bhp at 1500 rpm, and was marketed by a company called Cub Oil Engines. They were used in a variety of applications, including driving 5KVA War Department gensets, and I think, a small crawler tractor, before being superseded by a bit more powerful air cooled Enfield version.

Edited March 2, 2016 by Tony Dunkley