head skims and throttle springs

[scooby dooby doo]

a technical question to keep you entertained...

 

When an engine has a head gasket failure its quite common that the head needs skimming as its been damaged/pitted/become porous etc.

 

But skimming the block is MUCH rarer.

 

why 🤔

 

 

And a bonus question - where do i get a secondary throttle return spring from for the Jenvey TBs on on K series - running with a Bernhard Scouse airbox kit....

 

 

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[Peter Carmichael]

The mating surface for the fire ring is the thing that needs to be as near perfect as possible. The mating surface on the block is cast iron (whether against block or liners). The mating surface on the head is aluminium (more typically) and this has suffered the damage. Hence you skim the head.

[robmar]

Hoopy - I am have no real clue but what about the following as a numpty response

 

1. it is easier to skim a head as it is easily rmoved from the engine, withthe block still in place?

 

2. block made of different material? and hence less likely to be damaged?

 

3. maybe it is deisgned in this way as replacing the head is easier than replacing a block?

 

I am sure someone with some knowledge will be along to offer the real reason

 

rob

 

[robmar]

it took my numpty brain and fingers so long to type that that peter got in before me

 

but looks like my 2nd option was correct

 

rob 😬

[oilyhands]

Apart from the obvious logistics (engine would need to be out and completely stripped) and the point about the head being the part that is more easily damaged/warped.. skimming the block will cause the pistons to move WRT the top of the bore, skimming the head wont. On some engines this would cause piston/head contact which is a sure recipe for engine destruction. In the case of the K series skimming the block will simply increase the liner protrusion and increase the chance of the waterway sealing ring failing through insufficient clamping pressure and possible head warpage.

 

Oily

[scooby dooby doo]

so its the area directly around the piston that gets damaged, and in that area the block is effectively iron due to the liner. cheers Peter.

 

i realised that skimming the block is much more awkward with nasty side effects.

 

I'm just a little surprised that its one of those examples where, of the two things that could go wrong, the simpler one to fix is the one that goes.

 

So do you ever get damage in other regions where the interface is with the alloy of the block? ie what about failures where the waterways pass from head to/from block 🤔

 

 

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[Roger King]

Both the head and the block must be flat, but the head is less massive and has to deal with much higher temperatures than the block, so it is far more likely to warp.

 

Blocks can warp, but it is much rarer.

[scooby dooby doo]

aha ❗ makes good sense.

is the skimmimg generally to repair warping then 🤔 I thought mating surface damage was more of a problem 🤔

 

anybody able to answer the bonus TB throttle return spring question 🤔

please 🤔

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[oilyhands]

Skimming can be for either or both, it depends on the mode of failure.

 

Jenvey supply a linkage which mounts on top of the TBs, part of the linkage is a plastic rod which normally has a long coil spring sleeved over it, this acts as a secondary return spring. You will need to fabricate a small bracket below the TBs onto which you can attach a supplementary return spring, this can then be fixed to the throttle quadrant. If your cable isnt returning all the way it may be because the angle of the routing of the cable is too tight, or there is insufficient slack in the cable when the cable is at rest.

 

Oily

 

Edited by - oilyhands on 14 Sep 2002 07:52:02

[scooby dooby doo]

I wanted an extra spring as the throttle pedal feels a bit too light for my liking - ie the weight of my foot presses the pedal further rather than it being nicely balanced.

 

I'll have a dig on the jenvey site for some pictures - I can't quite visualise what your saying (or maybe a bit of coffee and reading it again would help )

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[scooby dooby doo]

ooh yes ❗

my blat last night with just me in an empty car showed just how much difference the TBs and new exhuast made. On the Finalnd trip i was full of luggage and a passenger so I think they were cancelling out the extra from the engine mods. although the throttle response was very noticeable.

 

Great fun 😬 and thanks oilyhands

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER