33 replies to this topic

[deanymini]

What tends to happen is someone hears something and then regurgitates that information in a similar thread. They have no research or evidence whatsoever, but jump on the bandwagon to appear to be informative and knowledgeable.

[mini-luke]

It may double the chances of the HG failing but that's hardly common anyway, not if both surfaces are flat and the proper torque is used

[jd24-7]

What tends to happen is someone hears something and then regurgitates that information in a similar thread. They have no research or evidence whatsoever, but jump on the bandwagon to appear to be informative and knowledgeable.

Thats about right!
Jd

[lsto]

I fear I have caused some disagreement among the forum...

I guess if you look at it in different lights then everyone has their own personal view, call it a bodge if you will but then again if you modify something from standard then technically thats a bodge because it was not how it was designed to be.

I'd rather call it engineering.

Thanks for everyones views, its nice to look at something from alternative angles.

[Alex_B]

The world is full of disagreements, thats life! Personally I enjoy it.

and well there is a big difference between engineering and bodging.

the difference is the person and reason for doing something, if you were doing it for a legitimate reason while taking account for all the angles and possible issues, thats engineering, using duct tape over a hole to get through a mot is a bodge.

thats how one thing can be both engineered and a bodge, like the decomp plate being engineered as a weak spot on production diesels, yet one man in a shed who cannot be bothered to machine down his head and using a decomp plate instead is a bodge.

and yes engineering something is a lot more rewarding, and I am now correcting a lot of bodges on my car with well thought out fixes.
probably why I am doing a degree in motorsport engineering

[JustSteve]

The world is full of disagreements, thats life! Personally I enjoy it.

and well there is a big difference between engineering and bodging.

the difference is the person and reason for doing something, if you were doing it for a legitimate reason while taking account for all the angles and possible issues, thats engineering, using duct tape over a hole to get through a mot is a bodge.

thats how one thing can be both engineered and a bodge, like the decomp plate being engineered as a weak spot on production diesels, yet one man in a shed who cannot be bothered to machine down his head and using a decomp plate instead is a bodge.

and yes engineering something is a lot more rewarding, and I am now correcting a lot of bodges on my car with well thought out fixes.
probably why I am doing a degree in motorsport engineering

It is not necessarily a weak spot?

Drop brackets are a weak spot- they allow more flex.
Adjustable camber brackets are notorious for snapping
Spaced drums put more wear on the bearing.


A decompression plate made from the correct material Will have NO adverse effect on reliability.

[Alex_B]

well decomp plates use two head gaskets which in my book doubles the chance of failiure and if something like over boost were to happen then the gasket would blow before a piston etc,

drop brackets do increase "flex" but only really when the steering lock is on and tightening the steering wheel nut has it ever been a problem for me personally,

camber brackets apart from KAD are crap, thats neither engineering nor a bodge its just plain bad design,
and spacered drums are originally for use with wheels with a different offset so using them with correct wheels will mean no more stress on the bearings, anyone using them with wrong offset wheels would increase wear but its not really a problem.

[JustSteve]

Doubles the chance of failure?



I'm going to buy myself a reliant robin now because With three wheels there's less chance of having a blow-out

[ANON]

you wont be getting a head and block sealed without a gasket though as it was designed with one in mind...

i have done it, it can be done but isn't practical to do.

well decomp plates use two head gaskets

WHAT!!! who the hell would use two head gaskets with a decomp plate??

[Alex_B]

I was under the impression that a decomp plate has a gasket on both faces, or is is just a thicker gasket?

if its the latter then there would be more of a chance of failure due to larger area of weaker material


and well done on getting a head and block to seal without a gasket, bet it took a while to skim both with the correct tolerances?

[ANON]

a decomp plate is pretty much a sheet of metal cut to match the block/head faces. as long as they are a decent quality then they are no weaker than what is already there, it's a myth that they are a weak spot. have a read of this...http://www.ferriday....on_plates.shtml. mike is a knowledgeable and respected bloke and has been around for a while.

head and barrel was easy to do without a gasket, just time consuming. valve grinding paste on a sheet of glass just turn it 90deg each way and did each one until it was an even colour/texture all over, the the same but with finer paste. when they were both done lapped them together, made a sleeve that was bolted to the head through the spark plug hole and into the barrel to keep them aligned.

built it back up and it was fine, no loss of pressure or coolant loss.

[Alex_B]

May I ask, why not just use a gasket? as I can imagine the amount of time needed to get it perfect

Edit: and thanks for the link clears it up in my head as I havent looked into decomp plates in detail and was just going by what I have been told and whats logical,

they do seem weaker when heat is concerned, obviously as they are a fraction of the width, logic would dictate that it would be the first think to go under extreme temperatures which still agrees with engineering in a weak spot, whether or not thats the reason why some manufacturers use them from the factory I dont know

Edited by Alex_B, 28 December 2012 - 11:05 PM.

[ANON]

got rid of the gasket as it was in a two stroke tuning book i had years ago plus it gave the perfect squish. was more of a 'let's see if that works' than anything else. did a lot on the engine as well, channelled the crown of the piston from the ports towards the plug etc...

they won't be weaker though, if it's an ally plate, used with an ally head, then it's no weaker than the head or pistons. if it's a steel plate then the pistons are 'weaker' than the steel plate.

[Wil_h]

There are two ways to use a decomp plate.

1) Sandwich it between to head gaskets
2) use a sealant and 'glue' it on to the head or block surface and assemble as normal.

Both will work. The single head gasket option is better but requires more consideration in assembly.

In some cases it is impossible to build an engine without one, but i personally would avoid it if possible. Pushrod issues is the main reason. Other than that you are creating a large piston to deck height, which is not ideal for engine efficiency and hence ultimate power.

And just to clear up a statement that has proliferated this thread, two head gaskets do not double the chance of failure, the failure rate would be the same. the reason for this is that gaskets have a life expectancy in the conditions they are working in, both gaskets will be working in the same conditions.

[supermotolee]

a company such as cometic will do a 3 layer steel head gasket in varying sizes i have seen minispeed supply them upto .120" that will drop compression fairly drastically. i would use this instead of a decomp plate no sealent required as far as i know as the outer layers are coated