squish

[a-teamlambretta]

Just changed head on an imola top end, squish is coming out at 0.80. Is this too close?. I am using packers top and bottom and don't want to add a gasket at the top, was hoping to get away without buying another packer. Cheers Simon

[mickyb]

wouldn't go below 1 mm, pref 1.2mm

[Donnie]

yep, 1.2 to 1.5

[Wack]

Fit a .5 base gasket= 1.3 mm

[rossclark]

Or a similar thickness head gasket, won't affect your port timings.

[hullygully]

use 2 head gaskets you gready twat

[a-teamlambretta]

dont want to use 2 gaskets as this may increase chances of leakage. will skim a bit off me head . but just incase mickey can you cut me some out of your cornflake packets , dont wnt to waist my own

[Eden]

I agree with Wack, just add a .5 base gasket.

[Jeff t]

Just rebuilding mine coming out at 1.1 I've a 0.3 head gasket, would you leave at 1.1 or fit the gasket

[dscscotty]

1.1 is fine, as long as head/piston have good profile?

[johnnyXS]

I think I've read of 0.8mm squish being used somewhere or other but as others say its bloomin tight with little margin for error.

It might be worth reading the 'Bang for Bucks' series of articles in Scootering mag as one of the early articles mentioned that a huge increase in power was found simply by reducing squish

[dscscotty]

0.8- is ok on race spec motors, we run 0.8 on production class with av gas controlled fuel etc, road motors a side of caution is advisable, some people read to much!

[johnnyXS]

0.8- is ok on race spec motors, we run 0.8 on production class with av gas controlled fuel etc, road motors a side of caution is advisable, some people read to much!

its a curse

[HxPaul]

The problem with road going scooters is,there not rebuilt on a regular basis and a build up of carbon in the combustion chamber of the head and the top of the piston can bring them very close together,that and the close squish of less than 1.00mm and they could be touching.

[Fast n Furious]

Generally speaking , it is safe and wise to limit the quish to 1.0% of the conrod length for sub 10,000rpm engines

[johnnyXS]

The problem with road going scooters is,there not rebuilt on a regular basis and a build up of carbon in the combustion chamber of the head and the top of the piston can bring them very close together,that and the close squish of less than 1.00mm and they could be touching.

I've been thinking about this and in my experience engines are to a large extent self cleaning .
Yes carbon can build up on the piston crown but it can only build up in the space that we give it. Using an engine through its full range and getting it thoroughly warm will prevent carbon build up to a large extent. Its the slow running pottering around Town commute milage that is so detrimental to engines

Once the squish area is filled with carbon on top of the piston it cannot build any further due to the constant detonation and squishing . The more squish depth we allow betweeen the piston crown and the head ....the more room we allow for carbon to build up. Once there is any appreciable carbon build up I would imagine it would have the effect of increasing the compression ratio ...giving potentially more power ...on the other hand the rough surface on top of the piston will slow down the flame travel under detonation which will have a slightly negative effect on power.

Come this August I will have been wrenching on Scooter, motorcycle and car engines for exactly 50 years and in all that time I have never read or heard of a single instance where carbon build up has been the direct cause of engine failure.
I'd be interested in others experience on this though

[Jeff t]

Well started some good debate, I'll be going with the 1.1 when I stripped it down the carbon was fairly minimal mainly around the crown of the piston but nothing excessive.

[johnnyXS]

I'll probably be going with around 1.0mm myself Jeff , after reading the power gained in Daryls recent Bang for Buck article . That was an astonishing increase.

[Fast n Furious]

The problem with road going scooters is,there not rebuilt on a regular basis and a build up of carbon in the combustion chamber of the head and the top of the piston can bring them very close together,that and the close squish of less than 1.00mm and they could be touching.

I've been thinking about this and in my experience engines are to a large extent self cleaning .
Yes carbon can build up on the piston crown but it can only build up in the space that we give it. Using an engine through its full range and getting it thoroughly warm will prevent carbon build up to a large extent. Its the slow running pottering around Town commute milage that is so detrimental to engines

Once the squish area is filled with carbon on top of the piston it cannot build any further due to the constant detonation and squishing . The more squish depth we allow between the piston crown and the head ....the more room we allow for carbon to build up. Once there is any appreciable carbon build up I would imagine it would have the effect of increasing the compression ratio ...giving potentially more power ...on the other hand the rough surface on top of the piston will slow down the flame travel under detonation which will have a slightly negative effect on power.

Come this August I will have been wrenching on Scooter, motorcycle and car engines for exactly 50 years and in all that time I have never read or heard of a single instance where carbon build up has been the direct cause of engine failure.
I'd be interested in others experience on this though

I can provide some testimony here Jonny.
It was back in 78/79.... A mate of mine, a couple of lockups down, serviced lammy's on occasion.
The rider (a really old bloke, who was probably actually younger than most of us now. LoL) said the scoot had no power.
When the exhaust was removed, it crashed to the floor.......... It weighed a ton. It was completely blocked up with carbon. The exhaust port was the size of a pea and the piston wasn't much better.
It was common practice in the days before 2 stroke oil when you had to use 20/50w or similar to do the job, leading to exhausts, pistons and exhaust ports to block up this way.
I can remember as a youngun having to decoke 2 stroke exhausts with a propane gas torch. Man did they stink. The neighbours loved me!
How times have changed.

[Warkton Tornado No.1]

Just changed head on an imola top end, squish is coming out at 0.80. Is this too close?. I am using packers top and bottom and don't want to add a gasket at the top, was hoping to get away without buying another packer. Cheers Simon

As somebody that has fallen foul of too small a squish & other head sealing problems, the rules that I apply for first class components are:

Road: Minimum squish 1 mm
Race (no piston bushes): Minimum squish 0.7 mm
Race (piston bushes): Minimum squish 0.85 mm
Road & Race: No cylinder head gaskets without exception

BTW. IMHO the object of aiming for the optimum, minimum, squish is to transfer heat from the piston crown to the cylinder head as well as optimizing the burn of the cylinder contents. It does have an effect on the compression ratio, but that is best controlled by other means, not by juggling with the finite squish figure .