squished heads

[richard rawson]

my scott silk has a dpy engine with a squished head it does not have a lot of compression it dies on hills could it be down to the cylinder head does anyone know how they work when you fill a standard head up with oil it takes 74mm of oil when you fill the squished head it takes 86mm of oil does that lower the compression would I be better off with standard dome being fitted, the recess where the spark plug fits takes the extra oil I have tried to upload a photo but the file is to big

[Stan Thomas]

Hi Richard,

I fear you may be thinking in the wrong direction!!

Firstly, unless there is a compression leak, in simple terms, the cylinder head as such cannot significantly alter the compression of your engine of its own accord. Also, I do not understand your terminology of a “squished” head, as a squish-band profile as such is impossible to obtain by virtue of the design of the piston deflector profile.

I also think that you are measuring the combustion volume of the head incorrectly – i.e. the combustion space with the head removed, as a volume of 74cc would provide only a compression ratio of 5.0:1 and 86cc only 4.4:1. whereas the standard compression ratio (allowing for slight variations) of a DPY engine is 7.2:1.

As the resolution of your problem may be more complex than can be encapsulated by postings on this forum, If you care to let me have a contact number, I’ll gladly discuss further if that would be of assistance.

Alternatively, I suggest you make contact with someone who really knows cylinder heads – for whom I refer to Alan Noakes, as he resolved my cylinder head problem by machining and fitting new combustion chamber “skulls” with a ten day turn-round and at very modest cost.

[BRIAN MARSHALL]

I think that the type of head Richard is refering to has an internal shape approximately following that of the piston top, instead of a plain dome/hemisphere. I was looking at one a couple of weeks ago in Geoff Hearn’s workshop. They were made by George Silk, and Colin Heath also had a small batch made years ago when he was vintage racing. In other words NOT a classic squish ‘panama hat’ shape combustion chamber.
Brian

[Stan Thomas]

I take your point Brian, but surely a cylinder head combustion space shaped to the profile of the piston (to raise the compression ratio) would not have combustion volume when correctly measured of 86cc?? notwithstanding such a combustion shape would reduce the volume compared to a standard profile, however measured.

I suspect Richard has mererly filled the combustion chamber of the head with it removed from the engine, as 74cc sounds about right when measured as a total space – which of course does not allow for the dispacement of the deflector.

Combustion volume can only be measured with the head in situ and with the piston at the top of the stroke, preferably with the engine removed, or at least with the bike jacked up at the front!!

Why?

Because the standard cylinder head combustion space is composed of two hemespheres of differing radii, and filling the combustion chamber to measure the volume with both wheels on the ground and the cylinders sloping forward may result in a trapped air-space and give a false reading. Trust me, I’ve been down that road – and got the tee shirt .

[richard rawson]

Brian is correct about the head jeff and I did the measurements with both types of heads jeff also said the correct way is what stan said having the engine complete the bores and rings are ok as there is not compression I thought it could be the head or porting but I am not that good at porting problems

[BRIAN MARSHALL]

Hi, As the cylinders slope forwards at 38 degrees, the bike has to jacked up by that amount to get the cylinders truly vertical before doing any measuring of combustion chamber volumes.
Brian

[Stan Thomas]

Sure you got that right Brian?

To raise the engine until the cylinders are 90 degrees to the horizontal would be 90 less 38 = 42 degrees.

[Stan Thomas]

Sorry – hit the wrong key – thats 52 degrees.

[BRIAN MARSHALL]

Depends on wheel diameters, state of front forks, etc., as top edge of crankcase sides may not actually be level to start with, so just raise front of bike until cylinders are truly vertical.
Brian

[Richard Moss]

Hello Richard,
Good luck getting it sorted out… It’s a lovely bike and worth the final push ( hopefully not literally).
It’s hard to visualise how the volume you mention could be possible in one of George’s heads given that the average tdc volume in my dad’s heads is thirty/forty something(IIRC).That’s production volume and not race volume. I’m at about 25cc now, but I know Colin got down to 19cc on Martins bike as I have some of the notes on his developments. I think Colin’s head was one from John Farrah’s pattern but I’m not totally sure. I know that the patterns were made without shrinkage taken into account and each hemisphere was a fair bit smaller than required as well as too close together. I machined one from scratch about 7 years ago and it took bloody ages. The fact that there was so much material to remove meant that you could get them closer all over though. My dad’s pattern which I’m pretty sure was used on the Silk Scott, has better centres as cast but also was made to have less fitting work. You can’t easily get less than mid to high twenties out of them. Still, now we are using expansion chambers so the effective compression ratios are likely to be substantially higher. In fact, I think that the compression ratios of (almost) modern racing two strokes are restrained to around a tenth of the cylinder capacity. Obviously vit would be easy to reduce that with an almost flat topped piston.
I do realise most of this isn’t useful.
But unless yours is not a high compression head (cast profile to match deflector) or it’s not a deflector piston (!) or oils too thin/rings too worn to measure or you are not at tdc, then 80 odd cc’s is hard to place. Looking forward to finding out what the answer is!

Richard

[scott596]

I hope this photo shows the type of head being talked about, the problem with this design is that there is always too much clearance down the side of the piston crown making the flame path just as long as a standard head so just because it is shaped like the top of the piston it is never going to be a proper squish head. Alan Noakes. scottworkshop@hotmail.co.uk

[James]

Couldn’t the head be made with a squish ring and the piston then machined to match giving a true squish? The ring does not have to be much say a .250″ width.

[Richard Moss]

Hi Alan,
I’m not sure my spirit will allow me to repeat the overly lengthy piece I’ve just written, and subsequently lost with another login prompt so I’ll have to just cut short!
The Moss pattern ‘high compression’ head that you picture has side clearance which is an intentional safety margin, given that its a sand casting and its primary function is to provide a direct replacement for a long stroke detachable head. Its always necessary , of course, to match such heads to an engine, which we do with an extendable conrod which my dad made to allow precise clearance setting. They increase compression and undoubtedly performance. Despite this, it’s necessary to minimise hand fettling as much as possible to keep costs reasonable. We both use the heads on our racers and he has been tested (pump petrol) at 45bhp. Of course there’s a bit more hand fettling required to get the kind of volumes we want for racing and my head is skimmed to allow me to have the piston sit deeper in the head which enables me to reduce the side clearance. They aren’t squish heads, but a fair reduction in burn times has been achieved.
The question is, what kind of performance improvement would a head so precisely matched as to qualify for the word ‘squish’ bring? I understand the benefits of flame speed, but my dad’s bike runs 19deg btdc optimum timing which is pretty good considering the lump you’ve got to cover. I guess the optimum tested timing of an engine is the main comparitive indicator of the burn efficiency, obviously maybe cylinder temperature would be reduced too as less build up on crown as ‘dead’ squish areas would remain cooler. Have you done a skull head that you would class as a squish head?

[scott596]

Hi all this post seems to have gone off at a tangent Richard Rawson asked a question that still has not been answered yet, why does his Silk head which is shaped internally and should be of higher compression, hold so much more liquid than a standard type head? and regardless of actual compression ratios the Silk head will always be of lower compression than a standard head when fitted to his engine.I will also have to correct Richard Moss as my picture is of a similar Silk head made and marketed during the 1970s and probably from the same batch as Richard Rawson`s, and although it might look outwardly similar to a Moss high compression head this one has never seen the inside of Rogers workshop, the Silk head in the picture was bought secondhand by me in the 1980s from John Farrar he had probably bought it new from Silk engineering. I knew George Silk when he moved to Darley Abbey and at that time George had many irons in the fire, lots of different components needed to be made, new staff taken on etc. and George could not oversee everything that was going on, obviously these heads escaped the inspection department, I have just taken the time to measure the head in the photo and have come to the conclusion that the pattern that was used to cast the combustion chambers was incorrectly dimensioned and no amount of machining will ever improve matters they are just too big. These are dimensions that I have taken from parts that I have in my workshop, Diameter of piston crown taken from random pistons including Silk =2.837″ Approximate diameter of standard Brum and Shipley combustion chambers =2.875″ giving .019″ clearance around piston if chambers are centralised on bores, the shaped chambers on the Silk head are not only much deeper than a standard head but the diameter measures 3.010″ giving a clearance of .086″ all around the piston and I must emphasize that the chambers are as cast and have not been machined in any way. So I have to say to Richard Rawson you do indeed have a very low compression head and your Scott engined Silk would benefit from improvements in this department.
Richard Moss asks if I have ever made any close fitting combustion chambers ? the answer is a long story for another time but the Farrar Noakes racing Silk that he has pictured on his website has them installed. Alan Noakes. scottworkshop@hotmail.co.uk

[richard rawson]

Thanks alan thats the answer to the question i have been looking for so if I have my head re domed will it lift the compression
as I cannot use any other heads as the water inlet and outlet are different

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