[jamit2006]

Hi All, I have hopefully a quick question. I'm contemplating doing the Squish Mod while I have the head off installing the Hitchcocks performance valves and CAM's.

Is there any additional consideration needed in taking the measurement for the squish mod with the valve and cam change?

I know StreetKleaver had an issue with the pushrods making contact with the cylinder head(?) after similar changes he made.

[allanfox]

Sorry, what's the squish mod? I thought the trick was to maintain the squish profile whilst raising the piston crown?

[SteveThackery]

Why would changing the squish zone be needed just because you've fitted new valves and cams?

Squish is a good thing. There are only two  reasons I can think of to reduce or get rid of it. Firstly, after fitting a new, high dome piston and finding the compression ratio is too high. Secondly, after skimming the head or barrel which will reduce the clearance between the piston crown and head.

Squish is one of the most important features of good combustion chamber design. Do some online research into it - it's a fascinating subject.

[allanfox]

Why would changing the squish zone be needed just because you've fitted new valves and cams?

Squish is a good thing. There are only two  reasons I can think of to reduce or get rid of it. Firstly, after fitting a new, high dome piston and finding the compression ratio is too high. Secondly, after skimming the head or barrel which will reduce the clearance between the piston crown and head.

Squish is one of the most important features of good combustion chamber design. Do some online research into it - it's a fascinating subject.

Exactly, plenty of posts on here (probably in iron barrel or AVL) about raising compression by skimming the bottom of the barrel and then skimming the piston crown edges the same amount to maintain the squish profile whilst raising the crown. I wouldn't just machine the squish off without really understanding the likely outcomes.

So on my AVL engine the piston is a flat top but I have an EFI piston (Enfield didn't make oversize for the AVL) fitted that is sunken in the middle (lower comp ration), to restore the comp ration i need to skim the barrel base say 1.5mm but then skim the piston top the same (only works on a sunken dome piston) so my squish profile remains the same but the bottom of the sunken part raise by 1.5mm and thus raises compression. If your piston was flat or domed you would only machine so far in from the edge to retina the piston crown albeit 1.5mm higher.

[SteveThackery]

To over-simplify: squish has two essential roles. Firstly, it generates beneficial turbulence in the combustion chamber which helps homogenise the fual-air mixture. Secondly it has a powerful quenching effect against detonation of the fuel-air charge in the parts of the combustion chamber furthest from the spark plug. 

Squish is one of the reasons we can run compression ratios well above 10:1 on the same low octane fuel that would cause pinking at 8.5:1 on the engines that were around in my youth.

[Taurim]

Read my subject about that. I had to modify the length of the pushrods after installing H's cams and shortening the barrel to raise compression ratio and having a correct TDC piston to head distance to have the squish effect.
I even carved pockets for the valves in the piston for added security.

From page 10 : https://forum.classicmotorworks.com/index.php?topic=29864.135

[mcplorer]

Read my subject about that. I had to modify the length of the pushrods after installing H's cams and shortening the barrel to raise compression ratio and having a correct TDC piston to head distance to have the squish effect.
I even carved pockets for the valves in the piston for added security.

From page 10 : https://forum.classicmotorworks.com/index.php?topic=29864.135

How did you remove the bottom tip of the push rod? Any tips/inputs would be very helpful as I am going to shorten the barrel height which might require to shorten the push rod height as well. Thanks

[Taurim]

I heated the tube of the pushrod to remove the tip with a plier.
For the reassembly I heated the tubes and put the tips in the fridge.

[mcplorer]

I heated the tube of the pushrod to remove the tip with a plier.
For the reassembly I heated the tubes and put the tips in the fridge.

Thank you, it helps. Do you know how much of slack is there in the hydraulic tappets means how much mm difference in the block height the tappets can take up without needing to shorten the push rods?

[Taurim]

The operating range and the way to measure it is in the Haynes manual if I remember correctly.
I will try to find that and scan it.

[mcplorer]

The operating range and the way to measure it is in the Haynes manual if I remember correctly.
I will try to find that and scan it.

That would be really helpful. Thanks

[Taurim]

In fact it is not in the manual.

The procedure is the same to measure intake or exhaust :
- rotate the crankshaft until the pushrod is at the lowest position possible.
- put the rocker assembly in place, rocker in contact with the valve and the pushrod, without tightening the bolts
- measure the distance between the head and the bottom of the rocker block with a feeler gauge (where the red line is on the photo)



- the distance must be between 1 and 2.5mm. Perfect distance is 1.75mm
(I asked HM what is the correct range)

If you shortened the barrel the distance is probably more than  2.5mm. You will have to shorten the pushrod.
If the distance is 3mm you should remove aproximately (3-1.75)*2=2.5mm to the pushrod length.

I hope my explanation is clear enough 

[mcplorer]

In fact it is not in the manual.

The procedure is the same to measure intake or exhaust :
- rotate the crankshaft until the pushrod is at the lowest position possible.
- put the rocker assembly in place, rocker in contact with the valve and the pushrod, without tightening the bolts
- measure the distance between the head and the bottom of the rocker block with a feeler gauge (where the red line is on the photo)



- the distance must be between 1 and 2.5mm. Perfect distance is 1.75mm
(I asked HM what is the correct range)

If you shortened the barrel the distance is probably more than  2.5mm. You will have to shorten the pushrod.
If the distance is 3mm you should remove aproximately (3-1.75)*2=2.5mm to the pushrod length.

I hope my explanation is clear enough 

Wow, this is super helpful. Thanks a ton 🙏

[Taurim]

By the way you probably have 2 longer pushrods provided with HM's performance cams which are tuned to match the reduced base circle of the cams.

If you shorten the barrel you will have 2 stock pushrod which are still too short and 2 HM's pushrods which are now too long 

[mcplorer]

By the way you probably have 2 longer pushrods provided with HM's performance cams which are tuned to match the reduced base circle of the cams.

If you shorten the barrel you will have 2 stock pushrod which are still too short and 2 HM's pushrods which are now too long 

Thanks Taurim but right now I have stock Cams and stock push rods but soon I will be shortening the cylinder block by 2mm. As per your calculations I need to shorten the push rods by 2-1.75 = 0.25mm right?