Distance Piston - Head - Fiat OHC

[Acki]

Hello Guy,

I need your help.
Actually I'm building a 1602cc OHC engine with two DCNF's carb's.
I think I show some pictures from the engine:
Head:

It's a 1.5 SPI Uno head with bigger inlet valves (39,5mm from 1.6 Tipo).

The pistons:

Lancia 1.5 pistons - compression height of 35mm and 0.6m oversize

The block:

It's a 1.5 SPI Uno block.


Crankshaft is a 1.4/1.6. To get 1602cc with the bore of 87mm.

The rods:

Uno Turbo Mk1 without CAT - shall be forged...

I have a piston overstand of 1.5mm.
I get buy three gaskets. In a thickness of 1.74mm, 1.8mm and 1.9mm.



My problem, are 0.2mm from piston to head enough?

The valve pockets have a volume of 5ml (I have to make them bigger for the big inlet valve - the engine gets a 316‚° camshaft).
So I have a CR of 11.48:1 with a distance of 0.2mm.

What are your experience with this? Shall I better make a distance of 0.5mm or bigger?
I think about this small distance because this makes a very small combustion chamber and so I have some HP's more than with the same CR and a higher distance. But I don't want to bump the piston again the head.

Thanks for helping.
I hope the style of the thread is ok :)

[Guy Croft]

Yes, 2/10 (0.2mm) is way too close and the pistons will defintely hit the head. Don't forget they rock from side to side at tdc so make sure you measure the highest point of the crown above the block by pushing the top of the piston over to one side.

Really, in my view 0.5mm is the absolute minimum proximity of piston to head (and gasket too) and me, I would prefer 1mm. Nothing worse that running it too tight and hearing that awful clattering noise - wondering if it's the head being clobbered by the piston, or valve to piston - or a bearing!

Don't forget to do a dry-build - you must put plasticine in all the VRs and check it with the belt on. You must have 100thou" vertical measured in plane of valve axis and 60 thou radial. Those are MY recommended minumums! But - use the wrong cam/valve spring combo and you'll get chronic valve float at high rpm and it won't matter what they are.

GC

[Acki]

Thanks for answering.
The valve springs are from the firm of the camshaft (Dbilas).
I think this should work without problems.

A dry build I will make to get the correct size of the valve pockets. 0.01mm and 0.006mm are enough? I thought about 0.5-1mm distance between valve and piston.

Do you have maybe a 2mm gasket? Then I have the 0.5mm distance and a CR of 11:1.
Else I will cut the pistons 0.3mm. I can use a lathe for this.

But I didn't want to come too low with the CR. I think I will lose power then... or?!

[Acki]

Hello Guy,

I have thought long time about the distance and the C/R.

I would try a distance of 0.84mm from piston to head.
With a overstand of 1.5mm of the piston (maximum is 1.7mm, minimum 1.2mm) I need a gasket of 2.35mm.
With this gasket I have a C/R of 10.53:1.

What do you thing? Is this enough to get a good powerful engine?
Or should I go high to 1mm distance? Then I have a C/R of 10.2:1.

What do you thing about the piston skirt? Can I make it shorter? Or does the piston overturn more? I thought about lowering the friction.

Then I have a other very stupid question. I have installed the crankshaft with the bearings and 1 piston.
When I try to rotate the engine I can't do this only with my hands. When I use a hand gear I can do it easy.
Is this ok? Or is something wrong with my bearing blocks?
I have installed them is this order:
.
..
...
....
no mark

Is this order correct? The numbers on the blocks are in the same directing as the number on the sealing surface.
I have looked in my Fiat books and I find nowhere some information about this.

How do you clean the block in the water channels? Do you clean them there?
I would remove the rust there but I don't know how or isn't it necessary?

Thanks for helping. I will make new pictures when I have made the bigger valves pockets (a engine overhaul firm will make me them) and I have painted the block. :)

[Guy Croft]

Tomasz, hi

I would try a distance of 0.84mm from piston to head.
With a overstand of 1.5mm of the piston (maximum is 1.7mm, minimum 1.2mm) I need a gasket of 2.35mm.
With this gasket I have a C/R of 10.53:1. What do you think? Is this enough to get a good powerful engine?
That's fine

What do you thing about the piston skirt? Can I make it shorter? Or does the piston overturn more? I thought about lowering the friction.
Never modify pistons skirts on cast pistons.

I have installed the crankshaft with the bearings and 1 piston. When I try to rotate the engine I can't do this only with my hands. When I use a hand gear I can do it easy. Is this ok? Or is something wrong with my bearing blocks?
I have installed them is this order:
.
..
...
....
no mark
Is this order correct? The numbers on the blocks are in the same directing as the number on the sealing surface.
I have looked in my Fiat books and I find nowhere some information about this.

Naturally the turning torque with a piston and rings fitted is higher than just the crank. But if it's way toght with just one in you're in trouble. The very first thing you should do is remove that rod and check the turning torque of the crank alone, then take off each main cap at a time and see if any is binding. If the turning torque drops, that one (there may be others) is the culprit. Then strip the crank out completely and check for any rubbing (severe polishing) at all on the bearings. I suppose it is possible to instal the caps in the right order but the wrong way round. This is important. Check this out and report back please.

How do you clean the block in the water channels? Do you clean them there? I would remove the rust there but I don't know how or isn't it necessary?
If you are concerned about corrosion deposits a solution of phosphoric acid will do it, but don't get it on the aux d/a brgs or it will eat them away.

GC

[Acki]

Thanks for your answer. I will make some pictures and then report you.
I have remove the rust as far as I could. Maybe I think to much about it.

[Acki]

Hello Guy,

I hope it's no problem that I write more about the engine in this topic. Or should I open a new topic?

Today I checked the valve pockets.
I have opend the valve 4mm (lift in TDC) and prepared the piston.
Then I put the head on the block with the old standard gasket and made the crankshaft a round to get a valve copy on the plasticine.
The pockets in the pistons are big enough (the plasticine wasn't pushed down till I saw the piston - I hope you understand what I mean). I only cutted the edges the polished the piston.
What do you think about this?

[SteveNZ]

Acki, you have 1.5L pistons which have a compression height to suit the shorter stroke. Can you get some 1.6L pistons? All problems will be solved.

[Acki]

The problem won't be solved because it's a 316° race camshaft with 4mm lift in TDC.
And I need the high compression height to come to a good C/R.
This pistons have the biggest valve pockets. All other pistons have smaller pockets. As far as I know...

But in the next step with this engine, we will install forged pistons.
But first we will try this pistons.

[Testament]

You could machine the pistons for clearance where required as long as they still have enough crown thickness afterwards

[Acki]

The piston crown is 5-6mm strong. Better I couldn't messaure it.
I only had to cut the edges a little bit.