Piston- Deck clearance

[Bart]

I am just doing a trial build of my 5L Rover engine.

Boost Performance crank, machined to 86.36mm stroke, 6 inch Chevy rods and Icon pistons, 1.25 inch compression height. 96mm top hatted block by V8 Dev

I have just been measuring my deck clearances, and got some weird results. Don't think they are 'passable'.

All below deck apart from No 2. In thousands of an inch. (The half thou's are from measuring to the nearest thou, rocking the piston and taking the average of two measurements)

1) 3 2) 0
3) 3.5 4) 2.5
5) 2.5 6) 7
7) 5 8 ) 7

It's the first time I have gone to the effort of trying to equalize my deck clearances. Looks to me like the right hand bank is not square, and a tapered skim of 0-7 thou would fix that? Not so sure about the left?

Would be very grateful for any advice about what to do in view of these measurements. If it wasn't for the 7 thou variation on the RHS, I think I'd just leave it, but that 7 though will equate to about 1.3cc chamber volume. (I haven't worked out what variation in compression ratio that equates to)

[GDCobra]

Do you have 100% certainty in your measurements? I'd be double checking and checking again before I removed any material.

Would it be possible to switch the piston & rods from cyl' 2 to 8 see if you still get the same values - This would check that the issue is not in the piston/rod build-up.

If you stripped the block would it be possible to bolt the deck face to a surface table and measure up to the main bearings?

[Bart]

It took some practise to get consistent measurements, but I am fairly confident in them now. I checked them several times.

I can't measure at the wrist pin due to valve cutouts and piston dishes, so I am measuring at the inner most edge of the piston, with a DTI and rocking to get two measurements, then averaging. I am zeroing the dti on the deck face adjacent to where I am measuring on the piston.

I don't plan to start any machining yet. I am going to mix and match rods tomorrow. I am limited to switching pistons around because the valve cutouts are not symmetrical, so I can switch 1 and 3, 5 and 7, 2 and 4 or 6 and 8.

Not confident I can close up that 7 thou difference just by mixing rods though? What would be an acceptable tolerance?

I don't have the equipment to bolt up to a surface plate or anything. Would have to go to the machine shop

[scudderfish]

Those numbers plotted don't give a straight line. Can you put a suitably long straight edge across the bank?

[GDCobra]

I can't measure at the wrist pin due to valve cutouts and piston dishes, so I am measuring at the inner most edge of the piston, with a DTI and rocking to get two measurements, then averaging. I am zeroing the dti on the deck face adjacent to where I am measuring on the piston.

I'm trying to visualise how you are performing your measurements but struggling, any chance of a picture?

I am limited to switching pistons around because the valve cutouts are not symmetrical, so I can switch 1 and 3, 5 and 7, 2 and 4 or 6 and 8.

At the moment I'm focussed only on the measrements to work out if the deck face is running at an angle to the crank axis rather than the pistons being in a 'workable' location. I don't understand why you can't put any piston in any bore to measure, maybe I'm missing something?
For instance could you not take a single rod/piston assembly, locate it in each bore in turn and take a measurement (ideally a few times)? That way you are taking the piston/rod out of the equation and effectively getting a comparative measurment between the deck face to the big end journal.
Personally I'd prefer to measure to the main bearing hence my previous comment just so that you have a full picture of the block geometry.

Not confident I can close up that 7 thou difference just by mixing rods though? What would be an acceptable tolerance?

I can't help on tolerance I'm afraid, I'd be more concerned (or at least as concerned) that the block was out of true and how it got that way than the difference this would have on compression not to mention any knock-on effects this may have although I can't think of any at the moment on an OHV set-up.

I don't have the equipment to bolt up to a surface plate or anything. Would have to go to the machine shop

Do you have a machine shop close by who could do this? it would be best to know the full picture with the block before sinking time and £££ into the engine build I'd have thought.

[Bart]

Hi,

Thanks for the replies.

I'll try and get a picture.

Yes, it occurred to me after my last reply that you meant to just switch the whole rod and piston assemblies to eliminate possibility of stacked tolerances in the rod and piston. Doesn't matter for now if they are not in their correct holes.

Yeah, I have machine shops near me. Also not far from Real Steel.

Plan for today is to check my deck flatness with a straight edge as Scudderfish suggested, then swap piston+rods around to see what happens. I'll report back.

Cheers

[Bart]

So I checked for deck flatness, and I am happy they are flat. They should be because V8 Dev skimmed them when they did the top hats.

I haven't attempted to switch any pistons over today, but I did devise a way to measure my deck heights, using a straight edge overhanging the deck, measuring the diameter of the crank nose and spigot bush boss to get the radius, then measuring the distance between the crank and the straight edge with an internal mic. Both decks 7 thou higher at the rear (obviously, from the points I measured from beyond the deck surface) Bang on 8.96 at the back, and 8.953 at the fronts. That looks kinda consistent with my clearance measurements on the RHS. Not so much with the left. I guess I am seeing a superposition of that taper in the deck, plus tolerances in rod/piston and possibly crank.

I can tell V8 Dev only took a very light clean up skim on the decks, which is what I asked for, so not their fault. I guess they indexed their machine on the old surfaces, which must have been on the cock from the factory.

I am going to strip out pistons+rods tomorrow, and put the same piston on each corner to measure the deck heights and eliminate errors in piston/rod length to see what that tells me.

[stevieturbo]

you do not measure by rocking the piston.

you measure at the centre of the piston where rock does not affect it.

And rock is a different thing altogether, affected by piston to wall clearance, heat, etc etc when running.

[Bart]

you do not measure by rocking the piston.

you measure at the centre of the piston where rock does not affect it.

And rock is a different thing altogether, affected by piston to wall clearance, heat, etc etc when running.

I can't, not straightforwardly, because my valve cutouts are over the centre line. (I am using wildcat heads). My pistons also have dishes. I have tried to use a depth gauge to measure the distance between the deck and bottom of the dish at the centre line, and subtracting the depth of the dish. I get similar results.

My depth gauge doesn't have a very big foot. I can try and put a straight edge straight across the piston to measure dead in the centre and substract the width of that.

However, on the RHS, I can feel that no 8 is deeper than 2, without measuring.