18 replies to this topic

[JonnyAlpha]

So I am in the middle of building a 1310 Engine (See project link in my sig below).

I am now looking at Cylinder Head options, but all the modders are maxed out and process are out of my range, so I am looking at what I can do in the mean time to make the best of what I have.

I am using P21253 Pistons which have an 8cc dish.

I have an Early MG Metro Head with the 35.2 and 29.2 Valves.

 

To achieve the target CR of 9.75:1 I had anticipated having the chambers enlarged slight by whomever was doing the head work, but as it stands this is not going to be the case and therefore I need to look at what I can do.

 

First task was to re-measure the combustion chambers (Not yet got a decent burette) and re-measure the Deck Height, which has not been done since the block came back from the machinist (s long while ago now).

 

Combustion Chamber

Anyway using a 20ml syringe, I have ROUGHLY calculated a combustion chamber 0f between 21 and 22cc (so it looks like its standard then at 21.4cc - this will be confirmed when I get a burette.

 

Deck Height

Now to the deck height. 

I brought each pair of pistons up to TDC using s DTI gauge and using a depth micrometer I found quite a variation in Deck Height.

When I sent it to the machine shop we were aiming for 0.005" Deck Height and he did comment when I picked it up that before he started the he noticed that the Deck was not level.

 

I have measured in four places around each piston and the results are as follows:

 

Piston No 1 - 7.5 / 6 / 6 / 5

Piston No 2 - 8 / 7 / 4 / 1 (Lowest point is on outer edge of cylinder, between piston 2 and 1)

Piston No 3 - 7 / 6.5 / 6 / 1 (Lowest point is on outer edge of cylinder between piston 3 and 4)

Piston No 4 - 6 / 6 / 3 / 2.5 (Lowest point is side of cylinder towards back side of engine)

 

Is this normal?

 

How do I use the above figures to calculate Compression Ratio? Do I take the average of each cylinder? 

 

[JonnyAlpha]

UPDATE

 

Looking back though one of my old posts here and @Coopermans CR post here

I have re-calculated everything (but may have made a mistake) and re-written the findings below.

For Deck Height, I took the average of the Deck Heights for each cylinder and then took the average of the 4 averages!!

 

Based on an early MG Metro (1989 Mk2) 1275 +40 Thou Rebore (1310) (2.821 Inches) and standard 1275 A+ Crank Stroke of 81.3mm

 

A - Piston to Top Deck Height (0.00525 Inches) =  0.5125cc

B - Piston Dish (P21253) = 8cc

C - Cylinder Head Chamber (To be confirmed) = 21.4cc

D - BK450 Head Gasket Compressed Volume = 4cc

E - P21253 Ring Landing = 0.75cc 

 

If the above figures are correct I have calculated the following:

 

Swept Volume (SV) = 327.5cc / 4 = 81.875cc (Swept Volume of one cylinder)

 

A+B+C+D+E = Unswept Volume (UV) 34.66

 

CR = (SV+UV) / UV 

 

CR = (327.5cc + 34.66cc) / 34.66cc

 

CR = 10.44:1

 

So if this is correct I need to increase the Combustion Chamber, or have the pistons modified, to reduce the CR

Edited by JonnyAlpha, 06 September 2020 - 03:11 PM.

[nicklouse]

Deck should be level.

 

when he machined it he should have made it level.

 

this is presuming that the pistons have the same pin to top distance and the rods all have the same center to center distance and are not bent and the crank journals are the same distance from the center line.

 

remember the pistons will rock on the pins so measuring front and back is not good.  Side to side should be the same.

[Turbo Phil]

You need to take the measurement of piston to block face across the gudgeon pin. If you do it the other way the piston can rock in the bore and give false readings.
As you already know your engine is 1310cc working out the compression ratio from the other figures you have is simple, this video shows you how.

 

Phil. 

Edited by Turbo Phil, 06 September 2020 - 04:12 PM.

[JonnyAlpha]

So across the gudgeon pin line I have:

 

Piston No 1 - 6 & 5

Piston No 2 - 8 & 1

Piston No 3 - 6 & 1

Piston No 4 - 3 & 6

 

So average are:

 

Piston 1 - 5.5

Piston 2 - 4.5

Piston 3 - 3.5

Piston 4 - 4.5

 

Mean average = 4.5

 

So by inserting this into my earlier calculations (following Turbo Phil's Guide):

 

1275 0.45 thou = 0.46 Piston to Block Clearance

 

A - Piston to Deck Height = 0.46cc

 

B - Piston Dish = 8cc (Piston Dish)

 

C - Chamber = 21.4cc

 

D - Gasket = 3.8cc

 

E = Ring Landing = 0.5cc

 

A+B+C+D+E = 34.16

 

 V + C / C = CR

 

327.5 + 34.16 = 361.66 / 34.16 = 10.59:1

 

So I have some research to work out the best way to get this down to 9.75:1

 

Here is a photo of one of the chambers:

 

 

And the chamber with the valves in:

 

 

And a view down the Inlet Ports:

 

 

And an exhaust port:

 

Edited by JonnyAlpha, 06 September 2020 - 08:06 PM.

[Compdoc]

I find the difference in deck height of the middle two cylinders, (5-7 thou), quite alarming, or is this generally accepted as within spec?

[Tones61]

Crank re grind problem I'd say,the deck will be flat if surfaced

[JonnyAlpha]

Crank re grind problem I'd say,the deck will be flat if surfaced

So are you say the crank may have been ground incorrectly and it’s now Knackered?

[sonscar]

Don't panic,the piston might not be totally machined correctly,the conrod could be bent,the bores may not be perpendicular to the deck etc etc etc.Whereas you want it to be the best it can be it is not a Formula 1 engine.(prepared to be shot down in flames by real engine builders)I would turn it over a few times and if worried pull the pistons and look for wear marks.Steve..

[PoolGuy]

Swap the pistons, see how the measurements vary, you’ll find out if it’s crank, rod, block or piston related? This is what dry build time is for imo, it can avoid unnecessary machining.

[Cooperman]

It makes very little difference, especially for a road car. From memory, every 10 thou down the bore is approximately 1 cc of volume.

The difference is almost certainly the result of centreless crank re-grinding plus the build-up of tolerances in rod length and piston pin height. Modern pistons are very accurate as a result of CNC machining techniques, but rod lengths are subject to BMC/BL/ARG/Rover tolerances, so a couple of thou can be expected there in worst case as a combined difference.

[Minigman]

24.66cc head volume required to get your 9.75:1 Bill. Can you take out the pistons and get 0.030” shaved off? That’s would save the head mods.

Edited by Minigman, 08 September 2020 - 12:09 PM.

[gazza82]

24.66cc head volume required to get your 9.75:1 Bill. Can you take out the pistons and get 0.030” shaved off? That’s would save the head mods.

 

I've got 24.166cc ???

 

 CompRatio.jpg   12.29K   1 downloads

Edited by gazza82, 08 September 2020 - 01:54 PM.

[JonnyAlpha]

Swap the pistons, see how the measurements vary, you’ll find out if it’s crank, rod, block or piston related? This is what dry build time is for imo, it can avoid unnecessary machining.

 

This is not the dry build this is the actual build. I did the dry build to work out what needed to be skimmed off of the heads. Pistons are now fitted to the rods!!

[JonnyAlpha]

Looking back through my project build here , during the trial build I had readings of 17.5 thou at cylinder 1 to 15.4 thou at cylinder 4. 

 

Between cylinder 4 and 3 I got the highest reading of 11 thou.

 

The machinist was instructed to take 0.008" off of the block, which he did. The only thing that has happened in the mean time is that I had the Conrods check, at which time they cleaned them up and reamed out the big and small ends to make sure they were circular, this may account for the differences?