Connecting rods and pistons (15/24)

Approved

As I already wrote in the topic about block preparation, the probability that you don’t have to sharpen the block is almost zero. Those. in any case, we will need pistons of a larger diameter (unless, of course, the block is sleeved). And this is where all the fun begins!

Carefully! The selection of connecting rods and pistons can sometimes break your brain)) In this topic I will try to describe the course of my thoughts in the process of searching for a configuration that would suit me. For a long time I did not dare to start writing this article, but I hope I will master it.

So what do we have?

Of the known values, we already have a crankshaft with its rise of 81mm. We also have a cylinder block, which from the factory had a height of 206.2 mm – now minus what we removed from it in the process of grinding the plane (0.15 mm)

The connecting rods I decided to put 135mm from m20b25 instead of 130mm, which were in the deceased b27 ETA. By slightly increasing the length of the connecting rod, I hope to some extent reduce the load on the cylinder walls, thereby increasing the engine life. While I may lose some torque and power (compared to shorter cranks), it’s not that big of a deal. There is no task to remove the maximum amount of power, you need a reliable, high-quality assembled engine for every day with slightly more interesting characteristics than the standard m20b25.

Of course, you can use lighter connecting rods, from m54b30 for example, but they already were from m20b25, and buying others is an unnecessary increase in the building budget, which is useless in this config.

It remains to pick up the pistons.

To begin with, for clarity, a picture from our German colleagues

On the left is the standard m20b25. In the middle, the piston was left from m20b25, installing the crankshaft and connecting rod from ETA. We see the piston shortfall to the edge of the block 1.6mm, i.e. 2mm would need to be removed from the block to get the same compression ratio as the original m20b25. But having removed 2 mm from the block, we will have to install a split gear in order to be able to set the engine phases. In my config, this is not an option, because. again inflates the budget.

After reading, counting and thinking, I decided on the compression ratio that I need to get – 10: 1. For calculations, you can use online calculators – such as rally-sport.rf/calculation_engine.html. Having scored all the available values ​​\u200b\u200bthere, we calculate with what compression height we need a piston.

Here, just in case, some numbers may come in handy:

– the thickness of the standard cylinder head gasket is 1.75 mm, the repair thickened 2.05 mm, the diameter of the holes for the cylinders is 85 mm

– the volume of the combustion chamber is 200 and 731 cylinder heads – 38 cc

– the volume of the combustion chamber is 885 – 42 (or 45) cc – I have Two digits are written, which one I don’t remember anymore. Better spill, measure just in case.

Then we take catalogs of pistons and look for the parameters that suit us. I settled on pistons from m42b18 with a compression height of 31.7mm. Of the benefits – they are lighter than the m20b25 and the lower oil scraper in them is type-setting.

I ordered Autowelt PN-50061 repair size 84.48 (+0.50). Yes, Mahle and Kolbenschmidt are cooler, but damn how much they cost…)) These, by the way, turned out to be very good in terms of workmanship.

Connecting rods and pistons
Connecting rods and pistonsParts pictured: PN50061
Connecting rods and pistons

To get the required CV of 10:1, according to my calculations, it is necessary that the pistons protrude 0.2 mm above the surface of the block. Accordingly, we remove the excess from them, since the thickness of the bottom allows – we mill by 1 mm (like that much, I don’t remember exactly).

Now the important point – you need to weigh the pistons.

I weighed it – the mass difference turned out to be small.

1 – 351.7

2 – 351.7

3 – 351.7

4 – 350.3

5 – 352.0

6 –

352.1 We deduce everything to zero, i.e. by 350.3

Connecting rods and pistons
Connecting rods and pistons

The fingers were all the same weight.

I didn’t cut the countersinks, because I don’t see much point in them on conventional pistons. In which case they will not save you from bending the valves – it was tested on my m20b27)) The only really working option is this:

Connecting rods and pistons

Pistons from Metric Mechanic. Who can afford them – put of course))

Now the rods. mine:

Connecting rods and pistons

We take to the turner, check the geometry. We check the lower heads for ovality and in 99% we correct.

Then I decided to blow my mind and hang the connecting rods “as it should” – weighing the upper and lower heads separately. Naturally, I didn’t have a ready-made stand – I tried to depict a kind of semblance from what was at hand from photographs and videos. The second or third version turned out to be more or less efficient.

Connecting rods and pistons
Connecting rods and pistons

В итоге так и не смог добиться от стенда внятного результата. Нет четкой повторяемости результата, опираясь на которую можно было бы что-то пилить. Что виновато — мои кривые руки или данная реализация стенда — непонятно. Потратил кучу времени и сил без толку. Хотя отрицательный опыт тоже опыт.

Взвесил все шатуны — получил разницу в весе в 2,9 г., что укладывается в допуски (4 грамма для шатунов).

Connecting rods and pistons

Решил все-таки оставить идею вывесить шатуны, т.к. очень велик шанс сделать хуже, чем поправить с такой работой стенда.

Чтобы минимизировать влияние разницы в весе провел небольшие расчеты.

Вес шатунов:

631,5

632,0

630,4

632,9

632,5

630,0

Сложил вес всех шатунов и разделил на 6 — получил средний вес шатуна — 631,55 г.

Умножил эту цифру на 2 — поршни у нас парами ходят — получил 1263,1 г. Это усредненный вес пары шатунов к которому нужно стремиться.

Потом подобрал пары по весу:

630,0 и 632,9 — 1262,9 в сумме

632,0 и 631,5 — 1263,5 в сумме

630,4 и 632,5 — 1262,9 в сумме

И в итоге раскидал эти пары по цилиндрам:

1 — 632,0

2 — 630,0

3 – 632.5

4 – 630.4

5 – 632.9

6 – 631.5 I

put a heavier pair on the edge – 1 and 6 cylinders, it seemed to me that it would be right.

Something like that. Maybe I’m a bad mathematician

Connecting rods and pistons

Connecting rods and pistons are ready.

Connecting rods and pistonsParts in the photo: 2000018

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