98 mm bore

[RSBea]

Can anyone explain why so many engines use a 98mm bore. As far backs the 1920s this bore appeared in a Vauxhall 4.2 litre 4 cylinder petrol engine, (designed by Laurence Pomeroy). The same dimensions are used in many current 4 cylinder diesel engines fitted to Fiat, Ford, Fendt, International and Perkins engines

 

[MikeHalloran]

Pistons are more difficult to make than a cursory inspection of their geometry would suggest. Some companies specialize in making them. If you're an engine supplier, and you compare the cost of making pistons yourself against buying them outside, someone like like, say, Mahle, can make a very competitive offer. If they happen to already be tooled up to provide 98mm pistons to someone else, their bid for your pistons will be even more competitive.

I.e., availability of existing tooling may tend to cluster numbers that might otherwise be random.





Mike Halloran
Pembroke Pines, FL, USA

 

[pickler]

i know for subaru 99mm bore was for more compact design and overall lower center of gravity.

 

[gruntguru]

I guess if you are tooled up for producing 98mm pistons, you can make them in 1mm oversize too.

je suis charlie

 

[Tmoose]

http://en.wikipedia.org/wiki/Golden_ratio

 

[RSBea]

The question I was trying to ask is why the combination of 98mm bore, four cylinders and 4.2 litre displacement occurs so often. This combination means the engines all share the same stroke, Mike Halloran's comment about being tooled up for a particular design might also apply to crankshaft suppliers.

 

[tbuelna]

I don't think there are many 4.2L I4 gasoline auto engines in production. However, it is common with diesel engines to use common bore sizes for a range of 4 or 6 six cylinder inline engines. Much easier and cheaper to change displacement by swapping out crank and rods than it is to tool new heads and blocks.

In fact, if you look at the range of engines produced by Detroit Diesel over the years, the model number (series 50, series 92, series 60, etc.) referred to the displacement of each cylinder in cubic inches. And the engines of each series all used a common bore size. With DI diesels, the combustion chamber is in the piston crown. And the size and shape of the chamber, and its relation with the injector nozzle, is critical.