Mini Mad
Posted 02 September 2013 - 06:48 PM
This is all hypothetical at the moment, just mulling things over.
I have some +0.030 998 plat top pistons and would like to fit them to my 850 block, this would the equivlant of a +0.090 overbore for the 850
Why I hear you ask, well why not. Would make an interesting high reving engine
Anyway, has anyone done this?
On Sabbatical
Posted 02 September 2013 - 07:08 PM
Do you mean to use the 850 crank? It would have very low compression, the pistons would be somewhere around 4mm further down the bore than intended at TDC.
One Carb Or Two?
Posted 02 September 2013 - 07:12 PM
i was always under the illusion that the 850 and 1100 were quite a lazy engine as in they have a long crank stroke so carnt really be revved hard 
Mini Mad
Posted 03 September 2013 - 05:30 AM
Do you mean to use the 850 crank? It would have very low compression, the pistons would be somewhere around 4mm further down the bore than intended at TDC.
No, pistons, not sure I mentioned crank ;)
i was always under the illusion that the 850 and 1100 were quite a lazy engine as in they have a long crank stroke so carnt really be revved hard
After the 970s the 850 has the shortest stroke of the std engines, the 1100 the longest
Anyhow, can we get back to my question, can I fit 998 pistons into an 850 block?
Camshaft & Stage Two Head
Posted 03 September 2013 - 07:32 AM
Up Into Fourth
Posted 03 September 2013 - 07:43 AM
Well its not a simply put no really. If you skimmed the block and the head to re-raise compression then its plausible. For all the cost and effort you might as well use a 998 block though.
Speeding Along Now
Posted 03 September 2013 - 09:37 AM
Bear in mind also that an 850 does not have cam bearings, whereas the 998 does.
Mini Mad
Posted 03 September 2013 - 09:53 AM
Yes, I had heard that
This may become a project too far
On Sabbatical
Posted 03 September 2013 - 04:55 PM
No, pistons, not sure I mentioned crank ;)
No, you said you were using 998 pistons. I asked about the 850 crank. If you were not using the 850 crank what would be the point? It would be a normal 998 engine. The block is the same casting it's just machined differently, all you would have done would be to go to a lot of effort to end up with the same thing.
Well its not a simply put no really. If you skimmed the block and the head to re-raise compression then its plausible. For all the cost and effort you might as well use a 998 block though.
I really don't think there is enough meat in the block deck to skim that far. As you say you could skim a bit off each, but by the time you have worked the head to open up the chamber, or gone to a Cooper head, you will have to skim a massive amount off to get it back down to the right CR and again I don't think there will be enough there. If you don't open up the chambers or go to another head then you'll be stuck with the original 998 type chamber design which is poor.
After the 970s the 850 has the shortest stroke of the std engines, the 1100 the longest
Short / long stroke aren't really objective terms, they must be considered in terms of the relationship between stroke and bore. The 850 has a shorter stroke than other engines yes but it also has tiny pistons and so is still a long stroke really.
Up Into Fourth
Posted 20 December 2013 - 08:31 PM
did they not fit 998 pistons in the mini X engines, you had to use stock parts back in the 60s, think you have to skim 1/8 of an Inch off the block. should rev to 9k if you sort the crank out as well
Uncle Cooperman, Voted Mr TMF 2011
Posted 20 December 2013 - 08:44 PM
The problem is also the crank as the 850 crank is weak compared to the 998 crank and to get an 850 to rev you need either the old 850 competition crank (and I've not seen one of those for 40 years!) or a new 850 crank made from a steel billet.
Can't see the point really as it would cost a small fortune.
Up Into Fourth
Posted 20 December 2013 - 09:10 PM
Its is doable, but simply not worth the expense and the weaker block that would result from such mods. Put simply reliable engine are ones that are in maximum material condition. This means minimal block decking and minimal bore size increase. This means using a 998 block as a starting point. In terms of reliability and cost wise its a no-brainer.
AC
Up Into Fourth
Posted 21 December 2013 - 06:27 AM
The problem is also the crank as the 850 crank is weak compared to the 998 crank and to get an 850 to rev you need either the old 850 competition crank (and I've not seen one of those for 40 years!) or a new 850 crank made from a steel billet.
Can't see the point really as it would cost a small fortune.
it just so happens I have such a crank. 
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