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Pistons


haggis
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Hi all, can anyone tell me if you can fit flat top Pistons in a block that previously had domed ones in? The only difference in the blocks seems to be the little recess around the bores. I have both heads from either piston type.,

Thanks hag

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Kevin's answer may need to be amplified. Domed pistons were fitted to 2L engines so that they could have exactly the same heads as 2.5L engines. The dome takes up the extra volume in the chamber but makes no difference to the stroke volume. So flat tops in a 2.5 will make for a very low CR, while the other way around it will be sky high. Ensure you know the stroke of the engine you are building.

 

The recess around the bores is also a later production change to improve sealing. "Recessed" gaskets have a tab at the back to ensure you use the correct one.

 

John

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Hi John, thanks for that, yes I read up on the domed piston, from what I understand the CR is also increased with the domed Pistons on the 2.0l and would be better for a modified engine? I have also read there are some 'burn' issues and that flats are best for modified engines and generally better although their CR is less. I would also assume that some head skimming would increase the CR on the flat version and this may be the best all round solution.

So basically I'm thinking:

Flat Pistons in domed piston block.(better block)

Recessed head gasket.

Shorter push rods.

Shorter cylinder head for flat tops skimmed ( by expert) to increase CR, as this suits the good parts I have out of two engines,

Does this sound like nonsense?

 

Thanks

Jason

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Has anyone ever done this? swapped flat top pistons & head on a domed piston recessed block? Im a bit nervous as my machine shop need me to make a decision and its going to cost lots, don't want to get wrong, arrrgh!.  I looked at the heads at it would appear the only difference other than height is there are water holes blanked off on the flat top version, does anyone have any idea if this matters, or I can drill them out maybe?

 

Any advice greatly welcomed.

 

Thanks

HAg

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James, Haq,

To say again, the domed pistons were fitted to 2L engines that had the SAME head as a 2.5L.

Bigger chambers as taller so fIt flat tops and the CR will drop to about 7.5. To use flats on a 2l that had original domes you must skim the head, a lot!

The volume of the dome is about 8mls. When the chamber volume is about 40mls that's 20% !!

 

As long as you understand this and do the necessary to the head there is every reason to choose flats. Get it wrong and you will have a very 'flat' engine. One that will run on paraffin but not very exciting.

 

John

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Hi john, thanks again, so if I have a shorter head, I.e the one that came with the flat top piston engine, then I can just fit this one instead of the taller head for the domed piston engine.maybe I should explain a little more.

I have two complete engines including heads one domed and one flat top Pistons, the block on the flat top piston engine is not good, but has brand new hepolite Pistons and a really good gas flowed head, the domed block is good but needs new Pistons and head, hence my dilemma. Good quality engine parts are in scarce supply and my machine shop have suggested using the new flat top Pistons if possible on the block that had domed Pistons in, hence my compatability questions between the two engines. The only difference I can see is the additional water way holes in the domed piston head.

Appreciate your help

Hag

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Warphi,

 

Yes, you are right, the chamber in the head is the combustion chambet. Score one.

 

Have you ever seen the inside of a Triumph combustion chamber? 

The walls are vertical - the shape is unchanged by skimming (reasonable skimming)

 

And do you know anything about combustion and CR?

The whole point of increasing the CR is that combustion at higher pressure is more efficient - if you can live with the NOx.

 

Do you know anything about decking the block?

The purpose is to equalise CR across the block - it will change it only fractionally

 

And as for "having the flat pistons domed" - fantasy, by adding  or taking away metal.

 

John

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Thanks JohnD, Seriously warphi1970? not sure that's gonna help me get any answers, but at least you hopefully managed to learn a thing or two.

 

Hag

Ps No one has said I'm stupid so I think my plan is a goer, just need to know about the different water holes in the head, to drill or not to drill is the question?

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Hag,

post some photos to show the differences.

If the holes are on the top of the head, outside the rocker cover then they are to accommodate different car heater arrangements.

With 4 cly heads they changed in the early 70s to remove the hole, when they changed the heater hose arrangement and put a Tee piece on the manifold. Transition heads were fitted with a blanking plug.

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Hi kevin, the holes are on the mating surfaces where the head gasket goes, I'll try and post some pics, tried before without success but have another go. The domed piston head has more water way holes than the flat piston head, although there are erosion marks on this head in exactly the same places. I'll try some pics hopefully that will help explain.

 

Thanks

Hag

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Hi Kevin, I've managed to work out how to download a picture but only one at a time!

 

This is the shorter head for flat top pistons. On the bottom row the two outer holes show signs of erosion from the block but no holes, the taller head block (domed pistons) has holes.

Can they or do they need to be drilled out? originally a GT6 MkIII engine KE (domed) in a MK1 body, they obviously have some potential overheating issues just wondered if this would make any difference. The head and Flat to pistons I want to install are from a MKii Vitesse HC (flat). Its just down to the best parts I have available. Thanks

Hag.

sml_gallery_341_88_1556495.jpg

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Another pic of the Flat top piston cylinder head water hole variation, in this one, the kind of large tear shape hole to left of the inlet valve is just a simple hole at the top (in the biggest bit) on the domed piston head, I honestly can't logically see there is any major difference as both engine blocks have the same holes in them, but I'm sure someone out there will have done something like this and can hopefully advise if there are any potential issues.

Thanks

Hag

 

sml_gallery_341_88_472939.jpg

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I have a late mk3 which should have domed pistons. The block has been re-bored twice (don't ask! :( ) The machine shop provided me with pistons both times and I'm 90% sure they're flat top. At the time I knew no better and didn't know they were wrong. This would probably account for my temperature gauge never getting to half way? Is it any slower? Not that I notice, these days.

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HI Doug. It defies my simple theory, if you changed the pistons to flat tops and didn't change the head the Compression Ratio would have too low and I would imagine it would have run like a snail. but you've had no problems, sounds like they either swapped heads or they are actually domed pistons and that 10% error has crept in. I've had a vitesse for 28years i've had the head off probably 10 times and I can't remember what pistons it has installed.

 

Thanks though, good to hear from you.

 

Hag

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Haggis, yes! I also have a hard time remembering. :unsure:  Although I do know the head is original. My mother (93) has just had an endoscopy test on her throat. An endoscope is what I need to check this out.

 

http://www.ebay.co.uk/itm/2M-USB-Waterproof-Endoscope-Borescope-Snake-Inspection-Video-Camera-UK-/161622197601?hash=item25a16f0561:g:RqoAAOSwstxU9T4O

 

Another must have tool.

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