14 replies to this topic

[vivalamini]

Stripping an engine I have and I just need confirmation of the size of the pistons... 73.5mm?? Or 74mm??

Thanks

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[wile e coyote]

Hmm - hope it's a strip for inspection and all's good - as another rebore without sleeving back  not likely.... bad news possibly... and I'd reccommend measuring the bore rather than the piston is that both 73.5mm and 74 are possible which on a standard 1275 stroke length of 81.28mm would give....

 

73.5mm bore = 1379cc (1380)

74mm bore  = 1398cc (1400)

 

.... although I've never come across the latter it's possible it seems from a table I picked up from somewhere! Bearing in mind that the piston will be slightly smaller than the bore then you may have just about the biggest A series engine possible without stroking it... that may have been done too - get measuring...

[mini13]

you need to measure about 10mm up from the bottom of the piston at 90 deg to the pin as pistons have a fairly generous taper so hen they are at running temp they expand top fit the bore, they expand a suprising amount! judging by your measurment they are 74mm

[minimans]

Looks like it will be 74mm to me, I have a new set at the shop I'll measure them up later. I'm not at the shop till tonight, If their 74mm they may be Triumph TR6 pistons which was common back in the day. As above I hope the bores are good as at 74mm you have no room for a bigger piston and even putting a liner in there is unlikely to work.

[vivalamini]

Thanks for the replies...

if the block they come out of is no good, I have a spare block that is 73.5mm (1380) would you say it's worth boring that out to 74mm to fit these pistons?

Or am I pushing it too big??? And just forget it & get new pistons to fit the 1380 block?

[vivalamini]

Looks like it will be 74mm to me, I have a new set at the shop I'll measure them up later. I'm not at the shop till tonight, If their 74mm they may be Triumph TR6 pistons which was common back in the day. As above I hope the bores are good as at 74mm you have no room for a bigger piston and even putting a liner in there is unlikely to work.

Newbie question alert....Interested to find out why would a liner be unlikely to work?

[mini13]

There's a max size you can line from, I know 73.5 can be done, not sure about 74.

[minimans]

Exactly, even if it was offset bored to 74mm there is unlikely to be enough meat between 3 and 4 to bore for a liner. Even 73.5 is pushing it.

 

as to your question of a different block at 73.5 I think that is the way to go for a road engine. reliability of head gaskets will be a problem for a 1400 and even at 1380 I'd offset bore and use an offset Cometic gasket.

[vivalamini]

Ok will check out 1400 block see how bad it is and decide. Don't want to spend cash getting it all machined and it proves to be unreliable and a waste of money. Your right about the gap between the cylinders... it's tiny.

[vivalamini]

UPDATE:

Checked the block today and bores look to be in really good shape, no lip. The camshaft looks good too has been balanced at some point in its life. Journals need looking at might get away with a polish. Helps I stored everything in a plastic bags and oiled it all.

If anyone can shed some light on the gearbox, firstly is that a cross pin diff?? And secondly what are the machining marks for? There's quite and few on the box. (See pictures)

Think I will try and build this engine as seems in too good condition to waste. If it blows I have a spare block..

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[imack]

That's a standard diff & standard factory machine marks in the gearbox case

[vivalamini]

That's a standard diff & standard factory machine marks in the gearbox case

Great that's cleared that up... cheers...

[Northernpower]

For future ref this is what a cross pin diff looks like. You can see the two extra planet gears sitting in a cross.

 

[Spider]

The top rings haven't been sealing too well.

I'd suggest looking in to the reason for that before going too far down the road with this block.

[Cooperman]

The best thing to do with this would be to find another block which has not been Rebores more than +0.020" and take it to the next size up. That way you know you will have a great engine after spending your money.
You could waste a lot on the one you have.