The V8 Owners Forum -

Hi All,
I have a 4.2ltr engine unfortunately it was stripped when i got it, so it means that i will have to rebuild it in standard form to measure for myself if I can't find the answer. In the speedpro book it mentions that the pistons are 40thou down the bore on a standard engine (I know TVR engines differ from this), but it doesn't mention the size of the engine they refer too. I'm looking for the height from the piston crown to the deck of the block.
Thanks in advance

.040" is typical but why not assemble the engine with one piston and rod installed?

Thanks Mike
It looks like I'm going to have to rebuild a cylinder, a bit of a pain when even the rods and pistons are parted

Maybe get somebody to turn down a piece of wood or nylon to use instead of the original pin? I've done this before now. An engineering shop would do it in minutes with aluminium.

Thanks Mike
I was in the garage thinking along those lines earlier.

K4000JB wrote:Hi All,
I have a 4.2ltr engine unfortunately it was stripped when i got it, so it means that i will have to rebuild it in standard form to measure for myself if I can't find the answer. In the speedpro book it mentions that the pistons are 40thou down the bore on a standard engine (I know TVR engines differ from this), but it doesn't mention the size of the engine they refer too. I'm looking for the height from the piston crown to the deck of the block.
Thanks in advance

As Mike said it will be around 40 thou. to a min of 30 thou.

Comp ratio of 4.2 engine is only 8.94/1

What are you plans for the engine and what are you looking to achieve?

Kevin.

Thanks Gents
I want to build a 7000rpm engine. I am looking at fitting a mechanical cam like the M238 or similar. I have heard and have now read on here that early TVR 5.0ltr pistons may be the way to go in getting the compression up. Any advise appreciated. I already run an omex 710 and Twin plenum, but I am looking to run throttle bodies in the future.
I have found some pistons which may be suitable but are 1mm taller(pin centre to crown height) and have a dish of 5cc in the piston crown, can anyone help with compression ratios if i use 36cc heads.
Thanks in advance

Name?

What pistons and are they standard bore size?

Kevin.

A factory 4.2 indeed had 40thou, a tvr 4.3 ran flush with the deck face, a few guys have used a 3.9 piston and skimmed the top of the piston down but this will leave a very narrow top ring land imho, 5cc pistons will definetly give you a high cr, if you can say where the 5cc piston will be in the bore and heads you are using i will work it out for you rob

An M238 might rev to 7000 but peak hp will come in way before that. I do have for sale at the moment a new Crower 50305 that is similar to an M256, it's got something like 256 @ .050 on the inlet and 260 @ .050 on ex. lift is in the .530-540 region. A lumpy cam I admit but it would get the power up where you want it for sure.

May be I am wrong but have this figures for TVR 5.0 piston :

Bore size (dia mm): 94mm
Comp height (mm): 38.5mm
Bowl (depth mm): 5.8mm
Crown thickness: 5.00mm

Which piston do you intend to use ?

Thanks chaps. I will try and get some more mesuring done and come back to you.
Cheers