minispares aus (hypertech 1360 pistons) overbore size?
Turbo Mini Forums

i bought some +60 pistons from aus minisport last month. they should take my block to 73mm, or 1360

has anyone else bought these pistons, and what did the engine shop rebore your block to?

the local machine shop will only overbore if you give them exact measurements for the overbore (which is fair enough), though these werent supplied by minispares aus.

i have emailed them but they dont always answer their emails that quickly, so anyone with experience of this size piston could be a help. hoping to get the block bored this week.

cheers

Plus 60 is NOT 73mm (1360cc)

Minisport (Aus) were selling Hypatech in +20, +40, +60 and 73mm and 73.5mm when I bought mine.

The 73mm are about +94, not a standard rebore size.

There is also a significant difference in the design of the +20,40,60 compared to the 73mm and 73.5mm, ie, the 73/73.5mm are superior to the +60

Having said that, any decent machine shop would want a piston in their hand to measure accurately and all you tell them is the skirt clearance you want.

There is a very long thread on here about the Hypatechs and which ones you should use....

Schrödinger's cat - so which one am I ???

I did exactly as Rod says and I have the 73mm pistons in my daily. I actually took all 4 pistons in and also got them fitted to the rods. Specified clearance and came back a couple of days later jobs a goodun

cheers for the reply. im pretty sure that i have the 73mm, that would take the cc to 1360?

i will try machine shop tomorrow, they usually like the block, piston and the paperwork that comes with the piston if they are using any other piston than their own.

the last email i had from them said '094 is std size, rebore size is + .015 thou'

not sure exactly what they mean?

It appears you have the 73mm ones (ie, about +094) which is 1360cc

What they probably mean by "rebore size is +.015thou" is the skirt clearance for the piston to bore - Except I would say +.015 thou is a typo.....

The recommended skirt clearance for the 73mm hypatechs is 0.0015" (or 1.5thou) from memory for normal use.

I had mine set at 0.06mm (2.5thou) for turbo use, but it's a personal choice.

Schrödinger's cat - so which one am I ???

if they have slots in the oil ring groove, they are +60, if they have holes, they are 73 or 73.5mm

Pistons are measured at the bottom of the skirt across the pin. That plus the skirt clearance of at least 0.002" should there abouts add up to the correct size.

yes, have holes, i remember now they are def. 73s.

they will be used in my na mpi daily driver. what do i need to ask for at the engine shop then? skirt clearance of 1.5 thou? how about the total overbore size?

have never had an engine overbored before.

cheers

You don't have to tell the machine shop the actual "overbore" size, just tell them you want a nominal 73mm but with a specified skirt clearance and give them one or more pistons for them to measure accurately.

They will then bore to the exact piston size plus your specified clearance.

From what you said earlier they are expecting that clearance to be stated by the piston manufacturer....

In which case I "think" Hypatech recommend 0.0015" but I decided it was too small and told my machine shop to give me 0.0025".

However, I suggest you re-confirm the 0.0015" because the zeros were in the wrong place in your earlier post.

However, the choice is yours - a good machine shop will recommend a clearance anyway once you tell them what the duty is.

EDIT - the other thing you should consider is offset boring... Vizard says it isn't essential until 73.5mm plus but I got my bore walls a hell of a lot more uniform with offset boring at 73mm.

Edited by Rod S on 17th Aug, 2009.

Schrödinger's cat - so which one am I ???

hi, cheers for the replys. i think im going to take the block and pistons down tomorrow, and see what they think. they are a bit stroppy, hance all the questions!

willo suggest something like a .0015 - .0020 skirt clearence.

will also ask for the offset bore.

with regards to the skirt clearance, what is best suited to different aplications? just wondering

the engine is mpi so have got to try to keep it within the strict emission tests. would skirt clearence help with this at all? cheers

iv just got back from the machine shop. the people there will not overbore an engine unless i am able to supply the exact size that i wish the bore to be machined to.

the pistons i have are +.094
specified clearence is .0015

is anyone here who works with engines able to shed some light on the exact size i need the bores to be taken out to?

just emailed aus minisport on this matter, but am waiting a response.

any further help much apreciated

cheers

Find another machine shop.

+1

If they can't measure the pistons up for you then somethings a bit amiss.

If something is worth doing, it's worth doing half of.

plus 2

Do not rely on quoted piston (and hence required bore) sizes.

Any half decent machine shop will measure your pistons if only to confirm in their own mind they are the size they are supposed to be.....

Sounds like the ultimate cop-out.... but Sir, you told me this bore size so it's not our fault if it siezed.....

Schrödinger's cat - so which one am I ???

yea, it is a bit of a cop out. there arent any other machine shops around here though. they also have a very good reputation.

how much should such an overbore cost roughly?

Scholars in my part of Suffolk were (a year ago) charging £18 per cylinder for a standard job ie, where they supplied the pistons so they didn't need to measure my own.

For my job, check measure my sizes, offset bore to my defined specification and machine the centre bearing cap differently to normal, they went to hourly rate.

Total was 120 + VAT but 1/4 of that was on the centre main cap work.

Schrödinger's cat - so which one am I ???

they quoted me £200 plus vat. minisport/minispeed etc charge under £100, though dont know if they want to suply pistons. have looked in yellow pages and will call a couple of other machine shops in the area for quotes tomorrow.

i thought what they were asking was a bit on the steep side.

isnt Jimster in your area? could ask if he uses anyone local.

How much !! ? I pay £50 for mine to be bored & honed & the jobs always spot on.

WWW.TURBO-MINI.COM

picked my block up this morning. have also phoned all of the engineering places in the yellow pages. the best i can get is £150 plus vat local.

also spoke to minisport, they are able to do the work for far cheaper (including delivery) than my other quotes. so looks like minisport it is. i know some people dont get on with they, but they have always been spot on with their products with me

You wont go far wrong with Minisports Machine shop, while some other products may be suspect

Anything over £100 for a simple offset bore is a rip off

Edited by Sprocket on 19th Aug, 2009.

will be on the phone to them tomorrow to organise things.

thanks for the responses and advice

think its a bit stange that nobody in the old endustrial parts of s wales will do an overbore for a competative price.

my local machine shop is about 80quid for an over bore if i supply the pistons, i called in today to have my block looked over

Yes i moved to the darkside

Instagram @jdm_brett

What was the verdict brett?

If something is worth doing, it's worth doing half of.

Yes i moved to the darkside

Instagram @jdm_brett