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Hi,

I am in the process of rebuilding an A+ engine for my road car. Its going to be running an SW5 cam at about 10:1

Has anyone on here used the Hypatec 73mm pistons? They are available from Minisport in austrialia.

I should know better but I bought an engine off ebay that was supposed to be a low millage 1380, When I checked it the main bearings were obviously new but had big scores running through them. I pulled the pistons and they had some scuffing to the skirts. They were also marked STD on the top. Having measured them they are 73mm.

If they are rubbish I will get rid, and bore out to 1380 and get some new pistons, but I don't really want to spend money I don't need to.

I am not after a high power high revving unit, reliable as possible for a warmed up A+ is the goal.

Any thoughts or suggestions appreciated.....

You need to measure the bore, not the piston - they'll always be smaller.
Depending on how worn and or scored the bore is, you might get away with a high and new rings.

GraemeC wrote:You need to measure the bore, not the piston - they'll always be smaller.
Depending on how worn and or scored the bore is, you might get away with a high and new rings.

Sorry I should have been clear, bores have been checked, the machine shop said they were at 73mm but about a thou over. After looking round on the net the Hypatecs look like the same piston that I have. Just measured the at the base of the pison @ 2.870 / 2.871 inch so they are a touch under 73mm but I only have cheapo calipers. I don't really want to pay for rings and a hone, if the pistons are garbage. If they are rubbish I would rather get hold of something decent at this stage. I only want to build it up once!

Try Keith Calver, I had some Aussie pistons off him and they are fine.

I have a 1360 Cooper S with the pistons you have. I have run it for a few years with a 2.76 CWP and two OD drop gears. It's runs fine and I have had no problems with it. I have driven it at 4000 RPM / 96 MPH. Sounds like the build may have been the problem. Steve (CTR)

I built many motors with the Hypatec pistons from Australia and although not overly light they are more than capable for high output motors. Graham Russell in Aus told me they are used in some race motors over there and he was originally involved with there design. He now has his own which are similar but a bit lighter. Bottom line, for the money an excellent piston.

Cheers
Dennis

I've been using them in all the engines I've done over the past 5 years or so now, while not a race piston or a high performance design, they really do stand up well not only to high revs (7k +) but also for long sustained periods. I can't fault them. As a bonus, they are inexpensive. They have common sized ring lands so a wide variety of rings from many manufacturer's fit them.

Prior to using these, I was using AE, but have found IMO, the Hypertecs to be better than the AEs.

Spider wrote:I've been using them in all the engines I've done over the past 5 years or so now, while not a race piston or a high performance design, they really do stand up well not only to high revs (7k +) but also for long sustained periods. I can't fault them. As a bonus, they are inexpensive. They have common sized ring lands so a wide variety of rings from many manufacturer's fit them.

Prior to using these, I was using AE, but have found IMO, the Hypertecs to be better than the AEs.

So it appears these pistons should be fine for my road engine. Its never going to see more than 6.5k!

Could you tell me what the final bore size that you use for the 73mm pistons? My machine shop has said that my bores are currently @ 73mm + "about a thou". Minisport aus have recommended a minimum clearance of 0.0015 they have not stated a maximum....

So do the bores need to be the piston size + 0.0015 or piston size + 2 x 0.0015? ie 0.0015 clearance on both sides of the piston?

Also what rings would you recommend?

Sorry about all the questions I just want to make the best job of this build that I can. I have never built an engine before and need all the help I can get!

The Piston to Bore Clearance suggested by Hypertec is 0.0015 to 0.0025" for these pistons. For road use, I tend to run 0.0017 to 0.0018". Don't make them too tight!

So, to be clear on this, that's 73.00 mm + 0.0017". A way of checking during assembly, you should be able to get a 0.0015" feeler in the bore and then the piston will feel a very neat fit in with it, you shouldn't need any force / pressure to do this (as you could be simply distorting the skirt). This Clearance is measured 0.500" up from the bottom of the skirt at 90 deg. to the Gudgeon Pin Bore.

The Hastings Plain Iron Rings and the Grant Plain Iron Rings I've found good.

Thankyou Spider and everyone else, your help is appreciated. I will get the machine shop to give me the exact measurements and then go ahead with the build. Looks like however built the engine last time had not got enough clearance, hence the scuffing to the piston skirt and scored bearings. Glad I won't have to pay out for new pistons, rings and a rebore. That money can go into having everything balanced instead.

Thanks again!