Piston Sets

[Glyn.G]

Hi,
Having acquired 3 good used barrels for my 75 GT380 engine build it is now time to order a set of 1st oversize pistons ( 54.5mm ). I have seen a few different sets on e-bay, eg Cruizin Image, IMD, and HMP, Prices range from £58.0 to £90.0. Has any one used any of these pistons with good results or is there anyone out there who has had any disasters.
I intend to have the cylinders honed out once I have the pistons and have found a reputable firm to do the job.

[Glyn.G]

Hi,
Just as a matter of interest how much can be safely honed out of a standard bore and still be run on a standard size piston.
One of the used cylinders I acquired has been cleaned up probably with a hand honer and measures 54.12mm, the seller reckoned it would be fine to run on a standard piston.
If this is the case I could probably clean out the other 2 cylinders to the same size and run as standard too. Or is it best to just have all 3 bored to 54.5mm.

[Zunspec4]

Hello Glyn,

I believe it will be worthwhile to go to the 1st oversize (+ 0.5mm). Oversize pistons may be easier to source and will be no more expensive than std. size.

Cheers Geoff

[Glyn.G]

Cheers Geoff,
I think that's what I'll end up doing. Just a case of choosing the piston supplier and a firm to do the job in the Derbyshire area.
Regards,
Glyn.

[markush]

Hi!

If you want to spare material of the cylinder liner, you can try to get aftermarket pistons with 54,25mm.
e.g. X7-pistons of TKRJ would fit. These are in their catalogue in 0.25mm steps.
(I have these pistons in my GT380, but with 55mm)

The absolute maximum piston play is 0.15mm.
With your 54,12mm-cylinder you can run, but it's far from "fine" and will rattle quite noisy.

gl
Markus

[Glyn.G]

Cheers Marcus,
I thought the cylinder would be near to the limit at 54.12mm so I've ordered some 1st oversize at 54.5mm from IMD and am arranging for a rebore at a not so local Motorcycle engineering company. Thanks for the info though it will be handy for any other project I get involved in.
Regards,
Glyn.

[Craig380]

I seem to remember that the height from gudgeon (wrist) pin to piston crown can vary on some aftermarket pistons for the X7 (Markush may know more about this), so it's probably worth checking this measurement against your old pistons.

[markush]

We wrote about it in this thread: viewtopic.php?f=2&t=9480&start=38
But Glyn has already bought IMD-Pistons. Would be nice to read how it goes with them...

[Vintageman]

how much can be safely honed out of a standard bore

The Service Bulletin and Service Manuals give the tolerances.

If the cylinders are decent, I hone very little just to de-glaze . A cylinder brush with scotch Brite like bristles may be better than a medium grit hone for latter takes off material fast. And, I can't control myself even why I try .

Don't try to take out all the "little" scratches or you just end up with loose piston, piston slap.

Else oversize it now... or later.

If decent (sounds like you may be at the limit?), i would de-glaze first and save the next size bore for later. But, the best is no doubt a correctly bored and fitted cyl, And, I use the word "Correctly" seriously. Your job shop needs good tools and must be paying attention!

It runs so much better blue printed, than a cylinder near the end of it's tolerance limits.

[Glyn.G]

Hi,
After reading the thread posted by Markus on the difference between the heights of the Genuine GT380 piston and the X7 piston I am now confused. Can anyone tell me if there would be a big difference in top end speed using the shorter X7 piston instead of genuine GT380 piston.
I will take some measurements of the IMD pistons when they arrive to make a comparison.

[Glyn.G]

Hi,
Well the Pistons have arrived and it seems they have sent a set suitable for the X7 ie all the dimensions match the measurements on the previous thread by Markus. They are 4mm shorter on the overall length than the genuine piston. Will this make a lot of difference?
Regards,
Glyn.

[markush]

Hello!

In theory you lose a little in the lower speed range and win something in the upper.
The (my) practice confirms this. The usable speed range is shifted by about 500-1000 r/min upwards.
So you have a little more power at the top and slightly less torque at the bottom. The engine character becomes a little bit more "peaky".

I can't tell exact values, I didn't made a Dyno run.
Look at this post:
If I understand that right, tz375 made a software-simulation with the shorter Piston and gets theoretical about 2 hp added.
(? 2 hp per cylinder or per engine? It feels more like per engine at my bike.)
viewtopic.php?t=9480&start=41

Markus