Help with piston clearance - 350 RA Douglas, Hepolite 447510 "slipper" piston

[cardan]

The motor of the TW (350 RA) is coming together.

I have a pair of NOS Hepolite pistons to suit. These are stamped on the crown: HEPOLITE REG. No. 447510. They are as shown in RA literature and the main feature is a central boss to suit the forked con rod found only in the Douglas RA/RW/TW bikes. They have other nice features like a drilling from the chamfer under the lower ring to the little end so that the scraped oil keeps things lubricated.

The pistons are finished and measure 56.70mm across the skirt (such as it is).

Can someone suggest a skirt-bore clearance for me to use?

The RW/TW is usually quoted at 57mm bore - 0.30mm (about 12 thou) seems like "racing clearance" for a 57mm cylinder. Would it be safe to run 6 thou clearance, or even less? I have no intention of racing the thing, flogging it, or even riding it very fast.

Advice based on experience with Hepolite pistons and "slipper" (not full skirt) pistons would be especially welcome.

Thanks in advance,

Leon

[cardan]

I notice 447510 is just the registration number for the trade mark "Hepolite", so that bit is not very interesting. What it does do is to date the pistons as post April 1924, as this is when the trade mark was issued.

Leon

[cardan]

I've scanned what info I can find on Hepolite clearances, and have decided on about 0.0035-0.004" clearance at the skirt. Please yell if you think I'm going to seize it.

For the record, the pistons are 56.7mm at the bottom of the skirt, 56.6mm under the lower ring, 56.5 on the land between the rings, and 56.35mm on the top land.

Leon

[Douglas52]

Hi Leon
Have you established whether or not the piston alloy is the Lo-Ex type? Phil Irving's book alludes to the use of non Lo-Ex alloys for racing use to take advantage of higher strength at temperature, at the expense of expansion.
Cheers
Steve

[cardan]

No I haven't Steve. Thanks for the timely warning and I will measure the expansion before I get too much further.

Leon

[cardan]

The cylinders are finished with 0.004" clearance at the skirts. With the pistons heated to 100C, they just fall down the cold (say 15C) bores under their own weight with no oil present. I have little experience with pistons like this, but it seems like this clearance might be OK for my purposes?

Leon

[Douglas52]

Hi Leon
I feel your clearance is fractionally on the tight side.
Heres my reasoning:
The book "Modern Motorcycle Maintenance" by Bernie Osborne of (Motor Cycling) states a clearance of 0.0015"/dia inch for solid skirt Lo-Ex (say 12% Si) pistons; touring use, air cooled. This would equate to a skirt clearance of 0.0034" for a 57mm solid skirt Lo-Ex piston. I would imagine solid skirt pistons would require a similar clearance to slipper pistons, all other things being equal.
However you probably have low/no silicon pistons which have a higher thermal expansion  coefficient.
A publication "Expansion Coefficient of Aluminum Pistons– Forged vs Cast Pistons:A Literature Search
Rev. 3 Fred Moreno, Chris Howden 31 March 2009" gives the effective thermal expansion coefficients of aluminium pistons of varying silicon content in steel(iron bores). The numbers compare well with other publications.
It indicates you can expect your piston expansion to be about 1.5 times that of Lo-Ex. So the clearance should be 1.5 X 0.0015= 0.00225"/dia inch, or 0.005".  In very rough terms, at about 0.3 thou skirt clearance, the piston fit becomes transitional and some seizing can be expected. This will occur when the skirt temperature reaches around 200 deg C  - not all that hot. With a 4 thou piston skirt clearance the seize temperature drops to around 160 Deg C.

If the 4 thou clearance is retained it would pay to warm up slowly in order to allow the cylinder to expand.

I expect someone in the group will be able to give give a more coherent explanation.

Cheers
Steve

[cardan]

Thanks Steve. Good food for thought, and to see some solid data is great. An extra thou or so might be wise.

In relation to fitting aluminium pistons, my experience is with those from JP. These have a bit of a reputation for nipping up, and despite the advice from the guys at the factory I always get the greatest clearance that I can convince them to give me. They are incredibly reluctant to give as much as 6 thou on a 500 (say 85mm bore) even though 6-8 thou is definitely required on a vintage engine. They are mostly worried that customers will come back complaining of piston slap.  Anyway, a friend has Panthers, which use a slipper-style piston, and although a JP piston needs 6-8 thou, a Hepolite piston in the same engine makes do with 4 thou.

It's hard when the pistons are of uncertain age and don't come with a spec sheet!

Cheers

Leon