1QwkSport2.5r

I'm rebuilding an OS FSR .25 ferrous p/l and the fit is fairly loose. The p/l in the engine now feels about the same - compression is soft and it's impossible to hand start on diesel. It won't hand start on glow either. So in an effort to not ruin the new p/l, should I "grow" the piston a bit first before running the engine to tighten the fit a little first? The ferrous Enyas I've run from new we're fairly snug fitting. I guess OS didn't set their ferrous engines up the same way even 25-30 years ago...

franchi

Hi:

How are you planning to "grow" this piston?

In the early 70's, I attempted to grow pistons but had poor results. I would have it squeezed down and then lapped in to a great fit.

Good luck,

Frank McCune

aspeed

The car guys pinch the sleeve, but these liners might be hardened steel. Plating the piston or liner could be worth the trouble. I would just use a starter unless it is for a race with pit stops.

1QwkSport2.5r

Growing the piston is not all that common these days, but it's been done in the not too distant past. Cast iron grows and hardens with time and heat. However if the initial fit is loose, it may not grow enough from regular running to get a good fit. The problem isn't so much with starting (though ease of starting go hand in hand with idle quality and speed) but with how well the engine idles and transitions. The new piston and liner fit is better than the original set from the engine, but it's still on the loose side. Seems to be pretty common with OS engines in general. Coming from having a bunch of lapped Enya engines that have a much tighter fitting piston, those engines start and idle significantly better than this .25FSR did. However I have not run in the new piston/cylinder yet.

Growing the piston involves heating the crown up bright orange and quenching in oil. The result is a slight growth in piston diameter and thus a better fit within the liner. However if it grows too much, it may need manual lapping to fit the liner. Usually the guys that didn't have much success with the process it seems either were working with a steel piston or didn't get the iron hot enough. In some cases it grows too large. This is based on what I was able to find/read on the Internet. I asked the question here to see if anyone has done it and what their results were to determine if it was worth trying on this new piston before I run it in. The cautionary posts on the other forum mentioned the grown piston will be much harder and as such may have issues getting the engine to break-in.

gcb

I would suggest you run it as is, even if you need to use a starter the first time. You may find that a hard flick will start it. I have a diesel that had almost no compression from new (MARZ-3). The instructions said to set the contra to just touch the piston at TDC, then back it off a turn. I got it started and after a couple of heat cycles it gained some compression.

You can always try the piston growth procedure if that does not work. Bear in mind that break-in also will smooth out the machining stresses from production. I would assume that growing a piston from new would add piston plus stresses so be careful or as previously mentioned, you will need to hand lap it. I have not grown a piston but most, if not all, cases that I have read about was with a piston that was worn-out.

Good luck with it.

George

1QwkSport2.5r

Thanks for your input. The engine ran on the old P/L, it just wouldn't idle below 3k for beans even with a big prop on it. The fit of the new P/L is a lot better than the old one, but not what it should be IMO. I'll run the engine without modification and break in the new P/L and see how it runs. I prefer to hand start diesels if possible, so I'm hoping the fit gets better so it will hand start and idle nicer. It'll be awhile before I get to running this engine - too busy with life at the moment.

ffkiwi

I know of no case where you can just 'improve the fit' simply by growing the piston slightly to tighten it-if you cherry bomb you grow the piston OVERSIZE for the bore. For a clapped out engine the method works a treat.........BUT (and its a big BUTT!!)-you have to lap the piston to the liner after you have 'grown' it-the process is a 'once and for all-all or nothing' job-you can't do it incrementally-test the fit and repeat the heating till its OK as it were. Secondly-its not a universal fix-it depends a lot on the cast iron originally used to make the piston-what type and what heat treatment it had had prior to-or during manufacture. In some cases the 'cherry bombing' has no effect other than making a clean piston dirty! The increase in size is the result of a crystal structure phase change within the cast iron-and yes-the resultant material is significantly harder-so yes-the concerns about running in etc are quite valid. In any case IIRC the method was stumbled upon almost accidentally-(George Aldrich???) when dealing with some Super Tigre models in the 1960s which apparently had 'soft' pistons which wore prematurely-and the supply of spares was inadequate or non existent. To me this would suggest that the original material was either defective or had been inadequately heat treated. Nearly 50 years on-who can say-Super Tigre may have been at fault, their metal supplier may have been at fault, it may have been a batch of material from a different supplier etc etc. All that is certain is that the method seems to work in a lot of cases-but not all!.........and unless you have the equipment and skills to lap the piston to the existing liner-or know someone who can-then there is little point in attempting the procedure. You also need very good measuring equipment to see what the liner is like-is it out of round, what taper if any is used etc-OS in common with many manufacturers set up their engines now fairly 'loose' to minimise the amount of running in required. Implicit in that manufacturing approach is that there will be less ability to absorb wear before the fit deteriorates to the point where starting and idling are affected-you can't have it both ways!

ChrisM
'ffkiwi'

TFF

I have never run one as a diesel, but every FSR I have ever messed with was "soft." If you got two piston liners sets, try it on the weaker.

1QwkSport2.5r

The new P/L (new old stock unit from eBay) is better fitted than the old set, but it feels soft. Really though, most OS engines I've had in my hands were pretty soft to start with. Probably why everyone likes them - light fitment means fast/easy break-in... Instant gratification anyone? I have a .60FSR (early ringed version which predates the .61FSR) that likely will take more time to settle than the ringless versions - iron piston or aluminum.

Sport_Pilot

It depends on if it had any heat treatment to start with as to if it will grow or how much. You also stand to make the piston too brittle and it would be hard to find another p/l. Suggest running it with 30% castor and on the lean side so that it will build up some varnish. That will make it fit a bit snugger. After that you can use regular oil with a normal needle setting.

Chris W

I would be leery of 'growing' the piston into out of round - both due to inconsistent grain structure and the presence of two (relatively) large gudgeon pin holes, internal webbing under the crown etc.

These all would distort what was once round into some sort of oval.

Chrome plating (as mentioned) is the best solution I have heard of (gleaned from an ex world champion Team Race mechanic) followed by getting a good hobby machinist to make a new piston.

Pinching a liner in my limited view would only address the fit at TDC ( as in reversing the bell mouth effect due to hard use ) and still allow leakage before that - but happy to be corrected on that one.

1QwkSport2.5r

I'm not gonna bother messing with the new P/L, but I may do it to the old set when I get around to it. Ringless OS stuff is setup so loose, it would take a lot of growing to be fitted properly initially - not worth the effort really. The .25 needs more heat cycles before its gonna be totally settled, but it does run pretty well. I'm not overwhelmed with power or running quality - this .25FSR is easily bested by my Enya .30SS.... But it was free to start with - $40 worth of piston/liner and bearings later, it's "okay". It should hand start a little better as a diesel now at least.