Not again !!??

[Hamish]

Does this mean I’ve broken another piston. 
 

very similar symptoms starts and runs great. 
can’t see much via compression tests or leak down.

exhaust emissions slightly dirty but pipe very sooty 

number one plug very sooty compared to others. 
 A lot of oil in catch tank after castle C 

endoscope shows dirty wet look on top of 1 piston and part of side bore 

if video works is that a crack around the piston ?

Edited April 28 by Hamish

[thebrookster]

Hard to tell Hamish (I hate those endoscope devices, they show you just enough to get you all concerned, but not enough to actually give you any answers lol).

Either a crack or ridge of carbon, however I agree that it does look suspiciously oily.

It's a head off job regardless, as there is only three possible sources of oil really. Removal of manifolds will eliminate valve guides however.

Phil

[flatter4]

Oh no Hamish
The sooty plug is indeed a portent...

As Phil says, it's head off to be sure - and probably pull a piston.
Is it the same piston that failed before?  I'm worried that there may be a detonation problem
Keep us informed,   Will.

[Hamish]

Well looks like my issues are not so obvious as a crack on the top. As Phil suggested a carbon line. 
 

my next task is to try and withdraw the piston(s) but that will have to wait for another session. As today was a little full on for a non working day !

this time I’m going it myself. A steep learning curve as I confess I’ve not gone this deep before with a wet liner. 
 

last time it was no4 I suspect no 1 this time  

time to change out all 4 this time I think  

 

Edited April 28 by Hamish

[Nick Jones]

Ah, bollocks - it was going so well 

Does look rather like a repeat of the previous problem.  Reckon you are finding the limit of the standard pistons!  Is there anything better out that that won't smoke your wallet?

[Hamish]

Sadly the next step would be forged pistons. in the region of  £1350.00 a set  

I am doing a “budget” repair with a set of pistons, rings and composite head gaskets as well as sundry gaskets.

the pistons may need machining to deck them. Which I’ll have to find a local machine shop. Been told I need 15thou clearance to top of liners plus that gasket thickness.

need to ask for big end bolt torque

need to buy a piston ring compressor fitting tool

i see Halfords still do them 

any good ?

 

 

I guess I need to compromise on revs !!?

https://www.halfords.com/tools/hand-tools/all-hand-tools/laser-piston-ring-compressor-696591.html?istCompanyId=b8708c57-7a02-4cf6-b2c0-dc36b54a327e&istFeedId=367c5610-f937-4c81-8609-f84582324cd6&istItemId=xxpmxi&istBid=t&gclsrc=aw.ds&&_$ja=tsid:|cid:17363835999|agid:|tid:|crid:|nw:x|rnd:7701894720434988961|dvc:m|adp:|mt:|loc:1006648&gad_source=1&gbraid=0AAAAADrJx6wTRhyBhoCbJAGhShMaF_LaG&gclid=Cj0KCQjw8cHABhC-ARIsAJnY12wCsJPdTVUno333bYRzFl6kF5BToHfnj7fVB8giKjpVL1KzH8FVGGUaAmsQEALw_wcB

 

 

[JohnD]

Hamish,

Those wide coils of spring steel that you wind up tight around the piston to squeeze the rings in to insert them in the bores are OK, but I've been most impressed by a 'ring compressor'.    A stumpy cylinder with a conical bore that you position over the bore and push the piston down into it.

Of course you need one that precisely matches the bore diameter!

JOhn

[thebrookster]

The taper bore style John is showing works very well, it is this design we use on the big marine engines.

You can either buy one (not sure where one would look mind you) or if you happen to know someone with a lathe it would be a relatively easy job to make one.

But otherwise, yeah the Halfords version will do the job perfectly fine, just a bit more faff. For a one off job, Halfords style all the way, however if you expect to do this a few times (and lets face it, engine rebuilds are a normal part of competing) then definitely go for John's option.

[Hamish]

Thanks guys. I have ordered the compression version from Halfords as it’s universal and I can lend it out to the local group if needs be. 
Cambridge motorsport do the specific sized taper versions at nearly £50 ( as well as the forged pistons !! )

 

[Nick Jones]

These are 89mm pistons?

[Martin]

I use the halfords Type Compressor since 40 years. Works fine.

when I see the piston in your pictures, seems it does have a rather high clearance as you can see  a lot of the piston ring. What clearance do they have?

[Hamish]

9 hours ago, Nick Jones said:

These are 89mm pistons?

Yes they are. They the type that are sold in a lot in kits eg 

https://www.trgb.co.uk/products/piston-and-liner-set-89mm?srsltid=AfmBOoqUMXiTxXzGdXSQfCkgS3E1sHkwdHZ_DyPd7plryv_RB3tntr7O

 

Or the forged versions cheapest found here

https://cambridgemotorsport.com/je-pistons/1777-triumph-tr2-4a-forged-piston-set-89mm.html

[Hamish]

4 hours ago, Martin said:

I use the halfords Type Compressor since 40 years. Works fine.

when I see the piston in your pictures, seems it does have a rather high clearance as you can see  a lot of the piston ring. What clearance do they have?

Suggested 15-20 thou

there is a big depth of piston above the top ring. 
 

but the block and liners have been decked so the pistons may need some work to get clearance.

 

i spoke to an older chap I know who has been in the racing and race engineering game for over 60years. He had a tr 2 or 3 and had help from Wilkie Wilkinson founder of Ecurie Ecosse To make it faster. I’ve seen him rebuild a F5000 engine and chassis panel work etc.  

he has given me some pointers re ring positions and gaps to look out for when I draw the pistons. 
 

he suspects ring gaps closing up I think. Or slack rings in the piston grooves ?!

 

[JohnD]

Ring gaps?  I thought that allowing 5 thou per inch of diameter was a tried and tested rule, based on the thermal expansion coefficient of steel.     Your engine might be under more stress than most, Hamish, I've heard of allowing more, but surely not less?

John

[Nick Jones]

Too tight ring gaps usually scuff the bores and breaks the tight ring(s).

Top one most prone as it gets hottest, though obviously it depends on gaps set at assembly.  As Will mentioned above, there’s a possibility that the piston ring land breakage is caused by detonation at full chat. Usually due to being a bit lean or a bit to much ignition advance.

[Hamish]

 a closer look this evening to measure the piston height relative to the liner deck height with a straight edge and feeler gauges suggest all very close to the recommended 15thou.

but check out number 2 piston top. 
the piston itself is like the surface of the moon in this small area. 
this would be about 7 o’clock stood at the front of the car/engine. 
 

thoughts ?

Edited April 30 by Hamish

[RedRooster]

Looks like the photos on Google when you search detonation. 

[hoffman900]

Looks like the start of detonation to me. 
 

If those are cast pistons, it would definitely be breaking your ring lands. That said, all those cylinders look like they have oil issues / pig rich. 
 

Detonation when bad will hammer your bottom end as well and could crack a block. I don’t think you have that. 

[andymcp]

On 4/30/2025 at 3:24 AM, Martin said:

when I see the piston in your pictures, seems it does have a rather high clearance as you can see  a lot of the piston ring. What clearance do they have?

I think Martin is referring to piston to bore clearance, not deck height?  The pistons do 'appear' (in the photographs) to be on the loose side.

My 500cc, aircooled, single cylinder, motorcycle has an 8 thou piston skirt to bore clearance, with an aluminium piston in a cast iron cylinder.

[flatter4]

 

Hamish, as you know the whole system needs to be looked at.  Be careful not to destroy evidence on the dismantling. The fuel system the ignition (what are you using - could be timing 'scatter' - jumping around at high revs) and the valves should be looked at too.
If there is signs of bore scuffing it could be ring gaps - as Nicks comment.  If you use a loupe / x4 (or more depending on your eyesight!) you should be able to see polishing on the ends at the gap where they'd touch (and a lack of carbon).  You don't mention scuffing, so probably not an issue.
Once the piston is out I'd hope it becomes clearer.  If a ring is broken it could have been assembly, or detonation (often accompanied by piston land breakage).

Don't worrk about the clearance at the top of the piston.  The piston is neither round nor straight.  The clearance on machining the bore is stated at the tightest point - this will be thrust / non-thrust at the bottom(ish) of the skirt.  This is also the most flexible part of the piston - so expands less and will 'move' more readily.  The top land will (should) have a high clearance cold as is expands most - due to being the hottest part of the piston and because it has the most material round it (it's solid).  Up to 1 mm diametrial clearance would not be surprising.

It's difficult to tell from the photos (though they are good!).  A few thoughts / questions:

I think the blue lines (wobbly sorry!) are the machining marks - if that's the case what is going on in the upper red circled area?  It looks like a depression
Is the lower red circled area deposits on top of the piston, or erosion? - scrape with flat scraper / end of steel rule......

Good luck and keep us posted.  You need to be back out there....
I'm hoping to get along and see you / others later this year.  Cheers,  Will.

[flatter4]

...oh and yes, the fitting sleeves are lovely - but really only justifyable for production or volume use.
Careful use of a conventional compressor is OK (I use one as attached picture).  The norm is to use an inverted hammer and top the piston though using the handle.  With some practice it works fine (though a small chamfer on the top of the cylinder bore helps - a good machine shop will do this when re-boring)

[flatter4]

#tap

 

[Hamish]

Thank you guys all wisdom gratefully received as you can see I’m a novice in this evidence based diagnostics. 
and strip and build. 
 

but given they are supposed to be fairly simple engines - even tuned, I  hope I can cope and with your collective guidance …………..

bill my old mate has told me the importance of noting things and positions and to mark parts on taking it apart. Digital pics should help.

today I negotiated what I believe is a good non list price for a set of forged pistons/rings/gudgeon pins. this was decision was based on the cost of an ordinary set of replacement rings to go on the 2nd hand pistons. Which I don’t need to buy separately now. The cost difference was significant but not horrendous and I’m not confident that my speed event driving style would change  so the forged set should cope with me better  but I still only have a std crank  

i only share this on the basis and hope that my better half doesn’t follow this thread. 

 

 

[Nick Jones]

12 minutes ago, Hamish said:

hope that my better half doesn’t follow this thread. 

 

Forged pistons are a good and worthwhile move IMO.

However, you do still need to be alert to the possible (quite likely) detonation issue.  The smoking gun (apart from ring land/ring breakage) are pock marks on the piston crowns, quite probably concentrated in one area, often on the far side from the spark plug.

The cause can be (usually is) as simple as the ignition timing at full chat being slightly too advanced. This can be inadvertently be caused by fitting new points or re-gapping the originals with a slightly bigger gap than was present when the timing was previously set on the rolling road. Bigger gap advances timing, smaller gap retards. As Will touches on above it’s also possible to have slightly different timing on different cylinders due to timing scatter. This is a mechanical distributor effect, worsened by wear and usually reduced by using electronic ignition. If the rules allow you might consider fitting electronic ignition and getting reset on the rolling road. It should remain stable indefinitely after that.

[Hamish]

Wow you guys are good. Thank you.

at Castle c I was running the engine builders set up +500 ish running in miles. 
 

I do have a 123 plus dizzy on the shelf, the engine builder doesn’t like them though. But I will do the rebuild with the points dizzy as it ran prior to strip down. And I don’t have a dizzy map for my engine for the 123 just some recommendations.