14 replies to this topic

[SirBallD]

Hi Mini-mates,

 

I have spend the past three months figuring out where to find the right rings for my pistons.

 

I've come to the conclusion that the pistons that I have is one of a kind (+/- 2). 

Engine information:

No engine plate

A+ type 998cc

Bore size 65,33mm / 2.572", indicating that it's a 1022cc (0.030 oversize).

Stroke 76,34mm

 

The identification and information on the pistons are as follows:

Flat top, four ring, circlip type pistons.

TRW 175-9798T (found on the inside of the piston)

21 6-8406 (also found on the inside of the piston)

+.03 (found on top of piston) which indicates that it's a 0.030 oversize, I guess.

Front JGO 06 (also found on the top of the piston). 

ø65.28 (also found on top of piston). I guess this indicates what bore size it's prepared for, as the diameter of the piston is 64.73mm (measured above the top ring)

Compression height approximately 33,84mm

 

1st groove width (top): 1,69mm ; depth approximately 3,07mm

2nd groove width: 1,69mm ; depth approximately 3,07mm

3rd groove width: 1,69mm ; depth approximately 3,07mm

Oil ring groove width: 4,04mm ; depth approximately 3,45mm

 

The ring that was fitted to the pistons upon disassembly has the following information:

1st ring thickness 1,56mm ; width/depth approximately 2,74mm ; square type with bevel on inside/top.

2nd ring thickness 1,59mm : width/depth approximately 2,82mm ; square type ring.

3rd ring identical to 2nd ring.

Oil ring thickness 3,93mm ; width/depth approximately 2,42mm ; coil spring loaded beveled oil control ring. I think it's called the GS type.

 

Now I have to decide whether to continue looking for a set of rings or go for a replacement of the pistons.

So far I've found Deves R33636-030 which comes close. It's multipiece oil ring (0.158") with two rails, one spiral spacer and an inner spring.

 

All help is appreciated as I'm very close to giving up. 

 

Cheers

Stefan

 

[Spider]

Your Rings are actually an Imperial Sized Ring, 1/16" and a 5/32" Oil Control Ring.

Quite a popular and common Ring Pack, however, just looking at your Ring Grooves, if those measurements are accurate, then your Pistons maybe worn. I'm not sure how you arrived at those measurements, they can be tricky to measure directly. If you haven't already, try measuring the grooves by using a Ring and  Feeler Gauge.

The clearance needs to be between 0.053 and 0.084 mm.

[mini13]

as they are 4 ring pistons, I would personally go for new pistons, probably +40's

[SirBallD]

Your Rings are actually an Imperial Sized Ring, 1/16" and a 5/32" Oil Control Ring.

Quite a popular and common Ring Pack, however, just looking at your Ring Grooves, if those measurements are accurate, then your Pistons maybe worn. I'm not sure how you arrived at those measurements, they can be tricky to measure directly. If you haven't already, try measuring the grooves by using a Ring and  Feeler Gauge.

The clearance needs to be between 0.053 and 0.084 mm.

I'm trying to measure the groove width with a vernier caliper, but I agree that it's very difficult to measure accurately. 

With the rings on the piston and measuring the clearance using a feeler gauge, I can see that the clearance at 2nd, 3rd and oil is within tolerance (between 0.053 and 0.084mm). The 1st ring clearance is out of tolerance at 0.127mm.

Does this mean new pistons? It just seems to be easier to find +20, +40 and +60, than +30. Is it because this +30 is obsolete?

 

@mini13, if I was to fit +40s, I have to re-bore the cylinders, right? 

[sonikk4]

Yes is the answer for going up to +40.

The other thing is here while you are concentrating on the piston ring issues, have you physically checked the bore for wear and ovality?? Ultimately this will drive your requirements.

[whistler]

Back in the day, any piston situation like yours and you would take the pistons to AES (engineering motor factors) and they would clean out the ring grooves and source the correct rings or modify the ring grooves and fit slightly different rings. Don't know if that is done nowadays by machine shops. Maybe AC Dodd would be able to help. I assume you've tried FW Thornton for those rings.

https://www.fwthornt...ixoC60UQAvD_BwE

[mini13]

yep, as said you'll need to rebore to +40.

 

Im working on the asumption that a 4 ring piston is going to be approx 40 years old+  so is likly to be worn, + 30 piston is probably going to be tricky to find, and the bores may well be work,

 

BUT! it is worth getting a machine shop to check the pistons and bores over if you dnt have the kit, you might get lucky and get away with rings and a hone, espicially if its standard spec

[Spider]

Just to pretty much echo similar thoughts to what the other guys have suggested here, there's a few options to salvage your +0.030" Pistons, but really you'd be better off replacing them with an oversize and having the engine bored.

I'd suggest too before buying Pistons, take your block to your Machine Shop and talk to them first. Going from +0.030 to +0.040 is going to be less than 0.005" off the wall of the bore, including honing allowance. Most shops won't bore that small amount, though some might hone it, which is a tricky operation to get right going that far.

[MiniMadRacer]

Would sleeving back down to STD be an option ?

[SirBallD]

Thank you for all the suggestions. I appreciate it very much.

 

I'll ask the machine shop to have a look at the block and the pistons and let them make a decision.

I would personally go for the rebore and new pistons +40. It's kind of a clean sheet.

 

What about the pin height when choosing new pistons? I have measured the pin height to be approximately 34.00mm (distance from top of piston to top of pin + distance from top of piston to bottom of pin. The measurement is divided by 2).

When fitted to the block, the piston was completely flush with the top of the block. 

Is the pin height different or is it standard when choosing stock pistons?

[mini13]

the pin height is normally pretty uniform, but there are some differences, often aftermarket pistons sit a little lower in the bore to allow for resurfacing,

 

basically you'll need to get it bored and do a "drybuild" to check the piston to block clearance. I gererally have my blocks decked to leave about 5thou clearance, but thats on large bore engines.

[Spider]

Most piston manufacturers do usually publish all these dimensions.

Just looking at the Hypertec Pistons, their dished 998 range has a Pin to Crown height of 34.08 mm.

[SirBallD]

I'm taking the block to the machine shop in the morning.
@Moke Spider, as you mention, when going from +30 to +40 is less than 0.005" so I guess they might advice me to have it rebored to +60.
Depending on what the machine shop advice me to have done, I'll buy either +40 or +60. I've been looking at the ADU3490 piston, as it looks like a decent upgrade from my four ring pistons.
Does anyone have any specifications on the ADU3490 pistons.
Just a simple question, with probably the most obvious answer. If the block is rebored to, let's say +40, should I go for the +40 pistons? I know it's a stupid question, but my "shelf of wrong spareparts" seems to be growing by the minute

[timmy850]

See what the machine shop says. Most want to have the pistons there while boring/honing to ensure they get the correct size to your exact pistons. Either you can supply them or you can ask them to source them (which will probably be more expensive, but they are responsible with getting the right ones)

[Spider]

When a shop does a bore, as Timmy mentioned, they'll want the pistons before starting. The size is only a nominal size and there is small variations between pistons sets and brands. Given the very small clearances, it's important to get it right.