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MikeH
06-23-2005, 10:49 AM
Now that my engine is dismantled, I'm thinking of having the sleeves bored out and going to 87mm pistons. Any recomendations on a supplier? Thanks, Mike /ubbthreads/images/graemlins/thirsty.gif

jsneddon
06-23-2005, 11:40 AM
Sleeves bored out?

Aren't they supposed to be replaced with the proper size?

MikeH
06-23-2005, 12:13 PM
My understanding is that the liners are the same external diameter, so that you can rebore the 86mm to 87mm. Mike

Dave Russell
06-23-2005, 12:29 PM
Increasing the bore by one mm will increase the displacement by about 3.25 cubic inches or 2.5 %. Considering the decreased cylinder rigidity, no room for additional boring if cylinders wear, & extra cost, hardly worth the trouble in my estimation.
D

jsneddon
06-23-2005, 05:56 PM
but to answer your question...

https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=36459

https://www.britishframeandengine.com/parts1.html

Alan_Myers
06-23-2005, 08:53 PM
Hi Mike,

Some folks choose to have their liners bored out. It is certainly possible. But it might not be very cost effective. The cost to have the machining work done, plus purchase a set of pistons, is about the same as some new sets of matched liners and pistons (usually including wrist pins, rings and figure 8 gaskets).

One concern with old liners is that they will likely have some effects of corrosion on the outside, which will probably reduce their efficiency exchanging heat with the coolant. There is also a possible concern that a liner has a hidden crack or other weakness.

Still, boring is an option. It's your choice and your car, after all! Someone who has access and skills to do the machine work themselves - or a buddy with a machine shop who might do it in exchange for a pizza and 6-pack - might find re-boring the old liners the more cost effective route.

Before seeking out a supplier, decide which piston you prefer.

The original pistons were AE Hepolite, but are next to impossible to find in any size.

AE Powermax are uncommon, too, but good pistons if you can find them, although the rings provided with them are very slow to bed in. The full set piston and liners are generally very well matched and seldom need any balancing or deck height adjustment. I can't say, though, if you can get the pistons alone. I've got a piston/liner set that are going to be installed in my car. I had pretty well decided to use the "hard" rings that came with it, since this is a street car and I figure they will give longer service than fast-bedding "racing" rings. But, I keep going back and forth with myself on this issue and still might switch to a set of Deves rings. The harder rings might be harder on the cylinder bore, too.

Mahle is a more commonly available piston brand and seem to be pretty good.

JE Pistons make some forged pistons for TRs, I'm not sure about 87mm/standard height though. I think they supply the "stroker" pistons to Cambridge motorsport, to be used with special, longer con rods. Expect these pistons to be more expensive. But, chances are JE can supply other types and sizes, too. I found a website for them, but don't have the link now.

If you do some searching, you'll not only find 87mm, but also 87.5mm, 88.5mm and 89mm which can be fitted without making any changes to the TR 4-cylinder block. Do note that 88 to 89mm require a stronger alloy liner, because it's relatively thin. Even 92mm are available, but need the block machined to accept the larger liners.

A larger bore piston will reduce compression slightly. It's possible to mill the head to compensate, if you wish.

One good and very knowlegeable source is Ken Gillanders at British Frame and Engine (www.britishframeandengine.com). He's an engineer and a walking encyclopedia of 4-cylinder TR info, has been building performance Triumph engines and racing TRs for something close to 50 years. He knows very well what works and what doesn't and can give you good advice. Ken's focus is on performance parts. He supplies a lot of things that simply can't be found elsewhere.

British Parts Northwest is another source. They offer 87mm pistons separately, but don't say what brand. www.bpnorthwest.com (https://www.bpnorthwest.com) is their website.

Cheers!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

Dave Russell
06-23-2005, 09:10 PM
[ QUOTE ]
Hi Mike,
A larger bore piston will reduce compression slightly. It's possible to mill the head to compensate, if you wish.
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

[/ QUOTE ]
Hi Alan,
Why so? I would think that increasing cylinder displacement with the same combustion chamber size would increase compression ratio. ??
D

Dugger
06-24-2005, 12:17 AM
The only way compression can be increased is by reducing the clearance volume (CV). When you increase the size of the pistons without shaving the heads you increase the cv, thereby reducing compression.

Geo Hahn
06-24-2005, 12:38 AM
I was under the impression that no math was required to participate in this forum...

Okay -- I have thought about it and I think it would either be the same or slightly more compression. The combustion chamber would be the same of course so the only change in the CV would be that little space (very little) left by the piston not quite reaching flush with the head and the thickness of the head gasket. That latter volume would increase but so would the volume swept by the piston -- by the same % increase. So the compression ratio would increase since the combustion chamber is the same and the rest of the volumes increase proportionally.

I'll admit I could be wrong as I have never really examined this -- in any case I think any change in compression would be very small with the 87 v 86mm pistons.

Dave Russell
06-24-2005, 01:02 AM
Hi George,
If the original & the larger bore pistons were flat top with the same deck height, compression ratio would increase by about 2%. If the new pistons were dish top or had different deck heights, the CR could be higher, the same, or lower, depending on dish volume or deck height change.

Dugger,
Compression ratio is defined as (displacement volume plus clearance volume) divided by (clearance volume). If clearance volume does not change but displacement volume increases, the CR increases.
D

Dugger
06-24-2005, 10:34 AM
Dave,
You are absolutely correct about that formula. However, the clearance volume does change linearly with the bore.

To prove my point, take a ficticious one cylinder engine with a stated 9:1 CR. Lets assume for ease of calculation that the bore is 3" and the stroke is 3" for a displacement of 21.21 cubic inches. Using the formula given, the CV would have to be 0.265 cubic inches to meet the 9:1 CR.

If you increase the bore by 2% (i.e., piston radius from 1.5" to 1.53"; this increases displacement to 22.06 cubic inches for the same 3" stroke. When you increase the diameter of the cylinder, you also increase the combustion gas chamber volume which is the definition of CV.
With a 2% increase in bore, CV increases from 2.65 to 2.76 cubic inches.

Then;
original -> 21.21 + 2.65 / 2.65 = 9

2% bigger bore -> 22.06 + 2.76 / 2.76 = 8.9

Therefore with the stroke and head unchanged, increase in bore decreases compression. Unless of course my calculator is lying to me.

martx-5
06-24-2005, 01:56 PM
...all other things being equal, when you increase the volume of the cylinder, the compression ratio will increase.

Edit: Here's some formulas and info (https://e30m3performance.com/myths/more_myths1/comp_ratio/comp_ratio.htm)

MikeH
06-24-2005, 02:16 PM
Wow guys, I'm not an engineer, just a lowly electrician. As I can't find anyone to rebore my liners, I've ordered a new set of 86mm standard which my machinist has assured me will work fine in the newly honed and ridgeless liners I will have when he is done. Thanks for all the imput. The idea to rebore came out of my 2004 Moss Motors catalog, where in a blue technical section above piston pricing, they said that it can be done with no ill effects.

Dave Russell
06-24-2005, 05:23 PM
[ QUOTE ]
Dave,
You are absolutely correct about that formula. However, the clearance volume does change linearly with the bore.
When you increase the diameter of the cylinder, you also increase the combustion gas chamber volume which is the definition of CV.
With a 2% increase in bore, CV increases from 2.65 to 2.76 cubic inches.


[/ QUOTE ]
If the piston is flush with the block at tdc, the only thing that could increase CV with a bore increase would be an increase of the headgasket diameter/volume. If you assume that the headgasket bore increases with the increased cylinder bore, you may be correct that the CV increases.

In reality, in every engine that I have seen, the headgasket bore is considerably larger than the actual cylinder bore to begin with. Thus, no need to increase the gasket bore diameter with the resultant increase in CV. So the CV does not usually increase with bore increase.
D

Alan_Myers
06-24-2005, 06:38 PM
Hi all,

Too much math going on here for me! Almost sorry I mentioned it!

My statement about CR being slightly less is based upon an explanation (from an engineer) that basically follows what Dave said in his response.

This was good news for me, since I'm installing 87mm in a motor that's got a skimmed head, about .125". I haven't calculated the actual compression ratio, but would suspect it's marginal for use with today's commonly available fuels. So, a slight reduction in CR might be welcome.

Of course, if CR is still too high or if other folks' assertions that increased piston bore = increased compression, it's always possible to install a slightly thicker copper head gasket to reduce it. I see Revington TR offers 1.2mm, 1.6mm and 2.0mm head gaskets. And, I know more than one can be used in extreme situations.

What I don't know is the thickness of the stock head gasket, or the compressed thickness of any of the head gaskets. There is also the shim steel head gasket, usually used to raise compression, though.

I think I'll start another thread about compression ratio.

Cheers!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

Simon TR4a
06-24-2005, 07:04 PM
Agree with Dave Russell, boring out the cylinder increases the compression because the engine displacement is greater but the combustion chamber volume is the same.
Alan, to avoid this beinfg a problem you could flycut the crowns of the pistons if they are thick enough, you need at least 200 thou of material left.
The thickness of a standard copper composite gasket when the head is torqued down is usually considered to be 1mm., or 40 thou.
The accepted way of verifying this is to measure the thickness of a used gasket at several places round the "fire ring" (the part exposed to the combustion chamber) and subtract 5 thou for compression by the torqued down head. (This last bit of info was given me by a technical support guy at Allied Signal, who make a good quality and popular gasket for the Triumph.)
Simon.

martx-5
06-24-2005, 07:24 PM
Alan,

Talk to Ken at British Frame and Engine. I'm restoring a TR3, and am putting in the 87mm pistons and liners. I talked to him about making about 125 hp, and he said to me the 87mm pistons, a mild cam, and milling .080" would do it. That .125" figure might be a bit much, but he can steer you in the right direction on what to do. Probably just a thicker head gasket. He probably can tell you what the compression ratio would be off the top of his head. He loves to talk and answer all your questions.

He also told me, add a header and you'll see 130 hp. /ubbthreads/images/graemlins/cheers.gif

Geo Hahn
06-24-2005, 07:29 PM
[ QUOTE ]
...What I don't know is the thickness of the stock head gasket, or the compressed thickness of any of the head gaskets...

[/ QUOTE ]

I just replaced the head gasket on the TR3A and have the old one right here. It is .048 thick (about 1.2 mm). That is compressed and miked right around the cylinder area.

And yes Dave... the TR head gaskets are 'one size fits all' so even my little 83mm pistons get a head gasket sized big enough for 87mm pistons.