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#702205 - 07/19/17 1:15 pm Wrist pin blues  
Joined: Aug 2006
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robertllr Offline
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robertllr  Offline
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Crozet, VA
I am very concerned. I sent my cylinder block (1970 TR6C) to a well-publicized guy on the west coast to be bored and fitted with new pistons and rings.

It is the new pins that are my concern: the walls are much thicker, the pins weigh 8 grams each more, and they are a much looser fit--in both the pistons and the small end bushes. (I did not renew the small end bushes, as they seemed fine.)

Removing the original pins when dismantling required heating the piston crowns, as did removing and installing the last set I did on my 1968 TR 120 back in the eighties.

Although I do not detect any play with my fingers with the new pins, they are an unnervingly easy slip fit, cold, into the pistons. In fact, they will fall right out of the pistons of their own weight if you hold the assembly on its side..

The micrometer sees this difference, between old and new, as 0.0001 in

The old pins are still a slip fit in the new pistons, but at least they don't fall out.

I'm am thinking about reusing my old pins. So two questions: should I be concerned with less than an interference fit between piston and pin; and is 18 grams of additional reciprocating mass asking for trouble?

Anyone ever heard of stock pins breaking? Why increase the wall thickness and weight? Oops, I'm up to four questions, now, aren't I Oops, now that's five... I'm stopping...

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#702216 - 07/19/17 4:59 pm Re: Wrist pin blues [Re: robertllr]  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
You haven't said what make of pistons.

Lack of an interference fit in the pistons is not usually any problem at all.

In my Brit bike experience, there are two fairly common types of gudgeon pin. One is parallel bored; the hole through the other type tapers to a thin edge at the ends and is lighter.

If the complete piston, pin and ring assemblies are significantly heavier than before, engine vibration will be altered and the difference may well be for the worse.


Amateur Loctite enthusiast.
#702217 - 07/19/17 5:18 pm Re: Wrist pin blues [Re: triton thrasher]  
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robertllr Offline
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robertllr  Offline
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Crozet, VA
The originals so indeed have internally tapered walls, while the replacements do not. Hence the extra weight.

The pistons supplied to me are JCC, 020 over, with Hastings rings. I didn't say that because I didn't see why that would make a difference. Why would it make a difference?

The piston and ring assemblies, old and new, sans pins, weigh exactly the same--262 grams.

So, I ask again, is 16 grams overweight--due to the extra pin mass alone--a significant detriment to the balance?

#702219 - 07/19/17 5:32 pm Re: Wrist pin blues [Re: robertllr]  
Joined: Dec 2004
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kommando Online content
kommando  Online Content


Joined: Dec 2004
Posts: 8,890
Scotland
The balance factor is 85% of the weight of the piston assembly and a portion of the conrod, call it 350g (rough guess) X 0.85 = 297.5g, 16g is 5% extra making the balance factor 80%.

#702220 - 07/19/17 5:33 pm Re: Wrist pin blues [Re: robertllr]  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
16 g is significant, but not drastic. No-one really knows the effect on how the bike will feel.

It will reduce the balance factor. You'll have more up and down force at origin, peak vibration is likely to be at a different rpm, but the real tangible effect is unpredictable.

Heavier pistons had a bad effect on the vibration of my pre-unit, but that's a different bike.


Amateur Loctite enthusiast.
#702226 - 07/19/17 6:52 pm Re: Wrist pin blues [Re: robertllr]  
Joined: Jun 2007
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HawaiianTiger Online content
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HawaiianTiger  Online Content

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Maui Hawaii
A machinist I admire would routinely fix light press pins to slip fit. Up to .001 clearance in the bush.
He said no oil could get in otherwise.
Cheers,
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
#702241 - 07/19/17 8:43 pm Re: Wrist pin blues [Re: robertllr]  
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RF Whatley Online content
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RF Whatley  Online Content
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North Georgia, USA
Different brands of pistons will have their own piston pin fit. That's just the way it is.


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#702246 - 07/19/17 9:38 pm Re: Wrist pin blues [Re: robertllr]  
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robertllr Offline
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robertllr  Offline
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Crozet, VA
My thanks to both "Scotties."!

I'm decided: I'm using the old pins. The stock bike was one of the smoothest Trumpets I've had. I'll stick with the original balance factor.

#702250 - 07/19/17 9:48 pm Re: Wrist pin blues [Re: robertllr]  
Joined: Mar 2005
Posts: 9,970
John Healy Online content
John Healy  Online Content



Joined: Mar 2005
Posts: 9,970
Boston, Massachusetts
As I posted to Triumph Rat Forum
Just some thoughts.
a. When measuring tenths of a thousandths the difference in the diameters can often be explained. One has to be extremely careful to make sure both the pin and the micrometer are all the same temperature. Holding the micrometer too long, or even leaving it on a bench exposed to a bright light can cause differences in the measurement you will get. The same goes for the pin by either holding it too long or exposing it to a warm heat source.

b. Since a lot of pistons manufacturers have gone from boring the piston pin hole to roller burnishing it, with the exact same hole and pin dimensions, the pin went from where you had to warm the piston to where it became a slip fit to where it became a slip fit of its own weight. It has to do with the improved surface finish and accuracy of the hole and modern methods of finishing pins.

c. the difference in weight of 16 grams (8 grams x2 pins) means the difference in the reciprocating weight is less than 4% (top end of rod, piston, pin and rings) of a stock assembly and less than 1% difference in the bob weight used when balancing the crankshaft. The piston, and thus the pin, most likely is from JCC. These piston assemblies have been used in these motors, as you received it, without any concerns for almost 40 years. The JCC pins are more than adequate for street use, and have been used in quite a few budget racing motors. If I was looking a get some real HP (60hp plus) I would be looking for a better pin.

d. Comparing REAL AE Hepolite (not Wassell JCC - Hepolite) Standard vs plus .020" pistons there is a difference of 390 grams (195 grams x 2). That is a big difference from 16 grams and these engines are typically bored without rebalancing with little, if any, difference in vibration. The weight difference for JCC pistons also increases with each oversize.

Quote:
Anyone ever heard of stock pins breaking?
Not in a street engine!


#702252 - 07/19/17 10:07 pm Re: Wrist pin blues [Re: robertllr]  
Joined: Mar 2005
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John Healy Online content
John Healy  Online Content



Joined: Mar 2005
Posts: 9,970
Boston, Massachusetts
Quote
The balance factor is 85% of the weight of the piston assembly and a portion of the conrod, call it 350g (rough guess) X 0.85 = 297.5g, 16g is 5% extra making the balance factor 80%.


the pistons are around .320 grams x 2 = 640 grams the top end of the rods are around 106 grams x 2 = 212 grams TOTAL 640 + 212 = 852 grams. 16 / 853 = 2% of reciprocating weight (both rods and pistons). But if it matters Robert has all of the kit so he can measure all of the bits and do the math. The stock 650 engine was balanced with two 689 gram weights, one on each rod.

BUT the new oversize pistons are going to weigh some where around 20 grams (x2 or 40 grams) for each oversize over standard. So if we are to worry, and few do, the increase in the weight of the piston is going to have a much bigger effect upon the balance factor than the 8 grams difference in the pins.


#702283 - 07/20/17 3:46 am Re: Wrist pin blues [Re: robertllr]  
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robertllr Offline
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robertllr  Offline
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Crozet, VA
I tried several scales. They all agree: the stock Hepolite removed from the bike (marked inside as such), with rings (de-coked, no pin) weighs 262 grams, while the JCC .020 over weighs less than a gram more.

With pin and circlips included, I measure a shade under 317 grams for one Hepolite assembly, though I can imagine a rough estimate of 320 per complete assembly is within bounds.

But no way am I finding anything even remotely like a 20 gram difference between the stock Hepolite and +1 oversize JCC for the piston with rings alone. Perhaps the discrepancy is due to comparing two different brand pistons/rings?

However, I find it fascinating to learn that modern piston manufacturing methods have resulted in pins that no longer require an interference fit. Thank you very much for that reassuring information!

#702286 - 07/20/17 5:43 am Re: Wrist pin blues [Re: robertllr]  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
Originally Posted by robertllr
My thanks to both "Scotties."!

I'm decided: I'm using the old pins. The stock bike was one of the smoothest Trumpets I've had. I'll stick with the original balance factor.


They'll be ok if they're the same length and not blued with heat.


Amateur Loctite enthusiast.

Moderated by  John Healy 


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