AR Piston

[gregorious77]

Found this http://www.ebay.co.uk/itm/171508472106 which is 66.4mm (AR Bore size) but as description says oversize I asked if it was for the AR and suitable for Nikasil bores. They said Nikasil is OK but std bore is 66mm I have now educated them and they have said it must be for the Mk 1-3 then.
My question to the experts on here is will it fit the AR? the size is correct for the bore but is their a difference in wrist pin size and location or height of the piston etc between the Mk 3 and the AR or is it just the bore that is different?

[knackeredMk1]

Wossner made 66.4mm pistons specifically for the AR.

66.4mm is not a generally recognised size for an MD24 engine as generally sizes go up at 0.25mm intervals but if you had the bore cut to fit a 66.4mm piston it would be fine.

It is my understanding that it is the metallurgy of the rings that determine suitability of bore rather than the make up of the piston but there are some materials that overlap.

[gregorious77]

I was thinking of refreshing my top end as she has done just over 12K miles of trail riding. Bore will be fine just wanted to put a new piston, rings and bearing in there.

[gregorious77]

Going to do a compression test and if low will buy this piston and slap it in.

[admin]

watching this - mines also on 11k and the last 3 yars have been flat out on track.
i planned on a rebuild at the end of this year to freshen it up

[gregorious77]

You done a compression test?

[admin]

I havn't no, not got a tester myself.
it just sounds "a little looser" than it used too 3 years ago prior to 1500 miles flat out on track lol.

If anything like my NSR it will all be within tolerance and a freshen would make a big difference.

I also noticed my fingers were starting to tingle more these days too - perhaps a sign of the balance shaft bearings ?

[gregorious77]

up the exhaust port.
http://i71.photobucket.com/albums/i147/ ... 142902.jpg
http://i71.photobucket.com/albums/i147/ ... 142912.jpg

Compression tester on route. I have a spare barrel here too which is tip top.
Bore should be fine and mine makes obvious slapping noises at lower revs. Balancer shaft done already.

[gregorious77]

Received this

Comp tester

comp tester.JPG (40.2 KiB) Viewed 10015 times

Took the plug out, done the compression test and got low reading as expected.

reading

comp reading.JPG (30.43 KiB) Viewed 10015 times

Reading should be around 177psi according to the book so she is down by 30%

Top end refresh required.

[gregorious77]

Ordered all the its today but LT said Wossner stopped making a piston for this bike and they went over to Prox and this requires a rebore and plate (Circa £150). I told them I found the 66.4mm piston and ordered one but not sure it will work now?
Seller said its not for AR and thinking about it why would LT use a TRX piston if they could simply use an overbore size std CRM Mk1-3 piston. I am thinking there must be a difference in pin ctr to top or skirt dimensions that makes it not work.
I have emailed LT this evening to ask why they dont use a std wossner in a rebored AR.

[knackeredMk1]

'Seller said its not for AR and thinking about it why would LT use a TRX piston if they could simply use an overbore size std CRM Mk1-3 piston. I am thinking there must be a difference in pin ctr to top or skirt dimensions that makes it not work.'

Why would an aftermarket manufacturer produce a piston that is not a recognised overbore size but exactly the original dimensions of a non-reborable cylinder?

Why would many owners of CRM ARs rebore and replate the cylinder lining to match standard CRM bore sizes when the original plating is damaged?

LT don't use CRM Wossner pistons on rebored CRMs because they are much more expensive than they can buy PROX TRX pistons.

The Wossner 66.4mm piston will fit in your AR bore if the bore is not worn and has not changed from the original.

[gregorious77]

Why would an aftermarket manufacturer produce a piston that is not a recognised overbore size but exactly the original dimensions of a non-reborable cylinder?
Good point and reason I suspect it is right

Why would many owners of CRM ARs rebore and replate the cylinder lining to match standard CRM bore sizes when the original plating is damaged?
To fit a TRX piston (not seen mention of fitting Mk1-3 piston has this been done?)

LT don't use CRM Wossner pistons on rebored CRMs because they are much more expensive than they can buy PROX TRX pistons.
They Quoted me a Wossner piston first thinking I had a Mk1-3 and said they used to use them in the AR but they are no longer listed

The Wossner 66.4mm piston will fit in your AR bore if the bore is not worn and has not changed from the original.

I have a spare good cylinder so it will fit one of them!


Time will tell I will have the parts soon and I can offer up to my spare cylinder. only unknown (to me) is pin centre to top of piston dimention of a Mk3 vs AR or TRX vs Mk3.

[knackeredMk1]

All correct answers .

The only thing that varies from TRX 250 (1987-89) pistons and MD24 and MD32 CRM 250 pistons is a window in the inlet side of the piston skirt on the TRX and the outer diameter of the little end bearing. (Some manufacturers mess about with coatings and lubrication holes)

[gregorious77]

Sweet news. LT were very interested in the availability of this piston so lets hope all is good.

[gregorious77]

Well seller refused to sell me the piston because LT told him it wont fit!!!
I told him to ship it anyway but he refused to do so point blank!
waiting on call back from LT to see what the difference between a TRX piston and a Mk1-3 CRM piston is. If there is no difference in dims there is no reason why it wont fit!