square four bore size .

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GuyWalton
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square four bore size .

Postby GuyWalton » Tue Jan 28, 2020 11:21 am

I have decided to rebore my 52 square four and fit new pistons . I rebuilt the engine but , at the time , decided to keep the original pistons as there was very little wear . One of the pistons was however corroded on the top edge and I wasnt that happy . Now that it is finished and it is still too cold to ride it I have decided to renew the pistons . So barrels are off and pistons are purchased . I am now looking at what clearance is right from what I can find it says 1 to 2 thou . This seems far too tight , especially as how hot it gets on the rear cylinders , I read somewhere 6 thou front and 7 rear but this seems too big . Does anyone have indication of what is best . Im using the low compression pistons that were for that year and not the 53 on higher compression pistons .
51 mk1 Areil Square Four
55 DB 500 BSA Gold Star
55 Matchless G9
67 BSA B44
06 Ducati 999R

john.whiting
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Re: square four bore size .

Postby john.whiting » Tue Jan 28, 2020 12:48 pm

The maker of the pistons is the one to specify clearance ......which is entirely dependant on design and material of the pistons.......I could quote one piston made from possibly a high expansion alloy where the maker specifies .020 " clearance....and I know from experience if you decide its too much and cut it to 015,the motor will seize as soon as you apply power....In a S4 it does seem reasonable to give the rears 001 greater clearance than the fronts.

roger.fellows
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Re: square four bore size .

Postby roger.fellows » Tue Jan 28, 2020 2:03 pm

There's probably some detail about this on Drags website, Bruce Longman has put information together. Have you looked in the knowledgebase Guy?

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Re: square four bore size .

Postby JohnnyBeckett » Tue Jan 28, 2020 3:43 pm

HI would get in touch with the maker of the pistons like john says modern materials expand different then the old pistons and drags website clearances mite not be right for your new pistons wen i do reboring i go by what the makers say and a lot of clearances have change over the years i do the boring then i make sure the rings gaps are right in the bores before i fit the pistons :!:

GuyWalton
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Re: square four bore size .

Postby GuyWalton » Tue Jan 28, 2020 4:00 pm

roger.fellows wrote:There's probably some detail about this on Drags website, Bruce Longman has put information together. Have you looked in the knowledgebase Guy?


Yes drags says 1/3 thou clearance . I have received the pistons today , from drags , and the information on them is 4 thou . So i will go for that all round . I have a new sunnen honing tool coming so I will wait for that before i do anything more . I have the barrels set up in my vertical milling machine and have just fitted and reamed the little end bushes . I have also fitted valve oil seals on the valve guides .
51 mk1 Areil Square Four
55 DB 500 BSA Gold Star
55 Matchless G9
67 BSA B44
06 Ducati 999R

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rschurr
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Re: square four bore size .

Postby rschurr » Tue Jan 28, 2020 4:04 pm

With my new piston set from IMP for my MKII the instruction was also 0.1mm (4 thou") clearance about 9mm below the piston eye.
1923 James Model 12 500ccm v-twin -- 1960 Chevrolet Corvette C1
1926 Douglas EW 350ccm flat-twin -- 1967 Chevrolet Corvette C2
1936 Motosacoche 500ccm single --- 1979 Chevrolet Corvette C3
1948 BSA A7 500ccm parallel twin -- 1955 Ariel Square Four MKII :!:

jj.palmer
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Re: square four bore size .

Postby jj.palmer » Tue Jan 28, 2020 7:19 pm

It's important that you compare the piston assembly weights between the old and the new, you may be changing the balance factor which may lead to engine vibration.
Before any rebore, crank grinding etc. I always check the balance factor and then use that has a datum for any rebalancing.

John P.

GuyWalton
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Re: square four bore size .

Postby GuyWalton » Tue Jan 28, 2020 8:30 pm

jj.palmer wrote:It's important that you compare the piston assembly weights between the old and the new, you may be changing the balance factor which may lead to engine vibration.
Before any rebore, crank grinding etc. I always check the balance factor and then use that has a datum for any rebalancing.

John P.


I went with the same low compression piston for the earlier mk1 so that it was like for like only 20 thou bigger .
51 mk1 Areil Square Four
55 DB 500 BSA Gold Star
55 Matchless G9
67 BSA B44
06 Ducati 999R

GuyWalton
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Re: square four bore size .

Postby GuyWalton » Fri Jan 31, 2020 7:53 pm

All done and pots back on . Forgot what a bastard it is to get the pistons back in the liners it is . Head on tomorrow .
51 mk1 Areil Square Four
55 DB 500 BSA Gold Star
55 Matchless G9
67 BSA B44
06 Ducati 999R

GuyWalton
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Re: square four bore size .

Postby GuyWalton » Sat Feb 01, 2020 5:55 pm

Heads on and engine is running . Had a bit of a chew with the ignition timing till i got my head around which way the distributer turns but got there in the end . Tank on tomorrow , all nicely painted , and thats it time for riding . Im not doing another till next year although I have a G9 and a B44 ill have to get moved on as I dont have the room for them all . so there will need a bit of fettling to makes sure they are still good .
51 mk1 Areil Square Four
55 DB 500 BSA Gold Star
55 Matchless G9
67 BSA B44
06 Ducati 999R


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