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Author Topic: Simple and lucid instructions to bump up compression,  (Read 324 times)

kylohere

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Simple and lucid instructions to bump up compression,
« on: April 21, 2014, 05:36:44 AM »
Greetings!

I know this has been discussed plenty of times in the past, but a lot of posts have been made assuming that the reader has some basic knowledge of working on the Enfield along with a good mechanical aptitude.

I was wondering if any of the stalwarts here could spare some time off their precious 'Bullet quality time' and lay out the process, step by careful step, of lapping the bore and head etc. to increase compression of a bone stock 350.

It is indeed a black art here in India and the versions I have heard honestly sound dubious.

Thanks in advance!

Mr.Mazza

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Re: Simple and lucid instructions to bump up compression,
« Reply #1 on: April 21, 2014, 05:44:14 AM »
The simplest and easiest way my bikes previous owner did was:
First, buy a hi-comp piston head and new rings for the engine, a flatter head gasket to get a wee bit more compression.
That's pretty cheap and easy way to get a compression from around 6:1 to around 8:1 (Rough estimation, I will test my compression with a gauge soon)
Even if you don't wanna mod it yourself, a good engine shop shouldn't charge too much.
Lizzy - 07 500 Deluxe ES - Red and chrome

kylohere

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Re: Simple and lucid instructions to bump up compression,
« Reply #2 on: April 21, 2014, 06:24:14 AM »
Greetings Mr. Mazza!

By the way, Mazza is Hindi for 'fun'!

Thanks for the quick reply. Doesn't the Iron Barrelled Bullet 350 have a compression ratio of 7.25:1 straight out of the box?

I was looking more in terms of lapping the top of the spigot and the recess in the head to go gasketless. Then may be cut off some metal at the base of the block which mates with the crankcase.

Some numbers regarding extent of metal reduction would be helpful.

Going to a machine shop is the easiest way but doesn't really give you the kind of kick that only doing it yourself will provide.

Oh, and I hope all of you had a wonderful Easter!

kylohere

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Re: Simple and lucid instructions to bump up compression,
« Reply #3 on: April 21, 2014, 06:26:07 AM »
Also, I assume that a bump up in compression might need some advancing in timing as well..?

Mr.Mazza

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Re: Simple and lucid instructions to bump up compression,
« Reply #4 on: April 21, 2014, 10:01:19 AM »
Going out of my league here mate sorry!
Maybe ace will be able to help, he is quite well knowledge in these matters, I don't get into heavy engine/cylinder mods, just simple replacement parts.
Lizzy - 07 500 Deluxe ES - Red and chrome

ace.cafe

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Re: Simple and lucid instructions to bump up compression,
« Reply #5 on: April 21, 2014, 11:18:52 AM »
The easiest way to raise compression is to eliminate the cylinder base gasket, and use just hylomar sealant instead. This raises the piston in the barrel by at least a half-millimeter, and should be good enough to get to 8:1 on a 350.
For using pump gasoline, that's probably all the compression increase it can handle with stock cams in it.

If you want to lap the head on to the barrel, use a lathe to cut the barrel spigot down just enough to allow the head to almost touch the barrel at the gasket mating surface, and use lapping compound on the spigot to lower it the rest of the way.
Use some hylomar around the pushrod tunnels to help seal the oil there.
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kylohere

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Re: Simple and lucid instructions to bump up compression,
« Reply #6 on: April 25, 2014, 05:47:48 AM »
Thanks guys!

Cheers and ride safe!

AgentX

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Re: Simple and lucid instructions to bump up compression,
« Reply #7 on: April 25, 2014, 07:25:23 AM »
This will increase compression, sure...but what else needs to adjust in concert with that??

ace.cafe

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Re: Simple and lucid instructions to bump up compression,
« Reply #8 on: April 25, 2014, 09:32:20 AM »
This will increase compression, sure...but what else needs to adjust in concert with that??

It's not a huge change, so maybe move to the next higher octane grade of fuel, and do the usual ping timing.
I don't know about the stock 350 piston, but it is smaller and lighter, so it has a better chance than the 500 piston.
Home of the ACE Fireball 535 Bullet,  Ace GP Hi-Lift Roller Rocker Head . Pistons, cams, etc. Highest performance Bullet engine mods available .  AVL mods. Redditch 700/750 Twin mods. UCE kit soon.

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High On Octane

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Re: Simple and lucid instructions to bump up compression,
« Reply #9 on: April 25, 2014, 12:20:05 PM »
I get skeptical every time someone wants to raise compression but does nothing to strengthen the bottom end.    :-\
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Bullet Whisperer

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Re: Simple and lucid instructions to bump up compression,
« Reply #10 on: April 25, 2014, 01:00:36 PM »
I get skeptical every time someone wants to raise compression but does nothing to strengthen the bottom end.    :-\
The 350 has an advantage here over the 500, in that the bottom end is as strong as a 500, but with a much lighter piston. Generally, I shorten the barrels by 2mm, from the base, this work reduces the combustion chamber volume by about 8cc on its' own at TDC with whatever piston is in place. Doing this and fitting a Meteor Minor Sports piston [big dome, see pic] will still only give around 9.5:1 C.R., something I have done many times. The 350 really wakes up at around 10:1, but the cutaway for the inlet valve in the piston crown, whatever piston is used will usually need cutting a bit deeper, once 2mm removed from the barrel base is exceeded and any head gasket done away with. The rewards for this extra hassle are well worth it, though, with a big increase in go for that extra 0.5:1 or so hike in compression!
 Although from our racing projects, pictured below [on the left] is a Meteor Minor Sports piston. There are a few 350 Bullets in the U.K. [and one in France] using these  ;)
 B.W.

High On Octane

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Re: Simple and lucid instructions to bump up compression,
« Reply #11 on: April 25, 2014, 01:03:40 PM »
BW - Thanks for that bit of information.  While we are on the subject, what material is the stock 350 rod made out of?

Scottie J
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Re: Simple and lucid instructions to bump up compression,
« Reply #12 on: April 25, 2014, 01:08:26 PM »
BW - Thanks for that bit of information.  While we are on the subject, what material is the stock 350 rod made out of?

Scottie J
Hi Scottie, the stock 350 rod is alloy and identical to the 500 rod, except the small end eye is a little narrower.