LCGB Forums

I'm doing a project at the moment and i'm struggling with the cylinder head hitting the frame and the ground clearance for the exhaust looks pretty grim.

My potential solution: Remove the frame tube under the loop and build a new section to hold the engine. This sorts the cylinder head issue but not the exhaust ground clearance. I'm thinking of raising the engine mounts higher on the new support to give more clearance. This would mean the angle of the engine (to ground) is greater. The usual angle looks to be about 10" , i'm thinking of doubling that.

Any thoughts of this causing any running issues ?

I have no idea what you're talking about. Perhaps some pictures might help?

Hi it would be useful to know what frame,exhaust,cylinder,manifold and rear spring you're using also have you got off set cones

I'm trying to sort some photo's but till then....

It's a Malossi MHR cylinder that i've grafted onto a series 3 casing. I've also fitted a reed valve crank induction.

After assembling the engine top end i've now found the cylinder head fins are quite a bit longer than lambretta ones (maybe 40mm ?). This coupled with a 110mm rod on a 60mm crank is pushing the head into the frame (series 2). I have tried off-set cones but it's still miles away. I measured the shock studs centre to centre and when the head touches the tube it measures 280mm. Adapting a shorter shock absorber would help but this pushes the cylinder down then i would have an issue with exhaust ground clearance. The spark plug on the MHR is also a problem. It is centralised on the head which is plum in line with the frame tube.....meaning if i could get it to fit it would mean dropping the engine to get to the plug

My solution is now to use a Ld frame. This has already had some of the tubing cut away in preparation for a series 3 engine. The Malossi engine would fit but the overall length of the engine is slightly longer than the Ld engine bay. By lifting the cylinder end higher this shortens the overall length of the engine, it also allows for better ground/exhaust clearance. Looking sideways, the engine would be angled at 45" (probably less but just so it helps to give an idea).

Hopefully when i load some photo's it will help explain things better.

thanks for the interest.

hi, ive been down this route with my rotax conversion, my barrel is slightly leaning downwards to give the head enough room, plus I had to grind some of it away to get enough clearance, crankcase induction was a similar issue, very close to the frame tube when engine is lifted an the back, to simulate the shocker being compressed, it was a juggling act, my exhaust is very low, I think about 4 inches at the downpipe, but ive only grounded it once up to now, but I never ride up or down a kerb and I avoid speed bumps

Untitled by stephen holt, on Flickr
Untitled by stephen holt, on Flickr

If I have the gist of the problem, & cutting drastic amounts from the head & utilising one of the very low spark plugs available are not viable options, then I would consider replicating the engine support tube & bolting it up with high tensile socket screws & nuts to the existing engine support tube (make sure the fasteners clear the 16 mm engine bar) to test the theory of the pivot point being moved backwards. For the sake of a bit of turning on a lathe & drilling holes, you can test revised geometry prior to committing to welding up.

(Back in the 70's, some racers revised the mount position, including one that moved his back. Of course, it wasn't very good for racing as, if anything, you desire a shorter wheelbase! Probably good for making handling less twitchy......Didn't Ron Moss do a longer wheelbase crankcase reed Avanti recently?)

Any longer S2 rear shock absorber should cope with the revised angle due to the revised engine mounted shock position.

I hope that helps.....

Due to my excellent description i think you've got the gist of my problem.

Yes, would the angle of the shocker be an issue and would the oil in the engine circulate enough. My engine is way to close to the frame to squeeze it in like Holty did.......i could consider a different head.....i've considered it and don't want to change the original MHR head ...so

i'm going for the angled optioned in an Ld frame and if i can get it to fit i'll then find any issues...or not !!

MickYork wrote:Due to my excellent description i think you've got the gist of my problem.

Yes, would the angle of the shocker be an issue and would the oil in the engine circulate enough. My engine is way to close to the frame to squeeze it in like Holty did.......i could consider a different head.....i've considered it and don't want to change the original MHR head ...so

i'm going for the angled optioned in an Ld frame and if i can get it to fit i'll then find any issues...or not !!

I don't think that with a large frame (S1, 2 & 3) that the shock absorber angle would present any issue.

(I had meant to ask whether you had considered skimming the deck (crankcase mouth) to allow the cylinder/head assembly to sit lower, possibly with a modified conrod or different compression height piston, but, no doubt, you have thought of that)

I suppose that with the Ld though, you will encounter many greater issues that it sounds like you are prepared to solve.....

Good Luck!

a suggestion that was made to me was to cut around the mounting tube but leave the weld on the front then peel the mounting away from the frame the required amount then re weld and add a gusset to give extra strength, maybe a combination will help, the shocker ive got fitted is about 10mm shorter than a standard one ,that gave much more room for the head, slightly less for the inlet, ive got my shocker set quite hard and the engine doesn't hit my manifold or bumpstop even though its pretty close, its still nice to ride.